The Open Sea: The World of Plankton
The Open Sea: The World of Plankton
Alister Hardy
The New Naturalist editors believe this to be the greatest general work on the subject ever written.Professor Alistair Hardy is truly obsessed by animals of the sea – devotedly enthusiastic about the nature of their adaptations and life histories, brilliantly critical in the examination of their mysteries, acutely lucid (and at the same time highly artistic) in his descriptions of them in his arresting plates.To describe the relatively unknown and mysterious world of plankton is a task that the greatest of marine zoologists might boggle at. Yet the plankton is to the sea what vegetation is to the land. The study of plankton is a complex discipline which few amateur naturalists have had the privilege to enjoy. Never before has such a synthesis of knowledge been attempted in a community of animals so mysterious, yet so important. Professor Hardy has grasped this problem in a new and exciting way; and at least the common reader can discern the pattern of life that dominates two-thirds of the world’s surface.
Collins New Naturalist Library
34
The Open Sea – Its Natural History:
The World of Plankton
Alister C. Hardy
EDITORS: (#ulink_e6e54ff0-6414-5b73-9650-9f68a5f32df3)
JAMES FISHER, M.A.
JOHN GILMOUR, M.A.
JULIAN HUXLEY, M.A., D.SC, F.R.S.
L. DUDLEY STAMP, G.B.E., D.LITT., D.SC.
PHOTOGRAPHIC EDITOR:
ERIC HOSKING, F.R.P.S.
The aim of this series is to interest the general reader in the wild life of Britain by recapturing the inquiring spirit of the old naturalists. The Editors believe that the natural pride of the British public in the native fauna and flora, to which must be added concern for their conservation, is best fostered by maintaining a high standard of accuracy combined with clarity of exposition in presenting the results of modern scientific research. The plants and animals are described in relation to their homes and habitats and are portrayed in the full beauty of their natural colours.
Table of Contents
Cover (#u82c38694-9eb0-577d-9fa4-3ab644beba53)
Title Page (#ua3d0bf02-d61d-5172-9d69-2d2f0063da7b)
Editors (#u33b5931d-34cc-591f-8e03-74f51ec6208b)
Editors’ Preface (#u15a5dc3c-e074-5bd3-b86f-385281f19702)
Author’s Preface (#ucb71242a-0b06-57a9-80b3-2ac33fa25869)
CHAPTER 1 (#ubcb89914-6a12-5f9a-961d-a32f7a64e31a)
INTRODUCTION
CHAPTER 2 (#u3397ab9e-de11-5382-badf-93137316dee4)
THE MOVEMENT OF THE WATERS
CHAPTER 3 (#uf28d2542-dfd2-59a4-8652-470397ba0dae)
PLANTS OF THE PLANKTON
CHAPTER 4 (#uf9e2ac0e-843a-5efb-963b-16fe015b8f29)
SEASONS IN THE SEA
CHAPTER 5 (#ub9d1ef9b-f5a9-51ce-96c7-37d3c4e2a8de)
INTRODUCING THE ZOOPLANKTON
CHAPTER 6 (#litres_trial_promo)
LITTLE JELLY-FISH AND LESSER FORMS OF LIFE
CHAPTER 7 (#litres_trial_promo)
SIPHONOPHORES AND THE LARGER JELLY-FISH
CHAPTER 8 (#litres_trial_promo)
MORE ANIMALS OF THE PLANKTON—BUT NOT THE CRUSTACEANS
CHAPTER 9 (#litres_trial_promo)
THE PLANKTONIC CRUSTACEA
CHAPTER 10 (#litres_trial_promo)
PELAGIC LARVAL FORMS
CHAPTER 11 (#litres_trial_promo)
THE PUZZLE OF VERTICAL MIGRATION
CHAPTER 12 (#litres_trial_promo)
LIFE IN THE DEPTHS
CHAPTER 13 (#litres_trial_promo)
PHOSPHORESCENCE AND PHOTOPHORES
CHAPTER 14 (#litres_trial_promo)
SQUIDS, CUTTLEFISH AND KIN
CHAPTER 15 (#litres_trial_promo)
PLANKTON AND THE FISHERIES
Glossary (#litres_trial_promo)
Bibliography (#litres_trial_promo)
Index (#litres_trial_promo)
Colour Plates (#litres_trial_promo)
Plates in Black and White (#litres_trial_promo)
Copyright (#litres_trial_promo)
About Publisher Page (#litres_trial_promo)
EDITORS’ PREFACE (#ulink_472ac1a3-e0a2-5cc4-97be-95d34706cd70)
PROFESSOR HARDY began his marine biologist’s life over a third of a century ago on his return from service in the first world war. After Oxford and a scholarship to the Stazione Zoologica at Naples, he soon became a member of the Fisheries Department in the Ministry of Agriculture and Fisheries; and, in the middle ‘twenties, served as Chief Zoologist to the R.R.S. Discovery expedition, to the Antarctic seas, making a special study of plankton. His subsequent professorships—first at University College (now the University of) Kingston-upon-Hull; next at Aberdeen; and since 1945 at the University of Oxford—have brought him the highest academic status and honours, but have not kept him away from his beloved sea. In the closing stages of the writing of this volume, as the editors well remember, he was correcting the typescript, and completing his unique and wonderful colour illustrations, on the deck of the latest Royal Research Ship, Discovery II, scanning the contents of each netting or dredging, sketching new or rare creatures of the sea before their colour faded, applying himself to his research with an enthusiasm excelling that of most naturalists of half his age.
If the editorial board were asked to select from Professor Hardy’s many scientific qualities that which has contributed most to the creation of this extraordinary book, they would perhaps settle for enthusiasm. Throughout The Open Sea it is quite apparent that he is devotedly obsessed by, and interested in, animals; he is eternally curious about the nature of their adaptations and lives, brilliantly critical in the examination of their mysteries, acutely lucid and at the same time highly artistic in his depiction of them in his remarkable plates. It was a welcome burst of enthusiasm that caused Professor Hardy to write so much and so well of the life of the sea that he has written us two books instead of one. It is the first of these, concerned with the general natural history of the open sea and the world of its plankton, that we here welcome. The second part of The Open Sea concerns the open sea’s fish and fisheries, and will be published some time in 1957 or early 1958; like the present book, it will be illustrated by Professor Hardy’s own colour paintings, which represent what no colour-camera has yet been able to catch, and by black and white photographs by that most distinguished marine biologist and skilful photographer, Douglas P. Wilson.
To most readers the subject of this first of Professor Hardy’s two contributions to our series—the world of Plankton—will be relatively unknown and mysterious; but here the enlightened amateur naturalist is shown how, with modest equipment he may investigate it himself. The world of plankton is a world of complex anatomy, much of which can be understood only with the lens of the microscope. The life-histories of the animals are also complicated; some of them are extraordinary. To describe the plankton of our seas, and to set it in its pattern of community, climate, sea-scene and season is a major task. Professor Hardy has brought vast knowledge and experience and scholarship to a synthesis never before attempted.
THE EDITORS
AUTHOR’S PREFACE (#ulink_e7956feb-8941-5258-be07-4c5f44a5e981)
ORIGINALLY it had been intended that the whole natural history of the sea, apart from that of the sea-shore and of the sea-birds already dealt with in the New Naturalist series, should be treated in one general volume. As the writing proceeded, however, it became clear that to do justice to the subject it would be impossible to include all its different elements within a single cover. There is the life of the plankton in almost endless variety; there are the many kinds of fish, both surface and bottom living; there are the hosts of different invertebrate creatures on the sea-floor; and there are those almost grotesque forms of pelagic life in the oceans depths. Then there are the squids and cuttlefish, and the porpoises, dolphins and great whales. In addition man’s fisheries now play such an important part in the ecology of our waters that they also must form a part of any general natural history of the sea.
Certainly there is too much material to go into one volume. There occurs, however, a fairly natural division between the teeming planktonic world and the other categories of life it supports: the fish, the whales and the animals of the sea-bed. This first book on the open sea deals mainly with the plankton; it aims at giving the general reader a non-technical account, save for the necessary scientific names, of its many remarkable animals and showing how, with only a little trouble, quite a lot of them may be seen and studied alive. Perhaps to some it may introduce a new world of life—a world so unusual that few of its inhabitants have homely English names at all. It is hoped, too, that it may be a guide to the plankton for university students who are beginning their studies in marine biology. The book also deals with the water-movements and the seasons in the sea; and it contains an account of the squids and cuttlefish, and of those queer creatures, including the deep-water (and often luminous) fish, swimming in the great depths only a little way beyond our western coasts. It will conclude by showing how a study of the plankton is helping us to have a better understanding of the lives of our commercially important fish. Later, and before very long, will come the sequel: a separate volume devoted mainly to fish and fisheries, but also including whales, turtles and other marine animals which are likewise, directly or ultimately, dependent on the plankton for food.
Before going any further I must thank the publishers and editors, not only for all the trouble they have taken over the production of this book, but also for the patience they have kindly shown over my delay in its completion. I accepted their invitation to write it in August 1943, some twelve years ago; it has, however, meant more than the writing. My excuse for its late arrival will be offered after I have made my main acknowledgment.
The value to the book of the remarkable collection of photographs by Dr. D. P. Wilson of the Plymouth Laboratory will be clear to all, but just how wonderful they are and consequently how lucky I am to have them as illustrations, may not at once be fully appreciated by those who are not yet familiar with the living plankton of the sea. Douglas Wilson has long been recognised as the leading photographer of marine life and his beautiful pictures in black-and-white and in colour which graced Professor C. M. Yonge’s The Sea Shore in this series of volumes will, I am sure, have been seen and admired by most of my readers. I, too, am showing some of his studies of the larger forms of life, such as those of cuttlefish or his unusual view of that strange jelly-fish, the Portuguese-man-of-war, taking a meal; it is, however, his photographs of the tiny plankton animals to which I particularly wish to draw attention here. Though they are taken through a microscope, they are photographs of creatures swimming naturally, very much alive and certainly kicking. Never before has such a series of photomicrographs of living members of the plankton been published; they are unique and will, I believe, be of immense value not only to marine naturalists but to all students of invertebrate zoology. They are the fortunate result of a remarkable combination; Dr. Wilson has brought his skill and artistry to work with that very modern invention the electronic flash. For the first time this device has made possible such instantaneous pictures at a very high magnification. It is not only that invention, however, which makes these pictures unique; while others will follow him, Dr. Wilson’s photographs will always have a quality of their own, because he is an artist as well as a scientist. He is not satisfied until he has produced a photograph that has an appeal on the score of composition as well as on that of scientific value. All his photographs except two (the stranded jelly-fish and squid) are of living animals. A few excellent black-and-white pictures by other photographers are included in some of the plates and these are acknowledged in the captions or the text.
It was my hope, and that of the editors, that in addition to his black-and-whites Dr. Wilson would have been able to contribute a series of colour photographs of the living plankton and especially of the richly pigmented animals from the ocean depths. At that time the electronic flash was only just being developed and he felt unable to attempt them. The movement of the ship at sea, he said, would prohibit the use of a long enough exposure to enable the deep-water animals to be photographed in colour by ordinary means; they quickly die and fade, and so must be taken as soon as they are brought to the surface. I had already had some experience in making water-colour drawings of living plankton animals on the old Discovery during the Antarctic expedition of 1925–27; the editors kindly allowed me to undertake a series of such studies to form the accompanying twenty-four colour plates. To obtain and make drawings of the full range of animals which I felt to be desirable, meant a considerable delay and this was added to an earlier postponement of my start on the book caused by my being appointed to the chair of Zoology at Oxford soon after I had accepted the invitation to write it. For several years the work of my new department and research to which I was already committed took all my attention.
All save seven of the 142 drawings in the plates were made from living examples or, in a few cases, from those taken freshly from the net when some deep-water fish and plankton animals were dead on reaching the surface. The seven exceptions, which are noted where they occur, were drawn from preserved specimens but with memories or colour-notes from having seen them alive; I should like to have drawn these too from life, but I could delay the book no longer. It may be of interest to record how the drawings were made. All the animals, except the larger squids and jelly-fish, were drawn either swimming in flat glass dishes placed on a background of millimetre squared paper where they were viewed with a simple dissecting lens, or on a slide under a compound microscope provided with a squared micrometer eyepiece; in either case the drawings were first made in outline on paper which had been ruled with faint pencil lines into squares which corresponded to those against which the specimen was viewed. In this way the shape and relative proportions of the parts could be drawn in pencil and checked and rechecked with the animal until it was quite certain that they were correct. The outline was then gone over with the finest brush to replace the pencil by a permanent and more expressive water-colour line; next all the pencil lines, including the background squares, were rubbed out and the full colouring of the drawing proceeded with. If rough weather at sea made such a course impossible, the living animal would be sketched in pencil, and painted, in perhaps one or two different positions, to give life-like attitudes and colouration without attempting to get the detailed proportions exactly right; it would then be preserved in formalin for accurate redrawing by the squared-background system when calmer conditions returned. The animals I have selected for illustration are mainly either those which are not included in the black-and-white photographs or those for which colour can add supplementary information. I have, for example, drawn some of the transparent but iridescent comb-jellies, but not the transparent and colourless arrow-worms or salps. I am most grateful to the Sun Engraving Company, who made the blocks for the colour plates, for the great care they have taken in making such excellent reproductions.
I must now make special acknowledgments in regard to these drawings. First I must record my thanks to Dr. N. A. Mackintosh, the Deputy Director in charge of the biological research of the National Institute of Oceanography, for kindly allowing me to accompany the R.R.S. Discovery II on two of her biological cruises in the Atlantic in the summers of 1952 and 1954. It is to the Discovery, with all her equipment of deep-water nets, powerful winches and laboratory accommodation, that 71 of these drawings are due, including all those representing the remarkable animals which live in the great depths over the edge of the continental shelf to the southwest of Britain. Without such facilities they could never have been made; actually three of them date back to earlier days when I had the honour of sailing south in the old Discovery in 1925. Next I must thank a number of kind helpers who have sent me living specimens of plankton in specially protected Thermos flasks from many parts of the coast: Mr. J. Bossanyi from Cullercoats, Dr. E. W. Knight-Jones from Bangor, Dr. Richard Pike from Millport, Professor J. E. G. Raymont from Southampton and Mr. R. S. Wimpenny from Lowestoft. Although I made many visits to different places to draw my specimens, there were still a number I could not get myself in the time available; these were supplied by these kind friends who were on the lookout for what I wanted at widely separated points. I am most grateful to Dr. Marie Lebour and to the Council of the Marine Biological Association of the United Kingdom for kind permission to reproduce some of her beautiful drawings of living plankton animals capturing their prey; these, which form my text-figures 26, 27, 35, 40 and 41, were originally published in her papers in the Association’s Journal in the years 1922 and ’23. Then I must thank Sir Gavin de Beer and members of his staff at the British Museum (Natural History), particularly Dr. W. J. Rees and Mr. N. B. Marshall, for kindly allowing me to make many of the black and white drawings in the text from specimens in the museum collections. I am similarly indebted to Dr. J. H. Fraser of the Marine (Fisheries) Laboratory at Aberdeen who has let me draw some of the beautiful plankton animals he has caught to the north and west of Scotland; and to Dr. Helene Bargmann and Mr. Peter David who have also kindly given me much help on looking out specimens from the Discovery collections for me.
With no less gratitude, I must make acknowledgments regarding the text. Apart from the more normal editorial comments and suggestions I particularly want to say how much I am indebted to my old friend—and once Oxford tutor—Dr. Julian Huxley, who read the whole book with the greatest care and made many valuable suggestions for its improvement. My typescript—how reminiscent of my undergraduate essays of 1919 and ’20!—was covered with his pencilled scribblings in the margin: “Surely you should refer to—, this might be made more emphatic” and the like; not all were adopted, but certainly a great many. The chapter on water movements was read by Dr. G. E. R. Deacon, and that on squids and cuttlefish by Dr. W.J. Rees; I am indeed grateful to them for a number of helpful suggestions they kindly made.
Finally I wish to draw the attention of those who are not scientists to a glossary at the end of the book giving a simple explanation of the few technical terms which have been unavoidably used; and for the zoologist I would point out that the authorities for the different specific names will be found quoted after these names in the index and not in the text where they are left out to avoid undue elaboration.
A.C.H.
CHAPTER 1 (#ulink_79bd5715-643a-5479-aa34-2f4332f09b89) INTRODUCTION
THERE is a very simple fact about the sea which makes its inhabitants seem even more remote from us than can entirely be accounted for by their being largely out of sight. To make my point allow me to imagine a world just a little different from our own.
Suppose for a moment that we live in a country which is bounded on one side by a permanent bank of fog. It is a grey-green vapour, denser even than that often known as a London particular, and it has a boundary as definite as the surface of a cloud so that it is like a curtain hanging from the sky to meet the ground; we cannot enter it without special aids except for a momentary plunge and as quickly out again for breath. We can see into it for only a very little way, but what we do see is all the more tantalizing because we know it must be just a glimpse—a tiny fraction—of all that lies beyond. We find it has life in it as abundant as that of our own country-side, but so different that it might be life from another world. No insects dwell beyond the barrier, but other jointed-legged creatures take their place. Unfamiliar floating forms, like living parachutes with trailing tentacles, show their beauty and all too quickly fade from view; then sometimes at night the darkness may be spangled with moving points of light—living sparks that dart and dance before our eyes. Occasionally gigantic monsters, equal in size to several elephants rolled into one, blunder through the curtain and lie dying on our land.
To make a reality of this little flight of fancy all we have to do is to swing this barrier through a right-angle so that it becomes the surface of the sea. How much more curious about its unfamiliar creatures many of us might be if the sea were in fact separated from us by a vertical screen—over the garden wall as it were—instead of lying beneath us under a watery floor. Who as a child has not envied the Israelites as they passed through the Red Sea as if marching through a continuous aquarium: “and the waters were a wall unto them on their right hand, and on their left.”? What might they not have seen? Because normally our line of vision stretches out across the sea to the skyline and carries our thoughts to other lands beyond, many of us tend to overlook this perhaps more wonderful realm beneath us, or we seem to think it must be too difficult of access ever to become a field for our exploration or delight.
The aim of this book is to give the general reader an account of the natural history of the open sea around our islands and at the same time show how he may, with only modest equipment, see something of this strange world for himself. The amateur naturalist afloat—whether on a yachting cruise, on a fishing vessel, or just out in a rowing boat—may see much if he has the right kind of quite simple gear and knows how to use it; he may perhaps also be lucky and make original observations which will be a contribution towards finding an answer to one of the many unsolved riddles of the sea. The book will also give a sketch of some of the factors upon which the success of our great sea fisheries depend. The lives of the different fish are like threads woven in a web of life—a network of inter-relationships between many various creatures large and small, as complex as any on the land. The story of fishery research, which belongs mainly to our subsequent volume, is so closely linked with this unseen web, that it is hoped an account of these less familiar animals may be as interesting to the fishermen as to the naturalist; indeed many fishermen are naturalists and have much of importance to tell the scientist.
As our title indicates, the book will deal with the open sea—the sea beyond the coastal waters. The life of the intertidal zone has already been beautifully treated in this series of volumes by my friend Maurice (C. M.) Yonge (1949). The sea-shore can be studied by direct observation as the tide recedes and has long been a happy hunting-ground for the naturalist; he can lift up the fronds of seaweed, turn over stones, probe into rock-pools and dig into the sand and mud. Our methods of studying the life of the open sea must be very different; it is far from ‘open’ to the investigator, being in fact a hidden world, but this makes its exploration all the more exciting. Deep-sea photographic and television cameras are important new developments which promise much for the future; they, however, as also submarine observation chambers like the bathysphere, must for some time to come be regarded as very costly and specialist equipment giving us here and there direct confirmation of what we usually have to find out by other means. The diving helmet and the aqualung may help us to see something of this enchanting world in shallow water, but for the discovery of what is happening over wide stretches of the underwaters of the open sea we must devise more indirect methods.
The fact that we can see only a very little way below the surface indicates a property of water, and particularly of the sea, which is of fundamental importance to the life it contains. Held up in a glass, water appears so very transparent that we are at first surprised to find how quickly light is absorbed in the sea itself and what a little distance its rays will penetrate. Measurements made in the English Channel off Plymouth show that at a depth of five metres (just over 16 feet) the intensity of light is less than half that just below the surface, while at 25 metres it is only an insignificant fraction, varying between 1½ and 3 per cent. This at once tells us that the green plants, which must have sunlight in order to live, will only be found in the upper layers of the water.
The one real difference, of course, between animals and plants is a matter of their mode of feeding. We know that an animal of any kind, whether mammal, fish, shrimp, or worm, must have what we call organic food: proteins, carbohydrates (sugars, starches and the like) and fats, which have been built up in the bodies of other animals or plants. One animal may feed upon another kind of animal which in turn may have lived upon other kinds, and perhaps these upon yet others, but always these food-chains, long or short, must begin with animals feeding upon plants. Only the green plants, with that remarkable substance chlorophyll acting as an agent, can build themselves up from the simple inorganic substances by their power of using the energy of sunlight (photosynthesis); they split up the molecules of carbon dioxide, liberate the oxygen, and combine the carbon with the oxygen and hydrogen of water to form simple carbohydrates, which are then elaborated into more complex compounds by being combined with various minerals in solution. On the land we are all familiar with this elementary fact of natural history; my reason for recalling it is to emphasise that it is of universal application. The plants are the producers and the animals the consumers as much in the sea as on the land. Indeed ‘all flesh is grass’.
Where then in the sea, we may ask ourselves, are all the plants upon which the hordes of animals must depend? They cannot grow in the darkness or dim light of the sea-floor, and the seaweeds, forming but a shallow fringe along the coasts, are of no real importance in the economy of the open sea. From the deck of a ship, or even from a rowing boat, we can see no plant-life floating near the surface; yet we know it must be there. Another little flight of imagination will, I think, help us to get some idea of the extent of this elusive vegetation.
Let us suppose for a moment that the herring is not a fish, but a land animal. We know that some three thousand million herring are landed every year at ports in the British Isles; these, together with all those landed in other countries, must be only a small fraction of their total number, for we also know that herring are the food of so many other abundant animals of the sea. For simplicity let us consider them to be feeding directly upon plants—and let us imagine them in their unnumbered millions sweeping across the continent. If we do this it needs no imagination to see that the countryside would be stripped of vegetation as if by locusts. Now let us think of the other fish in the sea besides the herring: the cod, haddock, plaice, skate and such that fill our trawlers (as distinct from the herring-drifters) to the extent of more than a million tons a year; then also think of the crowded invertebrate life of the sea-bottom. If all these animals were on the land as well, what an immense crop of plants it would take to keep them supplied with food! There are indeed such luxuriant pastures in the sea but they are not obvious because the individual plants composing them are so small as to be invisible to the unaided eye; we can only see them through a microscope. Their vast numbers make up for their small size.
As an introduction to all that follows let us consider the natural economy of the sea in its simplest terms. We have the sun shining down, its rays penetrating the upper layers of the water; we have the gases, oxygen and carbon dioxide, dissolved in it from the atmosphere; we have also the various mineral salts—notably phosphates and nitrates and iron compounds—continually being brought in by the erosion of the land, and there are minute traces of some essential vitamin-like substances. These are ideal conditions for plant growth. Just as these are spread through the water, so is the plant life itself scattered as a fine aquatic ‘dust’ of living microscopic specks in untold billions. In a shaft of sunlight slanting into a shaded room we have all watched the usually invisible motes floating in the air, floating because they are so small and light; these tiny plants remain suspended in the water in just the same way. Many of them are provided with fine projections like those of thistledown to assist in their suspension.
Feeding upon these tiny floating plants, and also like them scattered through the sea in teeming millions, are little animals. Crustacea, little shrimp-like creatures of many different kinds, predominate; mostly they range in size from a pin’s head to a grain of rice, but some are larger. There are hosts of other animals as well: small worm-like forms, miniature snails with flapping fins to keep them up, little jellyfish, and many other kinds which surprise us with their unexpected shapes and delicate beauty when first we see them through the microscope.
All these creatures, both animals and plants, which float and drift with the flow of tides and ocean currents are called by the general name of plankton. It is one of the most expressive technical terms used in science and is taken directly from the Greek πλavktov. It is often translated as if it meant just ‘wandering’, but really the Greek is more subtle than this and tells us in one word what we in English have to say in several; it has a distinctly passive sense meaning ‘that which is made to wander or drift’ i.e. drifting beyond its own control—unable to stop if it wanted to. It is most useful to have one word to distinguish all this passively drifting life from the creatures such as fish and whales which are strong enough to swim and migrate at will through the moving waters: these in contrast are spoken of as the nekton (Gk nektos, swimming). Actually when they are very young, the baby fish are strictly speaking part of the plankton too, for they are also carried along at the mercy of the currents until they are strong enough to swim against them. Photographs taken through a microscope of some typical planktonic plants and animals are shown in Plate I (#litres_trial_promo) and Plate II (#litres_trial_promo); they have been caught by drawing a net of fine silk gauze through the water. Their natural history will be dealt with in later chapters.
The simple sketch in Fig. 1 (#litres_trial_promo) shows this general economy of the sea in diagram form. A number of fish, including the herring, pilchard, sprat, mackerel and the huge basking shark, feed directly upon the little plankton animals; and so also, curiously enough, do the great whalebone whales, the largest animals that have ever lived. From this world of planktonic life, dead and dying remains are continually sinking towards the bottom and on the way may feed other plankton animals living in the deeper layers. For this reason the zoöplankton (animal plankton—Gk zoön, an animal) is not confined to the upper sunlit layers as is the phytoplankton (plant plankton—Gk phuton, a plant). On the sea-bed we find a profusion of animals equipped with all manner of devices for collecting this falling rain of food. Some, rooted to the bottom, spread out their branch-like arms in umbrella fashion and so look like plants; others, such as many shellfish, have remarkable sieving devices for trapping their finely scattered diet. Feeding upon these are hosts of voracious crawling animals. These and their prey together—worms, starfish, sea-urchins, crustaceans, molluscs and many other less familiar creatures—in turn form the food of the fish such as cod, haddock and plaice which roam the sea-floor in search of them. Finally comes man: catching the herring and mackerel with his fleets of drift-nets near the surface, hunting the great whales with explosive harpoons, and sweeping the sea-bed with his trawls for the bottom-living fish.
FIG. 1
A diagrammatic sketch illustrating the general economy of the sea.
We see how all-important the plankton is. All the life of the open sea depends for its basic supply of food upon the sunlit ‘pastures’ of floating microscopic plants.
Our knowledge of life in the sea has been built up step by step by many pioneer naturalists. Oceanography is still quite a young science; its beginnings were made only a little over a hundred years ago. It is worth while looking back.
The vast community of planktonic animals and plants was unsuspected till it was discovered by the use of a very simple device, the tow-net: a small conical bag of fine silk gauze or muslin, usually with a little collecting jar at its end, towed on a line behind a boat. In nearly all the text-books of oceanography it is stated that the tow-net was first used in 1844 by the German naturalist Johannes Müller, and I have myself been guilty of repeating this error. It is certain that Müller’s researches excited the scientific world and led many others to follow him; but our own great amateur naturalist J. Vaughan Thompson, when serving as an army surgeon in Ireland, was using a tow-net to collect plankton from the sea off Cork as early as 1828. It was there that he first described the zoëa, the young planktonic stage of the crab. A little later, 1833, he discovered the true nature of the barnacles and so solved an age-long puzzle. These enigmatic creatures, fixed to rocks or the bottoms of ships, had been thought to be aberrant molluscs. Thompson caught little undoubted crustaceans in his tow-net and found that they settled down to be transformed into barnacles. His classical discoveries were described in privately printed memoirs which he published in Cork; they are among the rarest items of biological literature. He showed that the plankton consisted not only of little creatures permanently afloat, but also of the young stages—larvae, as the scientist calls them—of many bottom-living animals; these latter more sedentary forms throw up their young in clouds to be distributed far and wide by the ocean currents, just as many plants scatter their seeds in the wind for the same purpose. Charles Darwin also used a tow-net before Müller, on his famous voyage in the Beagle; in his Journal of Researches (1845) under the date of 6 December 1833 he writes: “During our different passages south of the Plata I often towed astern a net made of bunting and thus caught many curious animals.” Today many forget that our famous T. H. Huxley, champion of Darwinism, began his career as did Darwin before him, as a great field naturalist; in 1846 he sailed for the South Seas as surgeon in H.M.S. Rattlesnake and by his use of the tow-net laid the foundations of our knowledge of those remarkable composite jellyfish-like animals, the siphonophores, which we will later discuss (see here (#litres_trial_promo)).
Another simple device, the naturalists’ dredge—a coarse netting bag on a rectangular iron frame—dropped and dragged along the bottom of the sea revealed another new world of life. It was first used by two Italian zoologists, Marsigli and Donati, in the middle of the eighteenth century, but it was another of our own great marine naturalists, Edward Forbes, who became the leading pioneer in this work; he began his dredging in 1840, both in British waters and in the Aegean Sea.
How deep in the sea can life exist? This became the subject of much controversy among scientists following the discoveries made by the use of an ingenious device invented by just a boy—a brilliant young midshipman in the U.S. Navy—J. M. Brook. He hit on the idea of attaching a quill to the sounding lead used in plumbing the ocean depths and so bringing to light a sample of the ooze from the bottom into which it had penetrated. It gave only a tiny sample—but how exciting! That was in 1854, and soon from all over the Atlantic basin, from any depth over 1000 fathoms, came samples of oozy sediment containing minute calcareous shells. These were shells of animals belonging to the group of the Protozoa (single-celled animals) known as Foraminifera and nearly all belonging to one genus, called Globigerina on account of the spherical form of their shells. This form of deposit has consequently become known as Globigerina ooze. Did the creatures which made the shells actually live at these great depths, or did the shells fall from near the surface when their floating owners died? That was the problem. It is amusing for us now to recall that most of those who held the latter and correct view did so on quite false grounds: they believed that it would be quite impossible for life to exist at these great depths and that therefore the shells must have fallen from above. A drawing of a living Globigerina is shown in Plate 2 (#litres_trial_promo).
Edward Forbes had considered there was what he called a zero of life at about 300 fathoms—a boundary below which no life could stand the great pressure of the depths. This fallacy was soon to be exposed. The laying of submarine cables was just beginning. In the Mediterranean one of these after a little use had parted and was hauled up for repair in 1858; it came up encrusted with bottom-living animals, some of them at points on the cable which must have lain at a depth of over 1000 fathoms. Once it is pointed out, the truth of the matter seems obvious: an aquatic animal should feel no ill effects of pressure provided it has no spaces or bubbles filled with air or gas inside it. All liquids are only very slightly compressible. A body made up of fluid or semi-fluid protoplasm, and covered with a flexible or elastic skin, will contract only very slightly even under the greatest pressure; its contents too will be of course at the same pressure as the surrounding water. With the stresses inside and outside the body perfectly balanced in this way, the animal can have a most delicate structure and make the finest movements just as well in the great depths as can one living nearer the surface. Even the seemingly rigid armour-platings of such animals as crabs are in fact made up of a number of parts separated from one another by thinner flexible joints, so that changes of pressure are equalised inside and out; the same applies to the starfish and sea-urchins, whose armour is actually not strictly on the outside of the body, but just below the skin.
Simple as the explanation seems to us now, the discovery of these animals living under great pressure came as a real surprise to most people. This was all the more extraordinary, for actually there was in existence a thoroughly attested instance of a remarkable starfishlike animal (one of the Gorgonocephalidae with branching arms) being brought to the surface from a depth of 800 fathoms; it came up entangled round a sounding line on Sir John Ross’s expedition to Baffin Bay in 1818. It had been forgotten or overlooked by the naturalists of a later generation, who also did not appreciate the significance of the dredgings reported by his even more famous nephew Sir James Clark Ross. Accompanied by the young Joseph Hooker, he had made a number of rich hauls from depths down to 400 fathoms during those great south polar voyages in the Erebus and Terror from 1839 to 1843. Unfortunately these important deep-sea collections, which contained marine invertebrate animals in great variety, were subsequently lost to science.
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The waters immediately to the north and west of the British Isles may perhaps be regarded as the cradle of oceanography; they became the scene of the pioneer deep-sea dredging expeditions in the naval surveying ships Porcupine and Lightning led by Dr. W. B. Carpenter and Professor (later Sir) Wyville Thomson. During the summers of 1868–70 they made nearly 200 dredge hauls over a wide area and reached a depth of 2,435 fathoms; as far down as they went they revealed a wealth of life and opened up a new world to the naturalist. Thomson’s great book The Depths of the Sea (1873) is still fascinating reading. It was their remarkable discoveries, together with the interest taken in the new venture of laying transoceanic cables and the consequent need for a more accurate knowledge of the ocean floor, that led in 1872 to the dispatch by the British Government of H.M.S. Challenger on her famous expedition; under the leadership of Sir Wyville Thomson she sailed on a three and a half years’ voyage to explore all the oceans of the world. The results of this magnificent venture filled more than 50 large volumes with a wealth of information not only of the life of the ocean and of the nature of the sea-floor as revealed by tow-net and dredge, but also about the physics and chemistry of the sea at different depths. Oceanography as an organised branch of science had come into being. Other nations followed the example of the Challenger and sent out similar expeditions.
Having mentioned The Depths of the Sea I must also refer to another great book of similar title which I believe will always be a classic in the literature of Oceanography: The Depths of the Ocean by Sir John Murray and Professor Johan Hjort, published in 1912. Murray was on the Challenger with Wyville Thomson and later, when Thomson’s health failed, directed the Challenger Office, seeing to the completion of all the work and the editing of the great series of Reports; Hjort, who died as recently as 1948, was the great Norwegian marine biologist and Director of his country’s fisheries research. I shall be referring to this book again, particularly in Chapter 12 (#litres_trial_promo), for it contains the results of a very successful expedition which the two authors made in 1910 in the Norwegian research ship Michael Sars to study the deepwater life of the North Atlantic. I draw attention to it here, however, because it is also a splendid introduction to our science in general, with a valuable chapter on the early history.
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Like the correction of the idea of a zero of life, the destruction of another early myth, the mystery of the ‘Bathybius’, is also amusing history. In 1857 H.M.S. Cyclops had made a line of soundings across the Atlantic with an improved modification of Brook’s apparatus for collecting samples of the sea-bed. The deposits of ooze were found to contain a strange gelatinous substance which was supposed to be a primitive form of life. Carpenter and Wyville Thomson again found it in their deep-sea dredgings. To give an idea of the universal interest it aroused at the time I will quote from ‘The Depths of the Sea’; Wyville Thomson is here referring to his deepest haul from 2,435 fathoms.
“In this dredging, as in most others in the bed of the Atlantic, there was evidence of a considerable quantity of soft gelatinous organic matter, enough to give a slight viscosity to the mud of the surface layer. If the mud be shaken with weak spirit of wine, fine flakes separate like coagulated mucus; and if a little of the mud in which this viscid condition is most marked be placed in a drop of sea-water under the microscope, we can usually see, after a time, an irregular network of matter resembling white of egg, distinguishable by its maintaining its outline and not mixing with the water. This network may be seen gradually altering in form, and entangled granules and foreign bodies change their relative positions. The gelatinous matter is therefore capable of a certain amount of movement, and there can be no doubt that it manifests the phenomena of a very simple form of life.”
“To this organism, if a being can be so called which shows no trace of differentiation of organs, consisting apparently of an amorphous sheet of a protein compound, irritable to a low degree and capable of assimilating food, Professor Huxley has given the name of Bathybius haeckelii. If this has a claim to be recognised as a distinct living entity, exhibiting its mature and final form, it must be referred to the simplest division of the shell-less rhizopoda, or if we adopt the class proposed by Professor Haeckel, to the monera. The circumstance which gives its special interest to Bathybius is its enormous extent: whether it be continuous in one vast sheet, or broken up into circumscribed individual particles, it appears to extend over a large part of the bed of the ocean …”
The ‘Bathybius’ however came to an inglorious end. It was shown by the naturalists of the Challenger to be a precipitate thrown down from the sea-water associated with the deposits by the alcohol used in their preservation, and T. H. Huxley made a public retraction of his earlier ideas.
Towards the end of the century came the founding of the famous marine stations, the first at Naples in 1872, and then those at Plymouth and Millport in this country and Woods Hole in America; in these laboratories and many more to be founded later, researches into the structure, development, physiology, life and habits of marine creatures of all kinds have been continued to the present day.
It then began to be realised that progress in oceanography was essential to a better understanding of fishery problems and to the development of a more rational exploitation of the sea. The rapid development of trawling, with the introduction of steam power and the replacement of the old beam-trawl by the much larger and more efficient otter-trawl, gave rise to some concern as to the possible depletion of the stocks of fish; this led a number of nations, our own included, to set up fishery investigations. As we shall see, those fears were indeed well founded. In 1899 King Oscar II of Sweden invited all the nations of Europe interested in sea fishing to send representatives to a conference in Stockholm; the discussions which took place led to the foundation in 1901 of the International Council for the Exploration of the Sea. The different nations began a series of investigations to form part of one great plan. In spite of the temporary suspension of activities in two world wars the work of the Council still goes on.
The scientists of the various fishery departments are not only enquiring into the natural history of the fish themselves, their life-histories, food and feeding habits, migrations, growth, birth-rates and so forth; but with continually improved equipment they are studying the distribution of the different planktonic forms upon which they depend, the conditions under which they live, the flow of the ocean currents, the physics and chemistry of the sea and the varying nature of the sea-bottom and its life. It was wisely realised from the start that in order to provide answers to such questions as: ‘Why are fish sometimes plentiful and sometimes scarce?’; ‘Can the future of a fishery be forecast?’; ‘Is this or that area being overfished?’ and so on, the natural history of the sea must be investigated in all its different aspects.
Everything the naturalist wants to find out about the conditions under which fish live he must grope for in this unseen world, often below a storm-tossed surface; he must always remember too that the sea is a very big place—he must work with a sense of proportion and perspective. He stops his research ship at intervals to let down his instruments on wires and ropes: some to take the sea’s varying temperature and to collect samples of water from different depths for analysis, others to measure the amount of light reaching different levels, and others again to record the speed and direction of ocean currents. He samples its life with all kinds of nets to estimate not only the varying quantities of the plankton but of the eggs and fry of the fish themselves. Building up a picture of life in the sea is like putting together a huge jig-saw puzzle made up of tiny pieces, but much more difficult. Not only have we a very imperfect idea of what kind of picture will emerge, but all the pieces to be fitted together are not on the table before us; they are lying about somewhere underneath it and we must feel about for them in the darkness. It is certainly a fascinating pursuit, but full of disappointments. Some bits of the puzzle—perhaps a stage in a life-history or some evidence of a migtation—can only be picked up during a short period of the year; before the missing pieces can be found, stormy weather may intervene and we must wait a whole year before we can try again.
In spite of these obstacles the picture of life in the sea is continually growing: the chapters which follow will endeavour to sketch an outline of what has been achieved. The amateur naturalist should not be discouraged by these difficulties for it is because of them that there are so many gaps in the story yet to be filled in. There are still many original discoveries to be made. And the difficulties add spice to the game; a golf-course would indeed be a dull one if there were no bunkers on it.
(#ulink_9afc2b6b-46b2-5c7f-b0d9-1ec96f0deda4) In the “Summary of the Scientific Results of the Challenger” Part 1, p. 79, Sir John Murray refers to the deep dredging of the Erebus and Terror. He says: “Sir James Ross was an indefatigable zoological collector, but it is to be regretted that the large collections of deep-sea animals, which he retained in his own possession after the return of the expedition, were found to be totally destroyed at the time of his death. Had they been carefully described during the cruise or on the return of the expedition to England, the gain to Science would have been immense, for not only would many new species and genera have been discovered, but the facts would have been recorded in journals usually consulted by zoologists instead of being lost sight of as was the case.”
(#ulink_cc99d3a0-0fef-5460-97aa-c036db547ab3) Sir John Murray also wrote an excellent little introduction to oceanography: The Ocean (1913). For those who wish to make a fuller study of the development of our science, and particularly of the early plankton investigations by the German Hensen school, there is J. Johnstone’s important book on Conditions of Life in the Sea (1908). Two other valuable introductions should be mentioned: Fowler and Allen’s Science of the Sea (2nd edition, 1928) which gives much practical advice on collecting specimens and the working of gear, and Russell and Yonge’s charming general natural history of The Seas (1928) which deals with the life of tropical waters as well as of our own.
CHAPTER 2 (#ulink_418798ab-0498-50b2-954e-b824ce9838fe) THE MOVEMENT OF THE WATERS
ONE OF THE MOST important features in the world of a marine animal is the movement of the sea itself. Apart from the wave-action near the surface and the to-and-fro tidal streams in shallow coastal areas, the whole water-mass is in continual flow as part of a greater system of oceanic circulation. Carried with the moving waters go the floating animals and plants. The main surface currents in the North Atlantic are shown in Fig. 2 (#litres_trial_promo).
In some places the sea may be richer in plankton and in others poorer—like contrasting regions of luxuriance and barrenness on the land; unlike them, however, these areas in the sea are day by day changing their positions in relation to the coasts and the sea-bed. Not only is the basic food-supply on the move, but the delicate and helpless young of many fish and bottom-living animals are carried in one direction or another. Clearly these water movements are profound in their effect. To understand the lives of the inhabitants of our seas we must first have a knowledge of the main average pattern of the ocean currents flowing round our islands. They are by no means fixed in an unalterable course: variations are continually occurring. Sometimes these changes have striking effects; occasionally some exotic creatures from warmer seas, such as the beautiful Portuguese Man-of-war (Physalia) with its iridescent float and long trailing tentacles, or the smaller blue Velella riding the surface with a little ‘sail’, is driven ashore upon the coasts of Devon and Cornwall. (see here (#litres_trial_promo) and Plate 5 (#litres_trial_promo)). Normally we find typical Atlantic water outside the western entrance to the English Channel, but at times there may be a marked incursion of water from the Bay of Biscay carrying with it these and other planktonic visitors from further south. It is probable that long periods of strong south-westerly winds may contribute to this movement.
FIG. 2
The surface currents of the North Atlantic Ocean. Drawn, with kind permission, from Admiralty Chart No. 5310 (1949), omitting some of the detail.
The general systems of the surface currents of the world have been worked out largely from the innumerable observations of navigators. Suppose a ship steers from point A on a particular course by compass which should carry it to a point B in, say, a day’s time; if at the end of the 24 hours the navigator finds by reference to the sun or stars that he has instead reached a point five miles to the south-east of it, then if there is no wind to account for his drift he knows that the surface water must be flowing in a south-easterly direction at a speed of five miles a day. The great pioneer in organising a systematic recording of winds and ocean currents throughout the world was Lieut, (later Admiral) M. F. Maury of the United States Navy who wrote the first text book of oceanography, The Physical Geography of the Sea in 1855, to be followed by many editions. The charts he produced of the wind and current systems of the world rendered immense service to commerce; before their production the average time of sailing between England and Australia was 124 days, but with their use it was reduced to 97, or the average passage to California (presumably from New York) was 183 days and this was reduced to 135 (from Maury, loc. cit.).
FIG. 3
Types of drift-bottle for investigating ocean currents, a, surface drifter, old type; b, bottom drifter; c, surface drifter, modern type; for further explanation see text.
Observations on the movements of ships are suitable for indicating the surface drift only in the wide open oceans. For working out the current systems in more detail in more confined areas science has made use of that romantic object, the shipwrecked sailor’s bottle with a message to his home. Thousands and thousands of these drift bottles have been cast into the sea at many different points. Each bottle displays through its glass sides, a notice in several different languages asking the finder to break it and read the instructions within; these tell him that if he will fill in and send off an enclosed postcard giving particulars of the place and date of recovery, he will receive a small reward. Many hundreds of these cards have been returned from various points along the coasts of different lands. In this way Dr. T. W. Fulton of the Scottish Fishery Board first charted the main surface movements of the North Sea at the beginning of the century. Later Dr. G. P. Bidder introduced bottom-drifting bottles, each provided with a trailing wire and weighted so as to drift along with this just touching the sea-bed; these are designed to be recovered by the many trawlers dragging their nets along the bottom of the North Sea and English Channel. Thus knowledge has been gained not only of the surface currents but also of the movements of the lower layers of water, which are often very different. Objects floating on the very surface of the sea may be driven by the wind much faster than will the true upper layer of water; surface drift bottles have now been improved by Dr. J. N. Carruthers (1928) who has provided them with little weighted ‘sea anchors’ to keep them drifting with the main body of water. Sketches of these different bottles are shown in Fig. 3 (#litres_trial_promo) and an example of their use in studying the drift of plaice eggs is shown in Fig. 5 (#litres_trial_promo). Quite recently oceanographers have started using water-proof plastic envelopes instead of bottles.
FIG. 4
Diagrammatic sketches showing the working of the Ekmann current-metre.
For the more exact measurement of the speed and direction of current flow at different depths special instruments have been invented to be used from anchored ships. One of the most successful is that of Dr. Ekmann. It is such a beautiful and ingenious device that it is worth examining in some detail to see how it works. It is suspended in the sea on a wire, as shown in Fig. 4 (#litres_trial_promo), and, being provided with a vane like a weather-cock, always points head on into the current. It also has a little ‘propeller’ which will be turned by the water flowing past it; this however is held by a catch until the measurement of flow is about to begin. On the underside of the apparatus, and held horizontally, is a circular box, like a large and rather deep pill-box. The lower half of this is divided into a number of compartments by partitions radiating from the centre like the spokes of a wheel; swinging above these compartments, and pivotting upon a spike in the centre, is a stout compass needle having a slightly sloping groove on its upper surface running from its mid-point to the end pointing to the north. In addition the compass box has a small hole in the centre of the lid and immediately above it is another little box containing a supply of bronze shot. To take a current measurement the instrument is lowered to the desired depth—perhaps 10, 50 or 100 metres; then a small brass weight, called a messenger, is threaded on to the suspending wire and sent sliding down to release the catch and set the propeller free to turn. After one, two or more hours as may be desired, the recording is ended by the dispatch of a second ‘messenger’, which moves the catch further over to stop the propeller once more, and the machine is hauled up to the surface. The number of turns the propeller has made in the interval of time is recorded on a series of little dials and so enables the speed of the current to be estimated; also after every so many revolutions a little valve opens and allows a shot to drop into the box below and be guided by the grooved compass-needle into one of its compartments. The compass-box, being fixed to the instrument, is always orientated in relation to the direction of the current; thus as the instrument swings in response to changes in current flow, each shot is dropped into whatever compartment happens to lie below the north-pointing end of its compass needle. By counting the shot found in each of the compartments at the end of a recording we know to what extent the current has varied during the time of observation and also the average direction of its flow. The direction is, of course, given by the angle between the mid-line of the box and that of the compartment which most often pointed north, as revealed by the shot in it. Such observations are made at several depths to show how the speed and direction of the water flow varies at different levels.
The upper layers of sea are nearly always travelling faster than the lower ones. The surface layer is affected by the wind; apart from this, however, the lower layers will be retarded by the frictional resistance of the sea-bed, just as in a river the water in the middle travels faster than that near the bank. We shall later see (see here (#litres_trial_promo)) that these differences in speed may have a marked effect on the distribution of plankton animals which make extensive day and night migrations between the upper and lower layers. In deep ocean basins we may even find currents at different levels flowing in opposite directions.
FIG. 5
Showing where drift-bottles liberated at A and B were recovered. A map reproduced from one in the museum of the Fisheries Laboratory at Lowestoft showing the results of one of many experiments made by the late Mr. J. O. Borley when investigating the drift of plaice eggs and larvae from the spawning areas to the coastal nursery grounds. The numerals show the number of bottles picked up at each point.
There are several different kinds of current-meter but most of them are similar in general principle to that described. Dr. Carruthers (1926) has devised a much more robust machine which is used on a number of lightships anchored round our coasts to record automatically the main drift of water for periods of a month at a time. It records the to-and-fro tidal stream as well as the residual current-drift. These instruments have given much valuable information on the variations in the flow of water through the English Channel into the southern North Sea (Carruthers, 1930). It is in the Flemish Bight that so many plaice congregate in winter to lay their floating eggs at the point where the Channel water enters; these eggs, and later the hatched-out fry, are carried by the current and so bring the new generation to settle down as small flat-fish on the nursery feeding grounds in the shallow waters off the Dutch and Danish coasts. Herring fry in vast numbers are also carried by the same current into the North Sea from one of the largest spawning grounds off Cape Gris Nez. The spawning migrations of many fish have been evolved in relation to the prevailing current systems; selection has naturally acted to preserve the offspring of those parents who migrate to lay their eggs at the point up-stream most favourable for their survival; eggs spawned elsewhere are less likely to be successful because they are carried to less suitable nursery grounds. Fig. 5 (#litres_trial_promo) shows the results of two of many experiments made with drift-bottles during the English fishery investigations into the life-history of the plaice. Those liberated at a point in the main spawning area in the Southern Bight have all been picked up on the Dutch coast, whereas those from the lesser spawning area off the Yorkshire coast are carried into the Wash, which is another smaller ‘nursery’ ground; the number against each point on the coast indicates the number of bottles found there. In some years a marked variation from a normal current-flow may well have a considerable effect on the relative success of a particular brood of fry; if the flow is weaker than usual they may not reach the best ground, if it is too strong they may be carried beyond it. The speed of flow of Channel water into the North Sea may be affected by the wind at a critical time apart from any variation in the fundamental current system; a prolonged westerly wind may accelerate the flow, and an easterly one may have the reverse effect. These are just some of the many factors we must take into account in puzzling out the possible causes for this or that unusual event in the fisheries; it is so often that such factors have an effect which is most marked in the fishery several years later, i.e. when the young fish of that brood will have grown to maturity—or failed to.
Without the use of drift bottles or current-meters some indication of the direction of current-flow may also be got by mapping the contours of varying salinity (measurements of salt-content) obtained by the analysis of water-samples collected at a number of different points in the area; for instance, a tongue of very salt water projecting into a less saline area might indicate a flow of ocean water into a more coastal region where the salt water has been diluted by drainage from the land. Indeed modern oceanography has developed an elaborate mathematical system for estimating the direction and relative speeds of water movements from a knowledge of the varying densities of the water at a number of different points. We shall later see how at times certain planktonic animals and plants characteristic of one particular type of water may be used as indicators of the incursion of such water into other areas: in fact one such example, that of the tropical Physalia and Velella reaching our coasts, has already been mentioned, and we shall discuss others at the end of the chapter. We are apt to think of the use of plankton animals as current indicators as rather a modern idea; I was interested to find that Alexander Agassiz in 1883 was emphasising the importance in this respect of the animals just mentioned. “This group of Hydrozoa,” he wrote “is eminently characteristic of the Gulf Stream, and wherever its influence extends these Velellae and Physaliae have been found. In fact these surface animals are excellent guides to the course of the current of the Gulf Stream—natural current bottles, as it were.”
In the Department of Natural History in the University of Aberdeen there is a cabinet containing a remarkable collection of South American and West Indian seeds picked up on the shores of the Outer Hebrides. They were gathered from 1908 to 1919 by William L. MacGillivray who was a nephew of a former Regius Professor; most of them he found on the West Sand of Eoligarry, Barra, where he lived, but some came from Lewis and the Island of Fudag. There are Brazil nuts, the seeds of a leguminous liana Diodea, the Virgin Mary nut, palm seed of different kinds, the pecan nut, the Calabar bean, nutmeg, the seeds of the Central American soapberry tree and many others. Altogether seventeen tropical species are represented. Just as these seeds are drifted to our shores, so also are the baby eels carried round by the ocean circulation from where they were spawned—from a small area situated between Bermuda and the Leeward Islands—and eventually scattered by the Gulf Stream to enter the rivers along the whole seaboard of Europe. This was the amazing discovery made by the famous Danish oceanographer Professor Johannes Schmidt. On many special voyages he plotted the distribution of the tiny eel fry all over the Atlantic until at last he could show that there is only one limited area where the very smallest and newly hatched young are to be found—a breeding ground some 3,000 miles from the rivers in which they grow to maturity. In Fig. 6 (#litres_trial_promo), I reproduce his map showing the spread of the fry of different sizes. Their drift round the ocean to Europe takes from 2 to 2½ years, and during this phase they are little flat and quite transparent creatures having the shape of a willow leaf. They used to be thought to be a separate species of fish, called Leptocephalus brevirostris, until the Italian naturalists Grassi and Calandruccio kept some in an aquarium and were surprised to find they turned into the common elvers, as the young freshwater eels are called when they ascend the rivers from the sea. The story is now very well known and I only recall it here because it is, to use Agassiz’s simile, nature’s greatest drift-bottle experiment and demonstrates so clearly the constancy of this vast current system; year after year, for many millions of years, the eel-fry must have been transported in this way with never a break in the sequence. How the adult eels navigate back to breed in this one place is one of the most profound mysteries of the sea; a discussion of this, however, belongs to a chapter on fish and must await the subsequent volume.
FIG. 6
The distribution of the common European Eel (Anguilla vulgaris) during its various stages of development. The contoured areas represent those in which the larvae of various sizes, 10, 15, 25 and 45 mm. are found; the line ul represents the limit of occurrence of unmetamorphosed larvae; the black bands along the coasts indicate the countries where the adult is found in fresh water (after Schmidt).
The causes of the great circulations of water—as distinct from mere tidal streams—are of three kinds; oceanographers, however, are still not fully agreed as to which is the most important: indeed all three play their part together. Primarily there are the effects of the prevailing winds over wide stretches of ocean, particularly the northeast and south-east trade winds blowing obliquely towards the equator from north and south respectively. These certainly take a great part in driving the equatorial water towards Central America, so that from the Gulf of Florida emerges the powerful warm Gulf Stream to flow across the North Atlantic and give to our islands and the north of Europe so temperate a climate compared with that of the corresponding latitudes of North America. The latter are cooled by the Arctic Stream of the Labrador Current. The Gulf Stream, or the North Atlantic Drift as it is more correctly called on this side of the ocean, has a profound effect upon our waters.
Although marine physicists are beginning to believe that the stress of the wind on the sea surface, together with the effect of the earth’s rotation, can give rise to slow movements of water in the deep layers of the ocean as well as near the surface, they have to consider a second kind of cause: the action of what are often termed Archimedian forces. These are the forces due to internal changes in the water-mass causing alterations in its density. Such changes may be due to the expansion or contraction of the water on being warmed or cooled; they may also be due to an increase in the salt-content caused by excessive evaporation of water at the surface, as in the tropics, or to a decrease in saltness caused by large additions of fresh water from melting ice or excessive rainfall. Whether these causes actually produce the deep water movements, or do no more than make the water take the path of least resistance, they have far-reaching effects, particularly in giving rise to vertical as well as horizontal differences in the great ocean basins. One of the most surprising discoveries of worldwide oceanography was made between the two world wars—indeed through the work of our own Discovery Expeditions and the German Meteor Expedition, in the Antartic and Atlantic Oceans; it is the fact that the heavy snow or rainfall and the melting ice in the Antarctic seas has an immense influence that extends across the equator into the northern hemisphere. It is so striking an example of these Archimedian forces that I cannot resist using it as an illustration.
FIG. 7
A section through the Atlantic Ocean, from latitude 55°S to 15°N along the meridian 30°W, showing the water of varying saltness (34–00/00 to 37–0 0/00) and the directions of the main water movements at different depths. It shows how the great Antarctic icecap extends its influence into the northern hemisphere. Redrawn in diagrammatic form from Deacon (1933).
The great ice-cap at the south pole dominates the oceans of the world. The Antarctic continent rising in a plateau to elevations of some 8,000 feet is covered with a sheet of ice many hundreds of feet thick; this is continually being added to by the frequent heavy falls of snow, and is constantly and slowly moving as a vast glacier to the coast and beyond into the ocean where it juts out as the floating ice barrier. This is shown on the left of Fig. 7 (#litres_trial_promo). At its edge this barrier from time to time breaks up into the massive tabular icebergs so characteristic of the south polar seas. This is freshwater ice, which on melting, helps to form a cold but light surface layer; in spite of being colder it is lighter than the normal sea-water because its salt content is reduced by the addition of the fresh-water. All round the pole this cold surface layer flows away to the north. Below this is water that is heavier because it is just cooled and not diluted with fresh-water; this sinks and forms a cold current, also flowing north but over the ocean floor. To take the place of these two streams of water flowing away from the pole, a mass of warmer water flows southwards and wells up against the ice, to be itself diluted, cooled and turned north again. The surface current thus formed continues till it meets warmer water which, although more saline, is lighter because it is so much warmer; the cold current now dips below this warmer water but still travels northwards and can be traced to a point some 30° of latitude north of the equator; it then sinks and joins in the return flow going south again to complete the circulation.
Here we have a striking case of waters at different levels travelling in opposite directions: a layer going south flows in between two layers coming north. We shall see in a later chapter how the behaviour of some plankton animals is adapted in a most remarkable manner to take advantage of this fact.
Dr. G. E. R. Deacon, who has done so much to increase our knowledge of this remarkable system (1933 and 1937), while on one of the Discovery expeditions took water-samples all the way along the path of this northward-flowing current after it had dipped below the surface; when he had analysed them he found something very extraordinary. We have said that this water was of low salt-content because of the melted ice; it also has a high oxygen-content because of a great production of planktonic plants in the polar surface waters (due to the rich nutrient salts and to the long hours of daylight in high latitudes). Now as this water travels north the salt-content increases by diffusion from the surrounding layers and the oxygen-content is lowered by the respiratory requirements of animals. As he went along the path of the current Dr. Deacon obtained a clear indication that the salt and oxygen values did not increase and decrease respectively in a perfectly steady manner as one might have expected, but in a series of waves. As far could be judged from the graph of the increase in saltness there were seven undulations in the curve from south to north; likewise in a graph of oxygen-decrease there were also seven undulations. Now the crests of the undulations of one curve corresponded in position with the troughs of the undulations of the other curve; in other words as we passed along the stream of water, regions of higher oxygen-content and lower salinity alternated with regions of lower oxygen-content and higher salinity. This water had come originally from the Antarctic surface layer in which during the summer more ice melts and also more plants are produced than in the winter. More ice melting means a lowering of salinity and more plants mean a greater production of oxygen. Clearly these regions of lower salinity and higher oxygen-content alternating with regions of higher salinity and lower oxygen-content represent the water which left the antarctic surface layers in past summers and winters respectively. Dr. Deacon tells me that he now fears that there are not sufficient observations along the path of the current to make their number quite certain; but since the indicated rates of water movement agree very well with most other estimates, he feels that they give us a fairly reliable time scale for this great circulating system. This water takes at least seven years on its journey from the antarctic to the northern hemisphere!
There is a somewhat similar system of a cold and less saline surface current flowing away from the north polar basin due to melting ice and the fall of snow and rain, but it is not so far-reaching as that from the south because this precipitation is less and in addition the Arctic Ocean is almost entirely enclosed by submarine ridges; this cold stream dips below the warmer water at the northern boundary of the Gulf Stream. It is partly to replace this cold water stream that the extension of the Gulf Stream—the North Atlantic Drift—is carried so far to the northward of our islands and up the northern coasts of Scandinavia. Here we see how these Archimedian forces may contribute to the North Atlantic system.
The third factor affecting ocean-currents is a much more subtle one, due directly to the actual spin of our planet. This deflecting force of the earth’s rotation is sometimes called Corioli’s force, after the French physicist, though in fact it was carefully worked out by his countryman Laplace 60 years before; it applies to the atmosphere as well as the sea. It is not a cause of the initial motion of the water but a cause of its deflection. A body of water moving in any direction is deflected to the right in the northern hemisphere and to the left in the southern hemisphere. The effect is greater towards the poles and reduced towards the equator; on the actual equator itself there is no such effect at all. It applies not only to water but to any moving object; we can perhaps understand it best by considering the effect upon a swinging pendulum. Let us suppose we could hang a fairly heavy weight, say of some 20 lbs., on a long string from a 100 foot tall gallows-like structure at the north pole; now if we set the weight swinging to-and-fro in the same direction as a straight line drawn in the snow beneath it, we should soon observe that its line of swing would deviate from the line in the snow. Its swing would be deflected in a regular fashion in a clockwise direction; even in ten minutes its path would be deflected 2½. Unperceived by us the earth and the gallows would be rotating in an anti-clockwise direction, but the heavy pendulum weight is swinging free and its path is not affected although the string at the top will twist. If we watched it for a full twenty-four hours we should see the path of the pendulum complete a deflection of 360° and once more for a moment swing directly above the line in the snow. If we repeated this experiment at the equator—drawing our line in the sand—we should see no such effect; if the pendulum was set swinging say north and south it would continue to swing thus as it would also continue to swing in any other direction in which we might choose to start it; here the earth makes no turning motion in relation to the swing; for the line in the sand and the line of swing are carried on together by the earth’s motion round its axis. At the south pole we should of course get a similar effect to that at the north pole except that the pendulum would appear to be deflected in an anti-clockwise direction. Swinging such a pendulum at places in different degrees of latitude will give a different amount of deflection. At a place situated in latitude 30° north the pendulum will swing through 180° in the 24 hours, for, speaking mathematically, the effect depends on the sine of the angle of latitude; in London at 51.5° latitude it will swing through 281°. The effect was shown very clearly by Foucault, the French physicist and inventor of the gyroscope, by swinging a hundred-foot pendulum at the Great Exhibition of 1851. This demonstration which is to be seen in the Science Museum in London and in a number of provincial museums is not difficult to set up in any building with a high roof or in any house that has a fairly wide staircase well above the entrance hall; it is an impressive sight to see in the matter of a few minutes the apparent change of motion of the pendulum, which really indicates the rotation of the hall itself or, indeed, the earth.
FIG. 8
The varying saltness (31.0 0/00 to 35.4 0/00) of the surface waters and the main circulation typical of the North Sea and English Channel in winter. Drawn from a chart kindly provided by Commander J. R. Lumby, of the Fisheries Laboratory, Lowestoft.
Just as the pendulum is deflected in relation to the objects in the hall, so any body of water in motion tends to be deflected to the right in the northern hemisphere and to the left in the south in relation to the surrounding land masses and the ocean floor; account has to be taken of it in every practical treatment of tides, wind drifts and ocean currents. Whenever a water-mass meets an obstruction, either a mass of land or an opposing water-mass, it will, other things being equal, turn to the right in the north rather than to the left, and vice versa in the south. There is another important effect. We have seen how through differences in temperature and saltness the water varies in density; the lighter water will naturally be on top. In a current system the water of a particular density—say the lightest water at the top—is not lying in a layer of uniform depth; owing to the earth’s rotation the lighter water is pushed more to the right-hand side of the current stream than the heavier water, so that imaginary surfaces separating waters of different density are not horizontal but tilted. It is from a consideration of the deflection of waters of different densities that the speed and direction of ocean currents can be mathematically worked out as mentioned earlier in the chapter.
With this slight introduction, intended merely to give an idea of the kind offerees at work to produce the circulatory ocean-systems, we may now briefly review the main streams of water in the seas around our coasts. Fig. 8 (#litres_trial_promo), is based on the account by Comd. J. R. Lumby (1932), hydrologist at the Fisheries Laboratory, Lowestoft, with a small revision which he has kindly made in the drawing for this figure. The water in the North Sea and English Channel is slightly less salt than the Atlantic Ocean water; it is typically coastal water diluted by freshwater drainage from the land. The Baltic has a much lower salinity still. A stream of Atlantic water flows into the North Sea from the north, mainly passing round to the east of the Shetland Islands to flow due south and not usually entering between the Orkneys and the Shetlands as was originally thought; a less powerful stream flows up the English Channel and enters it from the south. The northern influx is generally thought to flow on a broad front down the middle of the North Sea forming, as it goes, swirls off the coast of Scotland especially in the Moray Firth and in the region of the Firth of Forth. Dr. J. B. Tait of the Scottish Fishery Department has in recent years, however, put forward the view (1952) that the main streams are much narrower than formerly supposed—more like rivers flowing in the sea. Which is the correct view is at present by no means certain; some evidence from plankton distribution appears to support one view and some the other. Just before reaching the Dogger Bank the main stream, whether broad or narrow, appears generally to divide into three branches: one running south-westerly, another south-easterly and a third turning east to enter the Skagerrak. The south-westerly and south-easterly branches form large swirls in the southern North Sea as they meet the stream of water entering from the Channel. Another smaller swirl is formed outside the Skagerrak as the stream entering on the southern side meets the stream flowing out of the Baltic on the northern side.
(#ulink_f48d51e1-8bc0-56e7-b606-d6579a8e8558) The stream entering the North Sea from the Channel flows north-eastwards past the Dutch and Danish coast and some of it joins the stream going into the Baltic. Most of the North Sea is shallow, but there is a deep hollow running up the western coast of Norway to the north; it is along this Norwegian trough that the water leaves the North Sea—the less saline water from the Baltic on the top and the bulk of the North Sea water proper in the deep channel below.
The extent of the inflow of Atlantic water varies from year to year; such variations affect the distribution of the plankton and are likely to influence the distribution of the herring shoals which depend upon the plankton for food. In some years of exceptional influx numbers of plankton animals usually only found in the more open ocean make their appearance in the northern North Sea. There is some evidence to support the view that it is the pressure of this water from the north (produced by the main wind systems) which, apart from the occasional effects of local winds already referred to, controls the inflow of Channel water into the southern North Sea. If the pressure from the north is high it seems that the Channel flow is reduced; if it is weak then a larger influx from the Channel seems to take place. It is in the study of this inflow into the northern North Sea that the charting of the relative movements of certain Atlantic plankton animals in different years can be most helpful. We shall see in a later chapter (see here (#litres_trial_promo)) how, by the use of plankton-recording machines towed at monthly intervals by commercial steamships on regular routes, we can compare the areas of invasion of these more oceanic forms in different months and years. We shall find that not only does the extent of the Atlantic inflow vary from year to year, but the time of the advance of typical invading organisms will vary: in some years it may be a month earlier or later than in other years. There is an interesting suggestion now being investigated that the time of the appearance of the shoals of herring at different points down the east coast of Scotland and England, and consequently the time of the different fisheries, may be earlier or later in different years depending on whether this Atlantic inflow is earlier or later.
(#ulink_dc23d649-8c64-57d0-aa00-db1867ee1350) Whether this indication—it is no more at the moment—will be proved correct or not, there can be no doubt that the fluctuations that are found to occur in the water movements round our islands must have a profound effect upon the fish and other life inhabiting our seas.
A more definitely established connection between water changes and fisheries has been demonstrated at the western entrance to the English Channel. The water of the greater part of the Channel is like that of the southern North Sea—coastal water which is less saline and less rich in plankton than the Atlantic water that flows into it. This more coastal water can readily be distinguished from the more oceanic water by the presence of certain of these indicator plankton species—particularly two species of Sagitta, the slender transparent arrow worm shown in Plate IX (#litres_trial_promo); Sagitta setosa being found in the coastal water and Sagitta elegans in the more oceanic water.
(#ulink_adddf64c-ed23-545c-9c18-ed66cb5c3d2b) The boundary between the two waters formerly used to lie somewhere in the region of Plymouth where sometimes the plankton would have elegans predominating in it and sometimes setosa) during the investigation up to 1929 it was more usually elegans, indicating Atlantic water richer in phosphates and other nutrient salts. The importance of these salts in the economy of the sea will be discussed in Chapter 4 (#uf9e2ac0e-843a-5efb-963b-16fe015b8f29). Since 1929 the boundary between elegans and setosa water has lain much further to the west so that the water off Plymouth has been of the coastal type and much poorer in plankton. Since this date there has also been a marked reduction in the number of young fish of many kinds present in the plankton as well as a change in the herring fishing; since that time the herring which used to visit the Plymouth area around Christmas have not turned up in their usual numbers so that this winter fishery, once quite a prosperous one, now no longer takes place. An excellent account of this trend was given by the late Dr. Stanley Kemp in his presidential address to the Zoology Section of the British Association in 1938. More recently some other interesting differences between the elegans and setosa water have been discovered; these will be referred to later when the various chemical constituents of the water are being considered (see here (#ulink_e4b329bb-1a47-559c-8b6e-e322d668184d)).
FIG. 9
Map showing the distribution of three kinds of water round Great Britain each characterised by a different species of the arrow-worm Sagitta: serratodentata in open ocean water, setosa in coastal water and elegans in oceanic water mixing with the coastal water. The conditions are those which might be expected in the autumn of a year with a strong Atlantic influx into the North Sea from the north. From Russell (1939), but modified in the north in the light of more recent surveys and with some other details omitted.
Mr. F. S. Russell, the present Director of the Plymouth Laboratory, who carried out these studies on Sagitta (1935, 1936) and young fish (1940), made cruises to trace the boundaries between the different kinds of plankton. He has told me how very abruptly one type of water may give place to another. On one occasion he has said it was even possible to place the ship across the very margin between them, so that elegans water could be sampled from the bows and setosa water from the stern! Fig. 9 (#litres_trial_promo), shows the general distribution of the elegans and setosa water round the British Isles as it might be expected in the autumn of a year in which there is a strong influx of Atlantic water into the North Sea from the north; it is taken from another of Mr. Russell’s papers (1939). These different waters may also be sometimes discernible by a difference in their colour, a contrast of shades of blue and green making a line across the sea. In 1923, when on the staff of the Fisheries Laboratory at Lowestoft, I acted as observer in some attempts to locate shoals of herring and mackerel from the air. In flying from Plymouth to the western mackerel grounds we passed over a sharp line separating the green water of the Channel from the deep blue of the Atlantic; it ran on a slightly irregular course from the Lizard to the south-west as far as we could see to the distant horizon. Then while circling over the mackerel area we saw another equally definite boundary running from Land’s End towards the Scilly Isles separating the deep blue water from a more brown-green area lying to the north. At that time I could not interpret that striking pattern of colour contrasts; now on looking at Mr. Russell’s maps I have little doubt that the blue area I saw was oceanic elegans water lying between the setosa water of the English Channel and that of the Irish Sea. Fig. 10 (#litres_trial_promo) shows a comparison between my sketch of these colour boundaries, which was published in the official report (Hardy, 1924a) and Mr. Russell’s maps of the distribution of the setosa and elegans water in the same area but in different years (Russell, 1935, 1936). If these marked colour-changes can be correctly interpreted we may in the future find aircraft being used to make rapid surveys of the surface conditions in relation to the fisheries. The actual experiments in spotting shoals offish were not successful in these waters; in the southern North Sea the water was too opaque with the large amount of sediment constantly stirred up by tidal currents running over sand and mud banks; at the western entrance to the Channel the ocean surface was too much broken up by waves into light and shade to allow of any observations below it.
FIG. 10
Well defined areas of blue and green water (A) seen from the air during mackerel spotting tests off Cornwall in 1923 drawn from the chart by Hardy (1924) and compared (B and G) with the distribution of western and Channel water as indicated by the arrow-worms Sagitta, elegans and setosa, charted by Russell (1935 and 1936).
In the shallower waters—especially in the southern North Sea—we must not forget the influence of the tidal streams just mentioned; they may have a most profound effect in modifying the action of the main currents, especially when they vary so enormously in their force between spring and neap tides. At spring tides in certain places a mass of water may be moved for some thirty or forty miles in each direction.
The movements of water in the Irish Sea are also dominated by tidal currents; these flow into it from both ends and follow the general direction of the coast lines. Professor K. F. Bowden, who has given us such an excellent account of these tidal streams (1953), writes, “Knowledge of the non-tidal drift, however, is much less certain and is based on indirect evidence. It was recognised at an early date that the distribution of salinity indicated a north-going drift and in 1907 Knudsen estimated that the rate of flow was such that the water in the Irish Sea would be completely renewed in a year.” After saying that “this implies a flow through the Dublin-Holyhead channel at an average rate of just over a kilometre a day,” he later stresses that, although there seems little doubt about this average northward movement, “its magnitude, its variations and the degree of its dependence on the wind are still uncertain.”
We have now dealt with the main water movements round our islands; later in the book we shall see instances of more local effects and how upwellings and the mixing of waters may be important in producing a richer plankton. I will end the chapter by referring to some surprising and significant plankton records being made by Dr. J. H. Fraser (1952e, 1955) of the Scottish Fishery Laboratory at Aberdeen. In some years, over a wide area to the north of Scotland, he finds plankton animals which we should more usually associate with the latitudes of the Mediterranean; they indeed indicate a very unexpected movement of water. It now appears that some of them may in fact actually have come from the Mediterranean Sea itself.
FIG. 11.
A chart showing the northward flow of the ‘Lusitanian’ plankton; kindly provided by its discoverer Dr. J. H. Fraser of the Scottish Fishery Laboratory, Aberdeen.
It has long been known that a surface stream of Atlantic water flows eastwards through the Straits of Gibraltar and that this influx is balanced by an outpouring (at a lower level) of Mediterranean water of very high salinity; this spreads out from the Gulf of Gibraltar underneath the North Atlantic water and some of it is carried north up the edge of the European continental shelf. This movement is well summarised in Sverdrup, Johnson and Fleming’s important book The Oceans (1942, pp. 646, 685–6) which, for the serious student, gives such an excellent account of the main results of modern oceanography. Dr. L. H. N. Cooper of the Plymouth Laboratory has recently (1952) made a study of the distribution of this water to the west of the British Isles as it continues northward below the Atlantic water at a depth of some 600 to 1,200 metres. How far north it goes seems to vary greatly in different years; in some it appears to go no further than the west of Ireland, but in other years it flows onwards to upwell and spread over the continental shelf. Its course has been followed by Dr. Fraser by finding its typical but exotic fauna in his plankton nets; to the west he finds it deep down—but let me quote from his recent paper.
“It apparently follows the edge of the Hebridean Continental shelf, mixing on its western edge with open oceanic water, and upwells somewhat on the east side to overflow and mix with coastal water on the shelf. In some years this current may not reach Scotland or is too weak to be recognised, but on occasions it is sufficiently strong to continue into the South side of the Faroe Channel, though it only rarely penetrates beyond the north of Shetland. Frequently, however, it mixes with the coastal water on the shelf and the resulting mixture floods the area to the west of Orkney and often passes through the Fair Isle-Orkney Passage into the Moray Firth area.”
How lucky we are to have such a remarkable current carrying its rich and southern life far below the surface and then spreading it out, as it were, on our northern doorstep for our examination. Fraser calls this a planktonic ‘Lusitanian fauna’ and lists no fewer than 43 species characteristic of it; he has kindly prepared for me a chart of its typical distribution which is reproduced in Fig. 11 (#litres_trial_promo). Through his recent publications I have been able to add to my account some very interesting animals which I shall be describing in Chapter 7 (#litres_trial_promo) and Chapter 8 (#litres_trial_promo) and which hitherto I should never have dared to include as inhabitants of British waters. He defines his Lusitanian fauna as that which “originating in the outflow from the Mediterranean, has become modified by admixture with fauna from the area between the Azores and Bay of Biscay.” This work is an outstanding example of the importance of natural history in helping us to have a better understanding of the physics of the sea. I will give a final quotation from his work:
“The whole of this oceanic system to the north and west of Scotland overlies a south tending mass of artic or boreal water. The main flow of this water mass is to the west of Faroe from whence it thrusts southwards in deep water (below about 1,000m.), but part also penetrates the Faroe Channel where it is checked by the Wyville Thomson Ridge.
(#ulink_f5885b56-36c1-5d71-ad68-bb7fbd6399fb) Although this water affects the inflowing system where it mixes at its interface it is not of such importance as are the more massive cold water currents on the other side of the Atlantic.
“Each of the above water masses has a typical plankton fauna (see Russell 1939, and earlier works), which varies within certain limits, in the abundance and in the proportions of its constituent species from year to year. As these organisms are transported further from their natural habitat they gradually die as their limit of tolerance is reached, and they are replaced by other species through mixing either with other oceanic streams or with coastal water. The fauna of an incoming water mass thus gradually changes along its length; for example, few of the oceanic species noted off Scotland normally reach north-western Norway (Wiborg 1954). The degree of survival of the original fauna gives a measure of the purity of the inflow, and the relative life of the species less tolerant to various factors may give an indication of the type of dilution or change involved.”
(#ulink_534c11bf-fb7f-5157-a059-80d613c3cc0e) A valuable review of the changes in the southern North Sea, clue to variations in the influence of the low salinity water from the Baltic and that of the higher salinity water from the Channel, has been made by Lucas and Rae (1946).
(#ulink_bd866a47-5ab5-5497-aaf0-f45fbb2d3351) This will be referred to again in chapter 15, see here (#litres_trial_promo).
(#ulink_86f4bd14-62a8-5c14-b5bd-cb3514905b99) The difference between the two is shown in Fig. 42 (#litres_trial_promo).
(#ulink_067bab02-ceff-5f3b-bf76-329fc94211cc)see here (#litres_trial_promo).
CHAPTER 3 (#ulink_b58f47bc-0a94-5b5f-8c94-7f01869e6090) PLANTS OF THE PLANKTON
HAVING DISCUSSED the movement of the waters it might perhaps seem more logical to pass on at once to consider other physical characters of the sea and something of its chemistry before proceeding to deal with any of the life within it. On the other hand, since the plants of the open sea are so intimately dependent upon their physical and chemical background it will be more interesting if we know what kind of plants we are dealing with before we actually discuss the conditions which are most favourable for their growth.
The vegetation of the open sea must be floating freely in the water in order to be sufficiently near the surface to get enough light; the great difference between it and that of the coasts or land, is that it consists entirely of plants of microscopic size. They are, as we have already seen, part of the plankton: the phytoplankton. Each is composed of just one unit of life—a single cell—instead of being made up, as are the larger plants, of a vast number of such units. Instead of having various kinds of cells specialised to perform different functions in organs such as roots, stems, leaves and reproductive bodies, all these activities of life are carried out by just one highly organised unit. It does at first sight seem strange that there should not be even a few larger plants adapted for such a floating existence. There is the famous Gulf-weed Sargassum which, buoyed up by the little floats upon its fronds, is found in masses drifting round that great eddy of the tropical Atlantic—the Sargasso Sea; this however is not a true open-ocean plant for it has been broken away by wave action from the coasts of Central America and the West Indies. It continues to grow for a long time but never produces reproductive organs as it does when attached to its native rocks. In our own waters we may sometimes meet with patches of bladder-wrack, Fucus vesiculosus, torn from the sea-shore by storms and floating in the same way. They are but mutilated stragglers, out of place and lost in the open sea.
The microscopic plants must have some great advantage over larger plants in this floating drifting life. The smaller an object is the larger is its surface in relation to its volume. If we increase the size of an object—keeping its shape in the same form—the volume increases by the cube of linear measurement but the surface does so only by the square. This elementary fact is so important in the present discussion that it may be well to emphasise it by a simple concrete example. If we have eight small cubes of soap of the same size and press them together to form one big cube, the volume of this new cube will then be eight times that of one of the smaller ones, whereas its surface will be only four times as large. We have of course lost all the surfaces that were pressed and fused together. Inversely, the more we cut up our soap into smaller and smaller cubes, the more surface will each new cube have in proportion to its volume. A cube the size of one of our little plants will have a surface-volume ratio many hundreds of times as large as that of a cube with a side no more than an inch or two. A large surface-volume ratio is a great advantage to our little plants in at least two important respects. Firstly, the larger the surface in relation to mass the greater will be the frictional resistance to the water which will retard its sinking and so enable it to remain more easily in the upper sunlit layers. Secondly, since absorption must take place through the surface, the larger its surface in proportion to its volume the more readily will it be able to take up for its needs enough of the necessary mineral salts which may be present in the water in only very small amounts. This indeed may be the cardinal factor which has prohibited the development of larger plants in the plankton; but for this they might well have evolved bladder-like floats to support their larger mass, as some animals have done. Each tiny plant, as a single cell, can also take better advantage of the scattered sunlight than can a number of such cells massed together.
It is at first difficult to believe that these finely scattered and microscopic plants can really form a vegetation which has sufficient bulk to support all the teeming animal life of the sea: the dense populations of planktonic crustaceans, the vast shoals offish and all the invertebrate animals on the sea-bed. Yet we know this must be so. Some estimates of the actual quantities of plants present in a cubic metre of sea-water will be given later in the chapter; here, in passing, we will only note that, given suitable conditions, the amount of plant life produced under a given area of sea may well exceed that produced for the same area in a tropical forest. Just as our coal supplies are giving us the energy stored up in the great primaeval forests of some hundred million years ago—so is the energy in the petrol, which drives our motor and flying age, derived from that originally trapped from the sunlight by the tiny planktonic plants in the seas of long ago. According to current geological theory, the great supplies of mineral oil have been formed, in the course of ages, from the remains of marine organisms buried in sedementation under specially favourable conditions which are not yet fully understood. It is most likely that the planktonic crustaceans, whose modern representatives are so rich in oil, would in the past be the main contributors to the supplies of petrol we are burning up today; those crustaceans, of course, derived their energy either directly or indirectly from such tiny plants as we are now considering.
A microscope of sufficient power to enable us to see a great deal of this world of planktonic plants and animals need not be an elaborate one, nor need it cost much more than a good pair of field-glasses. We shall want some glass slides and coverslips, small dishes (such as watch-glasses), pipettes, i.e. old-fashioned fountain-pen fillers, for picking up very small plankton animals, and some glass jam jars; apart from that, all we need is a tow-net and line with which to collect the plankton from a rowing boat or any larger vessel that can be made to go slowly enough.
FIG. 12
A simple form of the plankton collecting tow-net.
A tow-net can be bought from the laboratory of the Marine Biological Association at Plymouth (address: The Laboratory, Citadel Hill, Plymouth) or it can be home-made. It consists essentially of three parts: in front is a hoop made either of light galvanised iron or strong cane and provided with three bridles of cord which will come together at a small ring or shackle for attaching to the towing rope; next comes the actual net, a conical bag made of a fabric which will act as a fine sieve; lastly at the end of the net is a small collecting jar, either a glass honey-jar or one made of zinc or copper with a slight lip. Such a simple tow-net is shown in Fig. 12 (#litres_trial_promo). For ordinary collecting purposes a hoop of 18 inches diameter will be quite sufficient. The net is best made of the silk ‘bolting cloth’ used by millers for sieving flour, but a good quality muslin will do if this cannot be obtained; with a mouth of 18 inches it should be almost five feet in length. If it is to be homemade great care should be taken in cutting out the material in order to ensure that a perfect cone is formed; if lop-sided it will not fish properly. It is a good plan to pin together a paper model to serve as a pattern; this will also enable one to see how best to use the material with as little waste as possible. Round its wide mouth a canvas or calico band is sewn for attachment to the hoop; it may either be provided with a series of eyes for lashing it on or it may be folded over the hoop and sewn to. enclose it, leaving gaps where the towing bridles are secured. At the hind end is sewn another canvas band to form a small cylinder, say 2½ inches in diameter, which will slip closely over the mouth of the collecting jar and be firmly held in position by a tightly tied tape. It is well to be provided with two such tow-nets; one made of the very finest material for the collection of the small plants—the finest bolting-cloth has 200 threads to the linear inch—and one of coarser material, having about 60 threads to the inch, for the capture of the somewhat larger animals. The coarser net lets most of the plants go through its mesh but filters a very much larger quantity of water more quickly and so captures the larger more active animals which are only rarely taken in the finer net.
To collect the phytoplankton the fine net should be towed just a little way below the surface. A weight, say a 71b. lead, is slung at the end of the rope and the net attached a little way above it. The essence of successful tow-netting is to tow very slowly, never at more than 1½ knots. If it is towed faster the water will not be filtered quick enough; the net will just push a mass of water in front of it which will prevent any more water entering it. A ten minutes’ tow may give quite a large enough sample. Most of the plankton will have passed down into the jar at the end as it is towed; a number of specimens, however, may still be sticking to the inside of the net as it is taken from the water, so that it should be carefully washed down from the outside with water from a bucket, to flush them into the jar.
Our sample will contain a vast number of both plants and animals. In this chapter we will concern ourselves only with the former, which are so small that they must be looked for with the compound microscope. After bringing our sample home and letting it stand for a little we should take only a few drops at a time with a fine pipette from near the bottom and place them on a slide under a coverslip; now we shall hunt with the low-power lens and then turn on the high-power to examine each new specimen we find. We shall not, of course, expect to find examples of all the different kinds in one sample but there may well be representatives of several of the more important groups. The most prominent members of the phytoplankton are the diatoms. They are unicellular algae differing from all other algae in having a cell wall which forms a siliceous external skeleton enclosing the cell like a glass box. The pigment bodies, or chloroplasts, which enable the plant to make use of the energy of sunlight are not the usual bright green of chlorophyll but a brown or brownish-green pigment closely allied to it. The siliceous skeleton is in two parts which fit together like the top and bottom of a pill-box; indeed some of the diatoms are just like a pill-box in form, but many others are drawn out into all manner of fantastic shapes. When first we see a sample of plankton rich in diatoms under the high power of the microscope it is like looking at a group of crystal caskets filled with jewels as the strands of sparkling protoplasm and groups of amber chloroplasts catch the light. Every plant or animal cell consists of a mass of protoplasm with a more or less central body, the nucleus, which appears to govern its life; it is characteristic of the diatoms that, in addition, the protoplasm usually has large cavities in it containing clear fluid. The nucleus is usually central and surrounded by a mass of protoplasm; radiating from this and forming an irregular network are protoplasmic strands stretching across the cavities like the spokes of a wheel to join up with a layer of protoplasm which lines the inner surface of the boxlike covering. More rarely, in some forms, the nucleus may be in the layer of protoplasm at the side. The pigment granules usually lie more or less regularly spaced against the cell-wall, where they are exposed to the light; if, however, the light is too intense, they come close together either down the strands to the centre or to some other part of the cell where they can partly screen one another from the harmful effects of the rays.
The top, bottom and sides of the glass-like box are not made of just plain sheets of silica; their surfaces are sculptured with all manner of striations, pits and perforations forming intricate patterns peculiar to the different species. This detail of design has always made the diatoms favourite specimens with microscopists, not only on account of their beauty, but because they are such excellent objects with which to test and display the quality of their instruments in the higher ranges of magnification. They are now being put under the electron microscope which can give a micrograph with a magnification of up to 100,000 times; this has at once revealed an arrangement of structure far more elaborate than that seen with the highest powers available in the optical systems (Hendy, Cushing and Ripley, 1954). Instead of there being just one system of pits or perforations in their walls, some forms are shown to have smaller and yet smaller ones, secondary and tertiary systems, on inner layers of silica; in others the wall is more like a basket of spiral threads intricately woven together. Some of the perforations measured were less than a ten-thousandth of a millimetre in diameter and the surrounding walls were of equal thickness. These delicate lattice systems have at least two important qualities for floating plants: they give strength with lightness and at the same time provide a framework for presenting a greatly increased surface area of protoplasm to the surrounding water.
FIG. 13
Diagrams showing the division of a simple pillbox-like type of diatom. a, a sketch of the cell before division has begun, b to d sections through the diatom after cell division to show stages in the formation of the new skeletal cell walls. Note that the upper cell in d is smaller than the original cell b.
Diatoms normally reproduce by simply dividing in two. The nucleus divides first and then the protoplasm becomes separated into two masses, each containing a nucleus, one at either end of the box; each mass of protoplasm now forms, between it and the other mass, a new valve as the halves of the pill-box are called. These new valves each fit closely their own part of the old box; we have in fact two pillboxes now instead of one, as is shown above in Fig. 13 (#litres_trial_promo). They may separate entirely, or in some species they may remain attached to form long chains. It will be realised that in this process of repeated division by forming new half-boxes within the old, the average size of the diatoms so produced will tend to get smaller and smaller; at each division, as shown in the drawing, one of the new boxes will be the same size as the old one but the other must be smaller. Thus we find a considerable range in the size of diatoms of the same species, but there must be a limit to this reduction. After a certain number of such divisions there is formed what is called an auxospore, by which the original size is recovered; throwing off the old valves the cell becomes a bladder-like mass of protoplasm within which new valves are formed two or three times the size of the old discarded ones. Some diatoms seem to do this at definite seasons whereas others do so only at intervals of two or three years. By taking sample measurements of the diatoms forming some of the dense concentrations which are carried by currents about the North Sea, planktologists have been able to identify individual patches in their wanderings by the regular decrease in the size of their component cells (Wimpenny 1936; Lucas and Stubbings 1948). Some of these concentrations, as we shall see in Chapter 15 (#litres_trial_promo) have been thought to have a marked effect upon the herring fisheries; their wanderings may therefore be of economic interest.
In addition to forming auxospores, diatoms may produce what are called resting spores when conditions become adverse. The contents of the cell become concentrated in a central mass which forms a new thick wall of a different but characteristic shape and the old cell-wall is discarded. They now either sink into the deeper water layers or right to the bottom where they will remain till more suitable conditions return; thus some may pass the winter in a resting state and then come up to start active life again in the spring. Planktonic diatoms are not definitely known to have any sexual phase, but in some a number of smaller spores (microspores) have occasionally been observed to be formed within the cell-wall and it has been thought that these may be gametes (sex cells), but this is not yet established.
Not all diatoms are planktonic; in the shallow coastal regions there are numbers living on the bottom where sufficient light reaches it; these have much thicker shells than the more delicate floating forms and are more uniform in character. What makes the planktonic diatoms so interesting is the variety of devices that have been evolved to assist in their flotation. It is not the purpose of this book to attempt a systematic treatment of the groups of animals and plants. Here we shall just refer to some of the more important kinds of diatoms in relation to their mode of suspension. Dr. Marie Lebour’s excellent book (1930) on the planktonic forms should be studied for a full account.
Although so very small they have a considerable range in size; a few exceptionally large ones may be over one millimetre in diameter and the smallest may be but a few thousandths of this. Sketches of examples of the different genera to be mentioned are shown in Fig. 14 (#litres_trial_promo) and Plate 1 (#litres_trial_promo), and some are also included in the photographs in Plate I (#litres_trial_promo). A number have the typical pill-box form, such as members of the genus Coscinodiscus and some of these are of comparatively large size; a few like C. concinnus may be just visible to the naked eye. These kinds have large vacuoles and would seem to be buoyed up by globules of oil. Members of this genus live singly, but others of allied genera may remain after division attached together to form long chains; the cells of Paralia and Guinardia are linked rigidly together by their valve surfaces, those of Thalassiosira form flexible chains as they are strung together by fine threads of protoplasm, and others like Lauderia are still more loosely held together by irregular strands of slime. The cells of Thalassiosira also produce such slime-strands but for a different purpose—around the margin of each valve (i.e. each ‘pill-box lid’) are a number of small hollow spines from which can be extruded long slender threads of slime so that they radiate on all sides like the strands of thistle-down and indeed act in the same way to assist in parachute-like support.
The remainder of the planktonic diatoms, while essentially built on this pillbox plan, have each valve or half-box modified into all sorts of shapes which increase their surface area in relation to their volume and so give greater frictional resistance to sinking. Some are flattened like thin sheets of paper and often twisted to some extent; these usually remain attached together to form long ribbons: such are Bellarochia, Eucampia and Streptotheca. Others are drawn out either into long thin hair-like forms such as Thalassiothrix longissima or into the more rigid pencil or needle-like members of the genus Rhizosolenia, pointed at each end; in the former the division plane between the two valves runs lengthwise along the thread whereas in the latter it occurs transversely to the long axis. Other forms again, such as Biddulphia and Corethron, increase their surface area by being provided with spines. This last method is developed to a remarkable extent by the many species of the genus Chaetoceros whose cells have four very long hair-like processes (two extending from each valve); long chains of these cells are formed and held together by their curving processes becoming interlocked with those of adjacent cells close against their point of origin.
FIG. 14
Some characteristic plankton diatoms not shown in Plate I (#litres_trial_promo), all magnified ×90 diam. a, Coscinodiscus concinnus; b, Bacillaria paradoxa; c, Thalassiosira gravida; d, Rhizosolenia styliformis; e, Paralia sulcata; f, Bellarochia maleus; g, Thalassiothrix nitzschioides; h, Streptotheca thamensis; i, Rhizosolenia hebetata (form semispina); j, Nitzschia seriata; k, Gyrosigma sp.; l, Chaetoceros curvisetus; m, Ch. convolutus. The actual length represented by the longer side of this figure is 1/20th of an inch.
There are many species of Biddulphia in our waters, but one, B. sinensis (Plate 1 (#litres_trial_promo)) is of special interest; it is now one of our commonest diatoms, often occurring in dense concentrations, yet it was unknown in European waters before 1903 when it was first recorded in the Heligoland Bight. It is a well known inhabitant of the coastal waters of the Indo-Pacific region, extending from the Red Sea to the coasts of China. It seems likely that it must have been brought, perhaps in ballast water, by some ship to the mouth of the Elbe; being tolerant of wide ranges of temperature and salinity, it found our waters congenial and spread rapidly. In the following years it was recorded further and further to the north until it reached a point a long way up the Norwegian coast where its further spread was probably checked by too cold water. More extraordinary, in view of the prevailing currents, was its spread down the Channel and into the Irish Sea; in the same year, 1909, it was reported for the first time both at Plymouth and off Port Erin in the Isle of Man. This would appear to provide evidence of an occasional reversal of the usual current flow up the Channel; indeed such a reversal has been suspected by some oceanographers on other grounds. Or was it transported to the western Channel and Irish Sea in the same way as it had apparently reached the Elbe? We may never know the answer to that. Some have maintained that Biddulphia sinensis must have been a native of our waters all the time and only just noticed at the beginning of the century; this, however, can hardly be so because of extensive collections that were made, particularly by the Kiel planktologists, throughout the eighteen-nineties.
In coastal regions we may find a number of typical bottom-living diatoms carried up into the plankton, such as the boat-shaped members of the genus Navicula. They are capable of a remarkable gliding movement, thought to be produced by a flow of protoplasm passed out through a slit in the wall of the cell. Gyrosigma is another bottom form often met with in the shallow-water plankton. Other related forms such as Bacillaria and Nitzschia are more planktonic but typical of coastal waters. B. paradoxa forms bands of long slender cells held together side by side like the planks in a raft yet each capable of sliding up and down along its neighbours; N. seriata, in contrast, forms long strings of narrow boat-shaped cells end to end with just their tips overlapping and in contact.
It is impossible to mention all the kinds of diatoms of our seas in such a general review, and I shall only refer to one other species, one which may well attract attention: Asterionella japonica (Plate 1 (#litres_trial_promo)). Its cells are rod-like but thickened at one end; by these thickened ends the cells remain attached to one another to form beautiful radiating star-like clusters.
In striking contrast to the brown-green colouring of the diatoms is the brilliant green sphere of Halosphaera viridis which may reach a size of nearly a millimetre in diameter; it and one or two closely allied species are the only representatives in the marine plankton of the Yellow-green Algae or Heterokontae. H. viridis is found over the whole Atlantic from the tropics to the far northern branches of the North Atlantic current off Spitsbergen. In autumn it is often brought into the northern North Sea in large numbers and is usually found floating very near the surface. It is exceptional in its mode of reproduction; it does not divide in two, but when full-grown undergoes multiple fission into a large number of small spores which break out of the surrounding envelope and swim, like the flagellates about to be described, by the use of whip-like locomotory organs. The full life-history has not yet been observed; whether after fusing with others or not, these spores must eventually give rise to the little green spheres which gradually grow to a full size again.
FIG. 15
Some flagellates of the plankton. a–h, Dinoflagellates: a, Ceratium fusus (×180); b, C.macroceros (×200); c, Protoerythropsis vigilans (×320) (note clear spherical lens against dark eye-spot); d, Dinophysis acuta (×400); e, Peridinium granii (×360); f and g, P. ovatum (×320), side and top view; h, Polykrikos schwarzi (×250); i and j, the Silicoflagellate Distephanus speculum living and half of skeleton (×320); k, a very small part of the large gelatinous capsule formed by the tiny cells of Phaeocystis; l and m, Coccolithophores (×1000): Coccolithus huxleyei and Coccosphaera leptopora; n–r, some of the smallest flagellates (×1500): n, Dicrateria inornata; o, Hemiselmis rufescens; p, Isochrysis galbana; q, Pyramimonas grossii; r, Chromulina pleiades. Original drawings except c from Marshall (1925), h from Lebour (1925), m from Murray and Blackman (1898) and n to r from Parke (1949).
All the remainder of the planktonic plants belong to the big assemblage of organisms known as flagellates of which there are many different kinds. A selection of the commoner forms is shown in Fig. 15 (#litres_trial_promo). They are all characterised by possessing at least one, and often two, of the motile whip-like processes termed flagella, with which they draw or propel themselves through the water and are thus able to keep up in the sunlit surface layers. These flagellates are claimed for study by both botanists and zoologists, for among them are indeed both plants and animals—and some which have the characters of both in one. Some possess green pigments allied to chlorophyll or even chorophyll itself, and so feed as true plants; others lack pigment and may feed either by absorbing organic substances through their surface or actually live as animals by capturing particulate food; yet again, others may combine the methods of plant and animal feeding. In this lowly group of organisms the animal kingdom has not yet become fully separated from the plant kingdom. However, most of the planktonic flagellates are in fact plants and most of them have a small red ‘eye-spot’ or stigma which is sensitive to light and so enables them to tell whether they are moving towards or away from the radiant energy necessary to build up their food. If you are able to obtain a plankton sample very rich in these small green flagellates you will be able to see how readily they are attracted upwards towards the light. Fill a tall narrow glass jar with the sample and cover the lower three-quarters with thick brown paper. Now if you leave it for half an hour in the full light of the window you will find on removing the paper that the top quarter of the jar is distinctly greener than the rest; the little flagellates from the whole jar have become concentrated in the sunlit zone. By standing a sample of sea water in the light you may be able to grow a more abundant culture of these little flagellates and so give a more striking demonstration of this experiment. There are some of the Dinoflagellates (members of the family Pouchetiidae) which have a much more elaborate light-sensory organ furnished with both a lens and a pigment-cup; indeed it might almost be called an eye. One of these is shown in Fig. 15c (#litres_trial_promo).
The Dinoflagellates are the most striking members of the phytoplankton after the diatoms and are usually present in large numbers; for a full account of them another excellent volume by Dr. Marie Lebour (1925) should be consulted. They have a cell wall made up of a number of plates of cellulose fitting together to form a mosaic and are characterised by possessing two flagella: one working transversely in a prominent groove which almost completely encircles the body like a girdle, the other projecting behind from out of a small longitudinal groove running backwards from the girdle. This latter groove is often protected by curtain-like membranes so that the flagellum may be withdrawn spirally into a sheath. Typically, as in Peridinium, they have a single spine pointing forward in front and two spines projecting backwards from the half of the body behind the girdle. The flagellum working in the groove sets them waltzing round as they are at the same time driven forwards by the other flagellum behind (Fig. 15 (#litres_trial_promo)); they screw themselves through the water. There are a great many species of Peridinium and closely allied genera with very much the same general appearance (Plate IIIb (#litres_trial_promo)); one related genus, however, Ceratium, is most striking in having the spines drawn out into long horns, with the two posterior ones usually curving forwards to give the whole body the shape of a little anchor. Sometimes, particularly in late summer and autumn, plankton samples may be full of Ceratium tripos or perhaps another of the many species of the genus distinguished by only small differences (Plate Ib (#litres_trial_promo)). Two species which are very common in our waters stand out in contrast to the rest: Ceratium furca (Plate 1 (#litres_trial_promo)) in which the posterior spines are rather short and point straight backwards, and C. fusus (Fig. 15 (#litres_trial_promo)) in which there is only one posterior spine, long and only very slightly curved, just like the anterior one. There is a remarkable range of colour in Ceratium from a bright green to a yellow-brown.
Dinoflagellates, like other flagellates, multiply by simple fission into two and, like diatoms, each daughter-cell retains one half of the old cell-wall and forms another half anew; but unlike the diatoms the new halves are not formed within the old cell wall and so there is no gradual diminution in size. Occasionally recently-divided individuals of Ceratium may remain adhering together to form little chains. The cell-wall grows in thickness but when the little plates of armour become too heavy they fall off to be replaced by new and extremely thin ones. The long horns of Ceratium are certainly organs to assist in suspension. Species in the warmer waters generally have much longer ones than those in colder waters. Warmer water is more fluid—or less viscid—than colder water, for the molecules move over one another more freely with greater heat-motion; in the tropics an object will sink twice as fast as will one of similar density and shape in the polar seas. In general plankton organisms, both plants and animals, are more spiny in the tropics to give them a greater surface resistance. It has been observed that species of Ceratium have the power of adjusting the length of their horns to the varying viscosity of the water; on being carried by currents into warmer water they grow longer horns, conversely if carried into a colder area they can shed parts of them. (Gran 1912).
Several species of Dinoflagellates can produce a brilliant phosphorescence. Many plankton animals are luminous and produce the sparks of light we often see in the water at night. In addition to such displays, however, we may also see a more general ghostly light or sometimes when out in a rowing boat our oar as it cuts the water may leave a trail of blue-green flame behind it; and even from the shore we may see the waves breaking in a flash of light. Such displays are caused by countless millions of dinoflagellates each glowing by an oxidation process as it is agitated in the water. Noctiluca is the most celebrated for this, but although a dinoflagellate it is curiously modified to be entirely animal in its mode of life and so will be described in a later chapter (see here (#litres_trial_promo)); other members of the group, however, particularly Ceratium, give almost as good a display. Once on a fisheries research trawler, having stopped at night to make some observations in the Channel, I looked over the side to see a small shoal of fish, most likely mackerel, lit up by each individual being covered by a coat of fire; they were being chased this way and that by some much larger fish similarly aflame. On putting over a tow-net, which came up brilliantly illuminated, the sea was seen to be full of a very small Peridinium-like dinoflagellate of the genus Goniaulax.
Two other genera of dinoflagellates occuring in our waters, and shown in Fig. 15 (#litres_trial_promo), will just be mentioned. Dinophysis has the part of the body in front of the girdle reduced to a minimum so that the girdle itself, with very pronounced margins to its groove, appears like a band round its very front or top; the posterior part bears a marked keel at one side as if designed to prevent the rotation which is normal to the group. Instead of spinning round it is thought to set up a vortex current by which it draws into the groove still smaller organisms as food. Polykrikos is a remarkable genus having a number of girdles, usually four or eight, placed in regular succession down the body; it is often spoken of as a ‘colonial form’ as if made up of several individuals which have failed to separate on division, but this can hardly be the correct view since the number of nuclei is always smaller than the number of girdles present. It appears to be an individual with a repetition of organs similar to the segments of an animal like an annelid worm. They are also said to feed like animals as well as like plants; they possess remarkable little capsules containing coiled threads which can be shot out like those found in the stinging cells of sea anemones and jelly-fish, and may possibly be used for a similar purpose—the capture of prey. In addition to all these forms with their different characteristic patterns of armour plating and spines, there are a great many so called ‘naked’ dinoflagellates which lack all such coverings; many of these are, for part of their lives, internal parasites in a number of different marine animals.
Among the small shells of Globigerina first brought up from the ooze of the ocean bed were found numbers of still smaller calcareous bodies, little plates, some oval and perforated, others round and bearing stout blunt spines; they were called coccoliths and rabdoliths respectively, and presented naturalists with a puzzle as to what they were. It was Sir John Murray who discovered their real nature by showing them to be plates which had covered the bodies of other little plank-tonic flagellates which were given the name of Coccolithophores.
(#ulink_915325a6-6fdf-5ea6-8c39-8c85b71158f3)Coccosphaera and Coccolithus (Fig. 15 (#litres_trial_promo)) occur in our seas. They are commoner in the tropics, although in the Atlantic water coming into the northern North Sea they may occasionally be so numerous as to give a milky appearance to the water and cause a chalky deposit to be left on the fishing nets as they dry. This is what the herring fishermen call ‘white water’ and generally believe to be a good sign for the presence of herring. A well-known herring skipper, Mr. Ronald Balls, who is also a keen naturalist, has recently written, under the pen-name of “Peko”, an excellent article on this white water in World Fishing (July 1954). He describes how this water gives ‘the queer impression of whiteness coming upwards: as if the light was below the sea instead of above it’. He then refers to recent views that the coccoliths are shields reflecting light from their owners which normally live in tropical seas where the illumination is too strong; ‘and here’, he writes, ‘was the perfect explanation of the fairy glow or white reflection that I had experienced long ago, and wrote about before I knew even that this organism existed’. As with the cell walls of diatoms, the electron microscope is showing that each little plate or coccolith has a much more complicated structure than was originally supposed; its base consists of radiating ribs like the spokes of a wheel and its rim is decorated with a frill like that with which a chef may decorate a ham. There are other similar little creatures, the Silicoflagellates, which form a delicate siliceous skeleton with radiating spines (Fig. 15 (#litres_trial_promo), i and j).
Herring nets, although they hang in the water near the surface, may often come out of it in a very slimy condition; this may be due to an excessive number of diatoms; more usually, however, it is due to globules of jelly large enough to be seen quite easily by the unaided eye. These slimy blobs are produced by aggregates of microscopic flagellates called Phaeocystis which colour the surface of the jelly in green patches (Fig. 15k (#litres_trial_promo)). All the meshes of a tow-net may be blocked with them. Dense concentrations of Phaeocystis, like those of diatoms, which cover wide areas of sea, have also been thought to have a deleterious effect on the shoaling of herring and at times to have led to a poor or delayed fishery. We shall refer to this again in Chapter 15 (#litres_trial_promo).
This completes the review of those planktonic plants we shall mention by name; but we have so far left out of account a vast number of still much smaller flagellates which have escaped capture by passing through the meshes of the finest net we can use. It is only comparatively recently that their influence in the economy of the sea has been realised. Their prominence was first demonstrated by the German naturalist Lohmann who examined the remarkably fine filtering mechanism, far finer than any gauze that man can make, used for their capture by some little plankton animals, the Larvacea, to be described here (#litres_trial_promo). They may, however, be extracted from a sample of sea water by centrifuging
(#ulink_a62ac299-94ae-50d9-a773-49ac31ea8893) small quantities of it in tapering tubes. If after such treatment the greater part of the water is carefully decanted, a drop of the remaining fluid may be taken up in a pipette and examined on a slide under the high power of a microscope; then they will be seen as tiny yellow specks jigging in the water Today a great many of these minute flagellates are being successfully cultured in the laboratory, notably by Dr. Mary Parke at Plymouth (1949). Five examples, sketched in line from her beautiful coloured drawings, are shown in Fig. 15 (#litres_trial_promo)n–r.
Smaller still, of course, are the bacteria which really lie outside the scope of this book; at present, very little is known about their occurrence in the plankton. Dr. H. W. Harvey (1945) states that their population density decreases on passing from inshore waters to the open sea and that in the ocean the greatest numbers are found where phytoplankton is abundant and in the water immediately above the sea-floor. They are found particularly in dense phytoplankton regions because of the undigested organic matter passed out by the animals which are eating more of the plants than they really require.
It is exceedingly difficult to get an accurate measure of the amount of plant life in a given quantity of sea water, even of the larger forms which are captured by a net. Although we can calculate the filtering efficiency of the net and know the quantity of water it should filter, we cannot be sure that it actually does filter this amount; in fact it rarely does, for to a varying extent under different conditions the meshes of the net become clogged by the organisms themselves and the filtering is much reduced. However we can get an approximate idea of the number of larger forms—the diatoms and dinoflagellates—in a given volume of water by using a net. Let us take an example. In 1907 Sir William Herdman and his co-workers began an intensive study of the plankton of Port Erin Bay in the Isle of Man which they continued until the end of 1920. Usually six times a week, every week for fourteen years, two standard nets of coarse and fine mesh were towed in exactly the same way over the same distance—half a mile—across the bay. Johnstone, Scott and Chadwick, who describe the results in their book The Marine Plankton (1924), estimate that for each such double haul “taking the two nets we shall not be very much in error (when all the conditions are considered), in assuming that 8 cubic metres of water were filtered through both nets.” The following figures, taken from their book, give the average number of the principal plant forms taken in such a catch during the month of April, i.e. the average of all the April hauls made over fourteen years:
That in round figures is 727,000 per cubic metre or about 20,000 per cubic foot. The number of plankton animals taken at the same time is given in Chapter 5 (#ub9d1ef9b-f5a9-51ce-96c7-37d3c4e2a8de) where the zooplankton is considered and may be compared here (#litres_trial_promo). The actual numbers present are estimated by using a specially calibrated pipette which takes up a known fraction of the sample; the fraction is spread out on a glass slide ruled in squares so that the number of plant cells can be counted below the microscope just as the corpuscles are counted in a sample of blood. We must remember two important things about the figures just given. Firstly they are for the larger microscopic plants; the very small ones are present in far greater numbers as we shall see in a moment. Secondly they are average figures for April over fourteen years; those for one year may be very different from those of another and the average figures for other months of the year will show still greater differences. There are marked seasonal changes in the plankton; but that is the subject of our next chapter.
The difficulty of knowing exactly how much water is filtered by a net when its meshes are becoming clogged by the organisms sampled, has been got over by an ingenious device invented by Dr. Harvey of Plymouth (1934). At the mouth of the tow-net he has fixed a little propeller which is turned by the water flowing into it; the number of revolutions it makes are recorded on little dials which measure the amount of water actually passed through the net. There is still, however, the difficulty of forming a true quantitative estimate of the plant life present. We can calculate, as we have just seen, the number of plant-cells in the sample; but these vary so enormously in size it is difficult to convert such an estimate into a measure of the total bulk of planktonic vegetation. Measurements of the volume of the sample can be made after all the plants have been killed by the addition of formalin and allowed to settle for several days in the bottom of a measuring jar; but this too is a very misleading estimate, because the various kinds, having different shapes, may pack together very differently: for example spiny forms take up more space than round or flat ones. However, these various methods do enable us to say broadly that one area is relatively so much richer in phytoplankton than another—always excluding the small flagellates which escape the net and must be estimated with the centrifuge. A more recent method of estimating the quantity of plant life caught in a plankton sample is to extract the plant pigment by acetone and measure the quantity present by matching up the samples obtained with a standard colour scale and expressing it in so many pigment units.
The late Dr. E. J. Allen (1919), when Director of the Plymouth Laboratory, made a simple but important experiment that gives us some idea of the vast numbers of little plants there are in the sea which are not caught by our ordinary methods. He had first perfected a method of growing them in bottles in a special culture solution, i.e. in sea water enriched with the addition of certain beneficial chemicals. He then took a sterilised quart-sized bottle and filled it with sea water from just below the surface about half a mile outside the Plymouth breakwater. This water he treated in two ways. The procedure may seem a little involved but it is worth following. Firstly he took four 10 cc samples of it and centrifuged them each twice with the result that he obtained an average of 14.45 organisms per 1 cc of water which gives us an estimate of 14,450 per litre. Secondly he took just ½ cc of the water he had collected and added it to 1,500 cc of his culture solution which he had previously sterilized; then after it had been thoroughly shaken up he divided this between 70 small flasks—a little over 20 cc in each—and placed them against a north window. After 10 days signs of growth were apparent. When they were finally examined there was not a flask that had not had some growth in it. He now recorded the different kinds of organisms in each. In two flasks there was only one species; in all the others there were from two to seven different species present, giving an average of 3.3 different kinds per flask. Thus at least 70 × 3.3 or 231 separate plants must have been taken up in the ½ cc originally added to the culture solution; that makes 464,000 per litre as compared with the 14,450 estimated by using the centrifuge! For comparison with the larger plant forms caught by the net in the former example we must express the number as per cubic metre: i.e. 464 million, or about 12½ million per cubic foot. Now this must be regarded as an absolute minimal estimate, for it is made by assuming that only one individual of each kind of plant recorded in a flask went into that flask at the beginning; this is most unlikely.
We begin to have some idea of the great wealth of plant life there is in the sea. Can we make it still richer by adding fertilizers in the same way as we increase our crops on land? Experiments have been made in that direction, but a discussion of them will come better in the next chapter, where we will deal with the various factors which govern phytoplankton production. For a more detailed and fuller account of the pelagic plants in general I would recommend for further reading the splendid chapter by Professor H. H. Gran in Murray and Hjort’s Depths of the Ocean (1912).
(#ulink_2b4c27ff-fcc8-5b2f-a31d-36f3f0c6dd4b) They were subsequently well described by G. Murray and V. H. Blackman 1898).
(#ulink_3aa26175-8e8a-51fc-a388-e5531838abd4) Subjecting to a force greater than gravity by spinning in a rotary apparatus: the centrifuge.
CHAPTER 4 (#ulink_a3346a5f-9582-5e90-b849-8ef1e018c7ee) SEASONS IN THE SEA
THE NATURALIST with a tow-net, if he can sample the plankton at different times of the year, will find contrasts between spring, summer, autumn and winter in our seas almost as striking as those in the vegetation on the land. These seasonal changes in the plankton have a profound effect on the lives of many fish. Just as we can tell the age of a felled tree by the number of concentric rings in its trunk representing summer and winter growth-zones, so we can tell the age of a herring by similar rings on its scales; these mark summer growth-periods, when its planktonic food was abundant, separated by lines showing where the scale, and the fish, had ceased to grow during winter when the plankton was scarce.
There is not, however, a simple and gradual increase in the plankton as spring advances into summer followed by a gradual decline in the autumn. Our naturalist with a tow-net will find some of the changes very puzzling at first sight. In British waters in the winter there is a general paucity of both animals and plants in the plankton; then as the sunlight grows stronger (the date varying in different years, but usually in March) there is a sudden outburst of plant activity. The little diatoms start dividing at a prodigious rate: in a week they may have multiplied a hundred-fold and by a fortnight perhaps ten-thousand-fold. The meshes of the tow-net are clogged by them and the little jar at its end is filled with a brown-green slime, a slime which under the microscope resolves itself into a myriad forms of beautiful design. Then as spring advances into summer the number of little floating plants steadily declines until by late summer there are surprisingly few. Some reduction in their numbers might indeed be expected, for the little animals in the plankton which feed on them are also multiplying as the season advances and the waters are warming up; with the increasing sunlight, however, we might have thought that the plants’ remarkable power of increase could largely keep pace with the grazing of the animals. Something seems to be preventing the diatoms from keeping up that rapid multiplication. In the autumn comes another surprise. As the days begin to shorten and the sunlight is getting less intense, when in fact we might least expect a renewal of plant activity, there comes a second phytoplankton outburst; it is not as spectacular as the spring maximum and not in every year is it of an equal intensity, but there it is—a definite surging up again of reproductive power. From this second peak of production, as winter approaches, the numbers fall again to the lowest level of the year.
This sequence of events was known for a long time before it was properly understood; it was only after much more had been discovered about the physics and chemistry of the sea that it was possible to see at all clearly the chain of cause and effect throughout the year. So important are these events that we must devote a little space to considering some of the more important elements in the physical and chemical background that will help us to explain them.
Let us first consider some of the physical properties of the water. At the very beginning, in the introductory chapter (#ubcb89914-6a12-5f9a-961d-a32f7a64e31a), we referred to the limited transparency of the sea and some figures were given to show how quickly light is actually absorbed on its passage below the surface. As might be expected absorption of light will be found to vary considerably according to the amount of suspended matter, either sediment or plankton, in the water. Far out from the land the water is usually much clearer than in the shallower regions against the coast where detritus and mud may continually be stirred up by the tides or brought in by drainage from the land. If we compare measurements of light made at different depths below the surface in the waters near Plymouth we find the penetration in inshore waters at Cawsand Bay to be only half what it is at a point some 10 miles S.W. of the Eddystone Lighthouse (Poole and Atkins, 1926). Taking the light entering the sea, i.e. just below the surface, as our standard, we find in Cawsand Bay that half of it has been absorbed at 2 metres depth (i.e. 1 fathom), some 75% at 4½ metres and 90% at about 8 metres depth; whereas at 10 miles out the same percentage reductions in light intensity are found at depths of about 4¼, 9½ and 17 metres respectively. At points in the open sub-tropical or tropical Atlantic, where the phytoplankton is very sparse, as in the Sargasso Sea, the corresponding depths might be increased four or five times. A very simple bit of apparatus known as the Secchi disc, which can easily be home-made, will enable you to compare the transparency of the sea at different points; you may well be surprised at some of the results you will get with it. Take a white painted metal disc, say two feet in diameter, and drill three equidistant holes near its margin; now take three cords each about 6 feet long, tie them to a weight, say a 7–1b lead, and then tie a knot in each at 3 feet from the lead; next pass the cords through the holes in the disc and bring them together as supporting bridles to be tied to a loop or eye at the end of the line which will suspend the whole device in the water as shown in Fig. 16 (#litres_trial_promo). If a metal disc cannot easily be obtained, a white dinner plate, with wire clips behind it to take the cords will serve the purpose quite well. To compare the transparency of the sea all you have to do is to stop your boat and lower the disc over the side at different places and find how deep it must go before you can no longer see it. The weight not only carries the disc down but, if the cords are properly adjusted, ensures that it is always kept horizontal. The line can be knotted at metre intervals to facilitate measuring the depth. It is well to raise and lower it about the disappearance point several times in order to make quite sure just at what depth it goes out of sight; at some places it may vanish in only 5 metres, at others it may be seen for as much as 12 metres or more.
FIG. 16
The Secchi disc for measuring the transparency of the sea.
In passing I may say that the Secchi disc is also valuable in helping us to compare the varying colour of the water. Here, of course, I am not thinking of the often striking and delightful changes of hue which we may see as light of different quality is reflected from a changing sky: when for example dark grey clouds give place to an open space of blue or when cumulus clouds dapple the sea with purple shadows. Reference was made in Chapter 2 (#u3397ab9e-de11-5382-badf-93137316dee4) to the contrast between the green water of the Channel and the deep blue of the Atlantic; such differences are due to the nature of the contents of the sea itself and are examples of what I mean by the varying colour of the water. A great variety of shades and hues may be found at different times; these are mainly due to the presence of different kinds of very small plankton organisms in exceptional numbers. The light reflected back through the water from the white background of the disc enables us to judge and compare these differences more easily than by just looking into the depths. Dense concentrations of diatoms such as Rhizosolenia and Biddulphia, or of the colonial flagellate Phaeocystis, may give a brown appearance to the water over large areas; the North Sea fishermen often call such patches of water “Dutchman’s baccy juice”. Some dinoflagellates may make the water almost red, coccolithophores may give it the white milky appearance referred to in the last chapter, and other small flagellates may occasionally make it a vivid green.
But let us return to the sunlight and these little plants of the sea; in order to flourish and grow they must produce more oxygen in the process of photosynthesis (see here (#ulink_6442f009-7e57-5b3f-89d0-645624049396)) than they use up in respiration. Plants, of course, breathe as well as animals. Some very significant experiments were performed in the Clyde sea-area by Drs. Marshall and Orr (1928) of the marine biological station at Millport on the Island of Cumbrae. They grew cultures of diatoms in glass bottles in the sea at different depths; these they suspended on strings from a long thin rod between two buoys at the surface and so kept them free of shadow. All their bottles were in pairs; one of each pair was exposed to the light and the other covered with a black cloth. In each bottle the oxygen-content of the water was measured at the beginning of the experiment and again at the end of twenty-four hours. An increase in the oxygen in the uncovered bottles showed the amount produced by photosynthesis less that used up in respiration; a fall in oxygen-content in the ‘blacked-out’ bottles measured respiration alone. By adding this oxygen-loss to the oxygen measured in the uncovered bottles the total oxygen-production as a record of photosynthesis could be estimated. The experiments were repeated as the spring passed into summer, and were also made on days which were overcast and on others which were sunny. As the sun went higher in the sky and the light became more intense the depth at which diatoms could produce more oxygen than they used in respiration increased from a depth of less than 10 metres on an overcast day in March to nearly 30 metres on a sunny day at midsummer. By far the greatest photosynthetic activity—on which their growth depends—took place, however, in the top 5 metres. In the waters round Great Britain we may now say that practically all the plant-production that matters takes place in the top 10 or 15 metres. This is one important clue in the puzzle of the seasons; we must now turn to temperature.
FIG. 17
The Nansen-Pettersen water sampling bottle: shown open and closed.
The water round our coasts varies in temperature from about 8°C in winter to sometimes as much as 17°C in the Channel in a warm summer. It is, of course, because the sea loses and gains heat so much more slowly than the land that we in Britain have so equitable a climate compared to that of an area in the middle of a continent. Two methods are used in taking the temperature of the sea. Down to moderate depths, say to 50 metres, the insulated Nansen-Petterson water-bottle, which is shown in Fig. 17 (#litres_trial_promo) is used; it is of metal and is sent down suspended on a wire to obtain samples of water both for chemical analysis and for temperature determination. It goes down with the bottom and top open so that water can circulate through it; then at the required depth a small ‘messenger’ weight is sent sliding down the wire to hit a trigger which releases springs to close it. Projecting through the top, in a protective casing, is the stem of a thermometer whose bulb is in the centre of the sampling bottle; its scale and mercury thread are visible through a slit in the upper casing so that it can be read as soon as the bottle is brought back to the surface. There are actually three walls to the cylindrical bottle, one inside the other, with a little space between; when the top and bottom are firmly closed there are thus two water jackets outside the bottle proper and these act as insulating chambers preventing loss or gain of heat in the water sample while it is coming up and the thermometer is being read. As soon as the temperature has been noted the water is run out from a cock at the bottom to be stored for later analysis and the bottle is opened ready to be sent down to another level. From much greater depths the bottle would take so long being drawn up that the insulation just described would not be adequate to prevent a change of temperature in the process. To get over this, special so-called reversing thermometers and bottles have been devised. The mercury tube of the thermometer, just above the bulb, has a loop and a kink in it, so that when it is swung rapidly upside down the thread of mercury breaks; as soon as this happens all the mercury that before was above the kink now runs to the opposite, and now lower, end of the tube. When it is brought up the height of this inverted column of mercury is seen against a scale which can only be read when the thermometer is upside down; it tells us the temperature that the thermometer was recording at the moment it was turned over. The bottle and thermometers (there are usually two to give check readings) are mounted in a frame which rotates when a trigger is hit by a messenger weight; the bottle, which before was open, is closed as it swings over.
(#ulink_4b8d8474-868f-5ef4-9b22-459132cbd589)
After this digression on thermometers, let us return to consider the temperatures of our seas with the passing of the seasons. Water, above 4°C, expands when warmed and contracts when cooled; so its density is altered: a given volume of cold water weighing more than the same volume of warm water. In winter the atmosphere is colder than the sea so that the surface waters are cooled and therefore sink beneath the warmer and less dense layers which were below; this is repeated again and again until after a time there is an almost uniform low temperature from top to bottom. The winter gales help in the process of mixing up the layers too. The sea, of course, is rarely so cold in winter or so warm in summer as is the atmosphere; as we have already noted, it gains and loses heat much more slowly. As spring passes into summer the air warms up and the radiant heat of the sun gets stronger, so we find the upper layers of the sea becoming warmer too; as they heat up they become increasingly lighter than the layers below and thus tend more and more to remain separated on the top because less and less are they likely to be mixed with the heavier waters beneath. This division between the upper and lower waters is called a discontinuity layer (or thermocline in still more technical language) and is usually set up at a depth of round about 15 metres. Let us take an actual example from the summer temperatures in the English Channel in July as found by the hydrologists of the Plymouth Laboratory. At depths from just below the surface down to 15 metres the temperature only varied from 16.5° to 15.82°C; but at 17½ metres it had dropped to 12.09°C and then, as it was sampled deeper and deeper, it remained practically constant to read 12.03°C at 60 metres. The upper layer was effectively cut off from the lower by this sudden drop in temperature of nearly 4°. A strong summer gale may destroy this discontinuity layer, but if it is not too late in the season it will soon form again. It is in the autumn that the air cools again and so the surface water loses heat; also the equinoctial gales stir up the sea and the more uniform temperatures of winter again become established from top to bottom. It will be noted that this warm summer upper layer corresponds very closely to the region (sometimes called the photic zone) in which the little plants get sufficient light to carry out effective photosynthesis. Two bits of the puzzle seem as if they would fit together; we require, however, yet another piece to go with them before we can see the explanation of the seasonal changes in the plankton. This last link concerns certain salts in the sea, and to them we must now turn.
First we must consider the general saltness of the sea; this, of course, is mainly due to the abundant sodium chloride which accounts for almost 77.8% of the total salt content. However there are many other salt constituents, of which the next more important, in order of descending quantity, are magnesium chloride (10.9%, magnesium sulphate (4.7%), calcium sulphate (3.6%, potassium sulphate (2.5%), calcium carbonate (0.3%) and magnesium bromide (0.2%). These proportions are actually those in which these different salts would be recovered from the sea on evaporation; their molecules as dissolved in the sea, however, would largely—some nine out of ten—be split up into their respective parts or ions: sodium and chlorine or magnesium and sulphate ions as the case may be. It is better to think of the salt constituents of sea water, as they mostly are in the sea itself, in terms of separate ions. We can tabulate the percentage proportions as follows, based upon a mean of 77 samples collected from different localities by the Challenger Expedition:
In addition there are minor constituents, for example iron, strontium, silicates, phosphates and nitrates, which constitute together only 0.06%. The degree of saltness of the sea, or its salinity, is usually expressed in terms of the total weight of salts in grams per thousand (°/oo) grams of sea-water; it varies in the open ocean from 34°/00 in polar waters, where it is low on account of additions of fresh-water from melting ice, to 37°/00 near the equator where it is high because of excessive evaporation of water. The North Atlantic surface water as it flows round our islands has a salinity of about 35°/00, but in the southern North Sea it is diluted to some 34.5°/00 by water-drainage from the land.
Now the important salts for our little plants are those which have only been mentioned among the minor constituents: they are the phosphates and the nitrates. Because they are present in such small quantities, it was a long time before accurate methods for their estimation could be devised; these were developed largely through the work of Drs. Atkins and Harvey at the Plymouth Laboratory just after the first world war. It had been realised that the plants of the sea must be limited, as are the plants of the land, according to Liebeg’s Minimum Law; i.e. so long as any really essential nutritive substance occurs in minimum quantities, plant production will be proportionate to the available quantities of it, even though there is a super-abundance of all other essentials. This seemed obvious enough but could not be proved until we had these more refined methods. It now became possible to measure the amounts of phosphates and nitrates taken up from the water by the little plants; it was shown that in our waters these salts could and did in fact limit their growth. The reproductive rate of these little plants grown in culture solutions was seen to fall off as the phosphates and nitrates were depleted and finally growth would stop altogether when they were entirely used up.
It is now possible to explain the seasonal cycle of events. In the winter, as we have seen, the waters from top to bottom are well mixed and their temperature is almost uniform. As the length of the days and the intensity of the light increases there comes a point at which the little plants can begin to multiply and they find a comparatively rich supply of phosphates available—about 40 milligrams per cubic metre of water. We have seen how rapidly they undergo fission when once they start. They are multiplying only in the upper, well illuminated zone; in the early spring these upper waters are being well mixed up with the lower layers by the equinoctial gales and there is a general reduction of the free phosphates as they pass into the plants. Presently, however, there develops an upper warm layer which becomes more pronounced as spring advances into summer; this is also the photic zone, in which alone the plants can multiply. The phosphates and nitrates are now being used up by the plants in this upper zone and are not being replaced by any mixing with the lower waters because of the difference in density between them. In fact the phosphates and nitrates are continually passing from the upper to the lower layers. The plants, which have taken up the salts, may either eventually form resting spores and sink, or may just die and sink, or more likely be eaten by the animal members of the plankton; these may themselves just die and sink or in turn be eaten by other larger animals and so on. The nutritive salts which were once present in the upper zones are now by late summer reduced to a minimum; they are carried in the falling bodies to the bottom or still locked up in animal life. Actually a good deal may be excreted back into the water by the animals,
(#ulink_01fdcb72-0ed5-537d-abcc-c78b4d4e5927) but as most of the animals only make comparatively short visits from the lower into the upper layers to feed on the plants at night, most of the excreted phosphates and nitrates will pass into the lower waters. Thus we see that so long as the discontinuity layer lasts, the plants, such as have not been eaten by the animals, are cut off from the richer phosphates and nitrates below. That is why their numbers decline so markedly as the summer advances and why they cannot reproduce at a rate sufficient to counterbalance the inroads made upon their population by the grazing animals. Down below, the supplies of phosphates and nitrates are being to some extent built up again by their return from dead animals broken down by bacterial action. Now, as the summer wanes, the upper layers are cooled again and the autumn equinoctial gales assist in a general mixing; the water richer in phosphates and nitrates is brought up from below towards the surface where once again we have a fertile layer while the sunlight is still strong enough to encourage photosynthesis.
Here at last we have the explanation of that autumnal outburst of phytoplankton which had for so long been such a puzzle. The time of its appearance and the quantity produced vary markedly in different years; it is usually not very long-lived and eventually the population of plants dwindles to a winter minimum as the light gets too weak to allow much active reproduction. The winter gales stir up the water and the nutrient salts are once again more or less evenly spread through the different layers of water. The temperature, too, is more or less uniform; the cycle is complete.
This brief account of the events throughout the year has dealt with the phytoplankton as a whole. If it suggests, as well it might, that all the different kinds of little plankton plants are increasing and declining together, as the seasons come and go, it would be giving a very false picture. There is in truth a succession of different forms which wax and wane in turn within this larger framework. As the summer advances and the quantity of the phytoplankton is declining, the dinoflagellates come to occupy a much more prominent part in the community; in late August species of Ceratium and Peridinium may be much more evident in the fine net samples than the diatoms. At the second autumn outburst the diatoms will swing back into prominence again. Then within these spring and autumn periods of production there is usually a fairly definite order of appearance of different species of diatoms as the weeks go by; not that one kind disappears entirely of course, but after a period of abundance the reproduction falls to a low ebb and the stock is maintained by only a few individuals or by the resting spores already referred to. The intensive work, already referred to (see here (#ulink_c8c1a1a8-3a7c-5c0f-9c97-cff4a75db940)), carried on week by week for fourteen years at Port Erin in the Isle of Man, has furnished us with a mine of information about these detailed seasonal changes at one place; and now the monthly plankton recorder surveys which will be described in the last chapter (see here (#litres_trial_promo)) are giving us similar information for a very wide area.
What makes one species give place to another? Why for example should Chaetoceros decipiens give way to Ch. debilis and socialis as the season advances or Rhizosolenia semispina be replaced by Rh. shrubsolei which in turn may leave the stage to Rh. stolterfothii? Whilst the grazing of the little plankton animals coupled with the reduction of phosphates and nitrates in the upper layers is bringing about the general decline in the planktonic vegetation it can hardly be controlling the rise and fall of the different species. Johnstone, Scott and Chadwick (1924) in discussing this seasonal sequence of species which they found in their long series of tow-nettings at Port Erin, made an important suggestion as to its cause.
“It is known that some bacteria are incapable of producing their typical effects (say in fixing elementary nitrogen from its solution in sea water) if they are present in pure culture. In order to function effectively they must be associated with some other organism which, by itself, cannot produce the effect in question. Probably such symbiotic relationships may exist on the great scale in the sea. The work of Allen and Nelson (1910) on the artificial culture of diatoms suggests this. In mixed cultures there is always a certain succession of species, one attaining its maximum when another has ceased actively to reproduce. The succession of diatom species during the period of the spring growth suggests that something of the same kind occurs in the sea.”
A similar effect has, of course, now been demonstrated by Sir Alexander Fleming’s great discovery that moulds such as Penicillium produce substances which inhibit the growth of bacteria. In my hypothesis of animal exclusion (in Hardy and Gunther, 1935), which will be referred to again in a later chapter (see here (#litres_trial_promo)), I have suggested that dense concentrations of planktonic plants may produce an effect in the water which is uncongenial to animal life and so account for the fact that animals are usually scarce in regions of great phytoplankton abundance. Dr. C. E. Lucas, my former pupil and colleague, now Director of the Scottish Fishery Laboratory at Aberdeen, has developed much further the idea of chemical interaction between organisms and stressed the possible importance of various substances given out into the water by different plants and animals as a result of their internal activities. Just as cells inside the body of an animal produce those various substances called hormones (or endocrines) which circulate in the blood stream to have profound effects on other parts of the body, so also may substances (ectocrines) be liberated from the body to have their effects on other organisms in an aquatic environment. The changed conditions set up in the water by one species may perhaps become both injurious to itself and at the same time more suitable to another kind which will follow it. Thus, among other interesting ideas, he gives strong support to this idea of seasonal succession: a chain of action, a conditioning and reconditioning of the water, as the year advances (Lucas, 1938, 1947 and 1956a).
Among the animals of the plankton there are also successive changes; particularly noticeable are the various broods of different species which follow one another, giving us in one month mainly adults and in another the young developing stages. The seasons are marked too by the throwing up into the plankton of the young larval stages of various bottom-living invertebrates, and also by the eggs and fry of different species of fish, all of which have their own distinct breeding times.
To return to more general considerations, it is interesting to compare the conditions as found in our waters at mid-summer with those in the surface waters of the tropics; we have the same heating up of the surface to form a discontinuity layer, but there it tends to be a permanent feature. In the open ocean in the tropics the phosphates and nitrates in the upper layers are thus reduced to a minimum all the year round and as a consequence the plankton of those regions is extraordinarily sparse compared with the more temperate or polar seas. This relative poverty of the tropical seas compared to our own was one of the surprising discoveries made by Victor Hensen’s German Plankton Expedition in 1889 and was at first disbelieved by many who thought, on false a priori grounds, that the warmer tropical seas must be richer in life than our own or the cold polar regions. A tow-netting in the tropical ocean may yield many more species than one in our own waters, but the total quantity of life is very much less; when we glance at a tropical sample at first sight nearly every specimen seems a different kind, whereas in one from our own waters there will be thousands of representatives of the same species. Plankton at certain places in the tropics, however, can be remarkably rich; this happens when deeper water with a supply of nutritive salts comes welling up into the sun-lit zones as against the coast or where a submarine bank comes near the surface and gives rise to disturbed water conditions.
The upwelling of water rich in phosphates and nitrates may well be very important in producing a more prolific phytoplankton in our own waters. There can be no doubt that the abundance of fish-food on the sea-bed which makes the Dogger Bank so renowned as a fishing ground, is due to the heavy rain of plankton showered upon it from above; there can also be little doubt that this rich phytoplankton in the surface layers is in turn produced by the upwelling of the richer phosphates and nitrates from below as the Atlantic inflow into the North Sea meets this large submarine bank set across its path (Graham, 1938).
A prolonged off-shore wind may have the effect of producing a heavy crop of plankton near the coast: it pushes the surface water away from the land so that its place has to be taken by water from below which wells up near the coast and thus again brings the desired nutritive salts up into the sunlit upper layers.
There are indications that at times other minor constituents of sea-water may have an effect upon phytoplankton. There is some experimental evidence that organic salts of iron and manganese will stimulate phytoplankton production; and it is suggested that such salts carried out by the drainage from the land may lead to an earlier outburst of reproductive activity among planktonic diatoms in coastal waters than among those further out to sea. Much information about these minor constituents will be found summarised by Dr. H. W. Harvey (1942, 1955) who has himself done so much of the experimental work.
For a long time there has been evidence that suggests that there is in the sea some trace substance at present unknown which is necessary before life can exist—some substance rather like the vitamins in our diet. Artificial sea-water has been made up to contain precisely the same proportions of chemicals that are known to be present in natural sea-water by the most exact analysis; yet planktonic plants will not grow in it unless about 1% of natural sea-water has been added to it (Allen, 1914). Quite recently the vitamin B
(Cobalamin) has been shown to be necessary for the growth of several marine flagellates and a diatom (Droop, 1954 and ’55) and its presence in natural sea water has now been demonstrated.
In addition to ‘trace substances’ which affect plant production, it now appears that there are some which are essential for the healthy development of delicate young animals. D. P. Wilson (1951) has recently shown a remarkable difference in this respect between the two types of water which we discussed in Chapter. 2 (#u3397ab9e-de11-5382-badf-93137316dee4): the more oceanic water characterised by one species of arrow-worm Sagitta elegans and the coastal water by another species S. setosa. It will be remembered that in the 1920’s there was usually elegans water off Plymouth; at that time Dr. Wilson had no difficulty in rearing the planktonic young of some of the bottom-living worms he was interested in. In the 1930’s, however, when the setosa water was over the area, he experienced much frustration in doing so. Thinking that there might be some subtle difference between the two waters, he made experiments to test his suspicions. He took the fertilised eggs of two different kinds of worm and of a sea urchin, and then divided each lot into a number of smaller batches; some he put into elegans water collected out in the Celtic Sea to the west of the Channel and the others into setosa water taken off Plymouth near the Eddystone. In the former water most of the larvae developed well, but in the latter they were abnormal or in poor health. “The experiments indicated,” writes Wilson, “that the Channel water lacked some unknown constituent, essential for the healthy development of these species, present in the Celtic Sea.”
Not infrequently a particularly rich outburst of phytoplankton is reported at a place where the waters of two current systems meet and mix. I have seen it particularly in the region of South Georgia in the sub-Antarctic where waters from the Weddell Sea and the Belling-hausen Sea meet in eddies on each side of the island. It is said to be a feature too of the boundary separating the North Atlantic current from the arctic water and it is not uncommon generally where oceanic and coastal waters meet and mix. Perhaps, on account of different plankton communities, each water has become deficient in some different but vitally important minor constituent; then on their coming together each will fertilize the other with the missing ingredients and so release an outburst of reproductive activity.
A good deal of interest was aroused in experiments performed during the war by the late Dr. Fabius Gross and his co-workers (1944) to see to what extent the growth offish could be accelerated by increasing the quantity of plankton in an enclosed sea loch by the addition of nutritive salts. The plankton was certainly enriched and an increased growth of the fish was recorded. It was in fact doing in a confined part of the sea what had already been successfully done in fresh-water fish-ponds; it is indeed a practice dating from ancient Chinese days. It has been suggested that it might be possible to add fertilizer to parts of the more open sea to increase the plankton in a limited area to provide a better chance of survival for the hosts of young fish that are expected to be developing there. But the open sea is a very big place and to do anything at all effective would need the provision of fertilizers on a scale perhaps too vast to be contemplated as a feasible proposition. Yet it seems that man does unwittingly influence the production of phytoplankton in the sea and consequently the yield of fish. In the southern North Sea opposite the opening of the Thames estuary there is frequently developed an area of a particularly rich growth of phytoplankton and here Mr. Michael Graham (1938) has shown an abundant source of phosphates and nitrates derived from the sewage of London. Dr. K. Kalle of the Oceanographic Institute at Hamburg has recently written a paper on the influence of this drainage from the Thames upon the fish population of the southern North Sea and this has been conveniently summarised in English by Dr. J. N. Carruthers (1954). He points out that the water from the continental rivers is carried quickly to the north-east by the current from up the channel; whereas that from the Thames is held up, wedged between two streams of oceanic water of higher salt content: i.e. that just mentioned and the Atlantic influx from the north. He estimates that 2,900 tons of phosphorus a year are carried from the rivers and when spread through the 171 cubic miles of English coastal waters south of the Humber amounts to an increase of 4 milligrams (0.004 grams) of phosphorus per cubic metre. Dr. Kalle then shows that the catch of fish in this region is per unit area ‘about double the corresponding catch made in the rest of the North Sea, in the English Channel and in the Kattegat/Skagerak region … and is about 25 times the catch reckoned for the Baltic Sea as a whole.’ He holds that two-thirds of this higher average catch may be attributed to the rich supply of nutrients from the population of our metropolis.
For a comprehensive treatment of the physics and chemistry of the sea in relation to plankton production the important books by Dr. H. W. Harvey (1945 and 1955) should be studied.
(#ulink_9b1c9b25-ca70-5b21-b304-2f2392cfbb39) When taking samples from a series of levels in very deep water several of these reversing bottles are generally used together, one above the other, on one wire at intervals of perhaps a hundred metres or more; as the messenger weight hits the trigger to reverse the first bottle, it also releases from below it another messenger which now slides down the wire to operate the second bottle and this again liberates a third messenger and so on to the bottom.
(#ulink_bc69a687-9dee-5558-8517-219d990aa9f2) See Gardiner (1937).
CHAPTER 5 (#ulink_6520a5dc-2d55-5488-9666-9b2074100333) INTRODUCING THE ZOOPLANKTON
THE ANIMALS of the plankton are by definition those which are passively carried along drifting with the moving waters; those other inhabitants of the open sea which are powerful enough to swim in any direction—the fish, whales, porpoises and the squids or cuttlefish—are in contrast referred to as the nekton (see here (#ulink_90dbbd09-52bf-5298-a614-64d2ae53d4e0)). The vertebrates therefore, except for certain primitive relations, will only be represented in the plankton by the floating eggs offish and the young fish themselves up to the time when they become strong enough to migrate at will instead of being just helplessly transported.
In spite of this limitation, and the absence of insects, I believe it is no exaggeration to say that in the plankton we may find an assemblage of animals more diverse and more comprehensive than is to be seen in any other realm of life. Every major phylum of the animal kingdom is represented, if not as adults, then as larval stages with the partial exception of the sponges; the sponges do indeed send up free-swimming larvae but they are in the plankton for so short a time that they can only be claimed as very temporary components of it. In no other field can a naturalist get so wide a zoological education and in few others will he find a more fascinating array of adaptational devices.
It is this great variety of forms, and the unexpected finds which are always turning up, which make hunting in the plankton such an exciting occupation. Except for the jelly-fish and some of the larger crustacea, it is of course hunting with a lens. Nearly every member of the zoo-plankton can be seen with a ×6 hand-lens or a simple dissecting microscope, and the most effective searching can be done with these. Before transferring any specimen to a slide for examination under the more powerful compound microscope, it is well to watch it for a time swimming in its own characteristic way in a small glass dish under the simple magnifier. To anyone who has never seen this life before it is difficult to convey in words a picture of the delights in store for him. I am indeed lucky to have the privilege of having my account illustrated and enriched by the beautiful photographs of living plankton animals taken through the microscope by my friend Dr. Douglas Wilson of the Plymouth Laboratory; they are quite unique and many of them have been taken by that remarkable new device, the electronic flash, which has for the first time made the photomicrography of such small and rapidly moving creatures possible. The naturalist will soon forget the absence of the insects in the wealth of variously shaped and often beautifully coloured crustaceans which are to be seen swimming rapidly in all directions. Tiny pulsating medusae—miniature jellyfish—swim into view; and here and there can be seen the transparent arrow-worms Sagitta which remain poised motionless for a time and then dart forward at lightning speed to capture some small crustacean. Then there may be delicate comb-jellies propelling themselves by rows of beating iridiscent comb-like plates and trailing long tentacles behind them. These comb-jellies and the arrow-worms belong to two phyla—i.e. major groups of the animal kingdom—which are found nowhere else but in the marine plankton. There are many different kinds of Protozoa, among which one order (the Radiolaria) is also entirely planktonic. The segmented worms may be represented by beautiful pelagic polychaetes and the molluscs by the so-called sea-butterflies (pteropods) which are really small snail-like animals with the foot drawn out into wing-like extensions to assist in their swimming and support. Most of these animals are permanent members of the plankton, spending all the stages of their life-histories drifting in the open sea; in addition there are, however, a vast number of the young, or larvae, of the bottom-living invertebrates which ascend to live for a time in the plankton and so distribute the species far and wide. These temporary members present us with some of the most striking adaptations to this floating life. Some of them are nearly always to be found in a tow-net sample from our surrounding seas which have such a rich fauna on their floor. Group by group—flatworms, segmented worms, different kinds of polyzoa, starfish, sea-urchins and, of course, the bottom-living crustacea—each has its own characteristic way of solving the problems of pelagic life. The plankton indeed presents a paradise for the student of invertebrate development; we shall devote a special chapter (Chapter 10 (#litres_trial_promo)) to a consideration of these larval forms.
Hitherto only a small minority of amateur naturalists have shared the delights of exploring the living plankton. Preserved samples, such as are often obtainable from marine laboratories for examination, are certainly full of interest; they can, however, never give the observer the same satisfaction as seeing this teeming world all alive. The professional marine biologist, engaged in investigating the relationship between plankton distribution and the fisheries, finds it very tantalizing to be able only very rarely to find time to stop and look at his captures before he must kill them; he travels to and fro across the sea taking as many samples at intervals as he can, in order to get the most comprehensive picture of conditions in the time available. Usually he only just has time to deal with the concentration, labelling and preservation of one set of collections before the ship arrives at the next position where another set must be taken; for the sake of understanding the fisheries he must always hurry on. In the past the amateur has often had an even more disappointing experience: having obtained a tow-net and hired a boat to take him out in the bay, he has returned home only to find that the wonderful sample of plankton he collected is now just a mass of dead or dying creatures crowded together at the bottom of the jar. Two modern inventions have altered all this: the Thermos flask and the refrigerator. If you have a Thermos flask, or preferably two, you can go to the sea, travel back by train for several hours and still have your plankton alive; if you have a refrigerator, or know a kind neighbour who will allow you to keep one or two 4 lb or 7 lb preserving jars in his, then you can keep your animals healthy for several days to be studied at your leisure.
I believe there are a great many people—and not only those who would call themselves naturalists—who would like to see something of this strange planktonic world, or show it to their children, if only they knew how. Anyone who goes to the sea can catch plankton quite simply. Those who can take a yachting cruise are particularly fortunate; they can study the changes in the plankton as they move from one area to another, can see the difference between the animals at the surface at night and in the daytime, and can try and find out just what organisms are making the flashing lights around their vessel in the darkness. Those, however, who can only take out a rowing boat may make very good collections, especially if there is water from the open ocean bathing their coast. If there is a pier sticking out into the sea and sufficient tidal current, as there usually is at some time of the day, quite good samples may be obtained by streaming out a net on a line and allowing it to fish for a quarter of an hour or so. Some may even think this preferable to a boat if the sea is a bit choppy! If you can only collect from a pier, or from a confined area in a rowing boat, you need not be too envious of your friends in the yacht, for fortunately the water is always on the move; a sample taken at the pier today may be very different from one taken only a few days ago and quite different again from one you may get next week. I have taken very good samples from some of the many piers built out to receive the steamers plying in the Firth of Clyde area.
To help those who do not know how to proceed I will give a few instructions. It is a good thing to have at least two Thermos flasks, so that you can keep at least two different plankton samples separate from one another. If you can manage it, it will be an advantage to start out with one of your flasks filled with sea-water that has stood in a jar in the refrigerator over night. Half of this you can pour into the other thermos just before you add the plankton sample collected. Thus in each flask the animals will be added to sea-water that has been chilled; it will keep them cool, inactive and in good condition whilst they are brought home. Details of how to make and use a tow-net have already been given in Chapter 3 (#uf28d2542-dfd2-59a4-8652-470397ba0dae). The net of very fine gauze suitable for collecting the small plants will also at the same time catch the very small animals, particularly the protozoa and small larval forms. For the capture of most of the zooplankton a coarser net having some 60 meshes to the inch is the most useful. If more of some of the larger animals are required, for example the larger crustacea and medusae, a still wider mesh net, say 25 meshes to the inch, should be used; this will filter much more water but let nearly all the smaller animals escape. The three nets of 200, 60 and 25 meshes to the inch will provide a very good equipment. Remember, as stressed in Chapter 3 (#uf28d2542-dfd2-59a4-8652-470397ba0dae), to tow slowly, at a speed of not more than 1½ knots. It is best to tow only for short periods—not more than five minutes at a time—which can be repeated if too small a sample has been collected. If the plankton is very abundant a longer haul will give you much too much so that all the little animals will be far too crowded together to live healthily for more than a very short time. If you have too thick a sample, pour a lot of it away and only take home in your flask a small part of it, diluted as much as possible with more sea-water. It seems hard to pour most of it back, but you will be sure to have sufficient of the commoner kinds and a few kept in good shape will be better than a great many in poor condition.
I must now give some idea of the actual numbers of animals you may expect to get. Here (#litres_trial_promo) I gave the figures for the diatoms and dinoflagellates taken in two 14-inch diameter tow-nets hauled for half a mile across the bay at Port Erin in the Isle of Man; they were averages for several hauls a week during the month of April over a period of fourteen years. For comparison I now give in the accompanying table the corresponding figures from the same source (Johnston, Scott and Chadwick, 1924) for the more important elements of the zooplankton in the same series of hauls.
The corresponding average totals for the months of June, August and October were 39,105, 38,812 and 35,631 respectively. Since it was calculated that approximately 8 cubic metres of water were filtered by the nets during towing, this gives an average of about 4,500 animals per cubic metre or some 120 per cubic foot of sea-water during the summer months. It must be remembered that these figures are averages and that individual samples may vary enormously from week to week. For comparison it may be interesting to give the average figures for the total plants of the plankton—the diatoms and dinoflagellates—recorded from the same series of net hauls for the four months April, June, August and October; they are in round figures 5,815,000, 6,674,000, 107,000 and 485,000 respectively. It must be remembered, however, that there will have been much larger numbers of the still smaller plants, the tiny flagellates referred to here (#ulink_21cacdb4-e2f9-576c-86e5-ce66f10ad48e), which will have passed through the meshes of the net and so not been recorded. To give the number per cubic metre we must again divide by 8.
If you have time, and the sea is calm enough, you should pour your plankton haul into a dish and examine it with a pocket lens as soon as it comes up; then with a wide-mouthed pipette you can pick out from it into another jar some of the rarer animals that you particularly want to study. After that you can more light-heartedly pour away most of the sample before putting the remainder, together with the rarities you have picked out, into your Thermos for transport home. The most useful dish from which to pick out specimens is one of the large oblong photographic dishes made of white porcelain and used for washing whole-plate negatives; half of the bottom of this can be covered with black paper so that you have a contrast of backgrounds to enable you to see both the darker and lighter forms more easily. All the jars, dishes and pipettes you use for living plankton must be kept thoroughly clean and never be used for samples that have been preserved with formalin or other chemical fixatives. These small animals are delicate in constitution as well as in form.
The majority of plankton animals tend to come up towards the surface at night and sink down into the deeper layers during the day (Chapter 11 (#litres_trial_promo)). Very rich samples of plankton may be collected by simply towing the net just below the surface at night; in the daytime, however, if you are over deep water you may have to send your net down to 15 to 20 fathoms to get a good haul. To reach this depth you will require a good length of line—50 to 60 fathoms—and you will also require a much heavier weight, say a 20 lb lead, to take your net and all this line down. Care must be taken, of course, to know just how deep the sea is at the point where you are working so as not to run the risk of trawling the bottom with your net and either bursting it by filling it with mud or tearing it to ribbons by dragging it over a rough bottom. If you have not a chart you can consult, you should take a sounding with your lead and line before starting.
It is often very interesting to take a series of samples from different depths at the same place as near together in time as possible to enable you to study the depth distribution of the various animals; if you repeat the series again at night you may be very surprised at the different results you will get. As you let the net run out on its line to a deeper level it will fish very little on its way down, for it is moving backwards with the water as it runs out and sinks; when you haul it up, however, at the end of a tow, it will of course fish all the way up. This difficulty is got over in modern oceanographic practice by having in front of the net a special closing mechanism which is operated by a brass messenger weight sent sliding down the cable; this releases the bridles when a trigger is struck and the net falls back to be closed and held by a throttling noose which passes round it behind the mouth. The net is thus hauled up to the surface closed like a sponge-bag with the strings drawn tight and you know that all the animals in it must have been caught at the actual level at which it was fishing. A simple example of this arrangement is shown opposite in Fig. 18 (#litres_trial_promo). These devices, however, are perhaps rather elaborate to be practised by the amateur, especially as a smooth steel cable is required for the messenger and this means the use of a winch; they will not be further described but full information about them will be found in the descriptions of the equipment used on the Discovery Expeditions (Kemp and Hardy, 1929). To minimise the effect of catching whilst hauling up an open net, it is well to make rather longer hauls with it down; the time taken in coming up will then only be a small fraction of that during which the net was fishing at its proper level.
If you are going to take a number of such hauls for study you will soon accumulate far more material than you can hope to keep alive successfully; in this case it will be best to keep only a small part of one or two samples fresh and preserve the rest for study dead. The living plankton will give you the greatest pleasure in studying the swimming movements and behaviour of the animals; the dead samples may nevertheless give you interesting information about the depth distribution of the same animals, which you could not otherwise obtain. The best general preservative for plankton, and the easiest to use, is formalin, i.e. a 40% solution of formaldehyde in water; this, which can easily be obtained at any chemist, can be added to the sample in quite small quantities to give a mixture (about 5% formaldehyde) strong enough to keep it indefinitely in good condition. Remember always to reserve separate jars for preserved samples—never mix them with those used for fresh; a good plan is to stick a red label on them for danger! The dead formalined samples can of course be concentrated into a smaller space; 1lb honey jars with screw-on tops are convenient for their storage. If you are going to keep the samples for any length of time it is well to use what is called neutral formalin, i.e. that to which just sufficient borax—from 5 to 10 grams per litre—has been added to neutralise its acidity; ordinary formalin nearly always contains formic acid which if not so neutralised will very soon dissolve away the calcareous shells and skeletons of many of the animals.
FIG. 18
Sketches of a simple release mechanism for closing the mouth of a tow-net before hauling it to the surface. A, the rig of the net when towed; B, enlarged view of release gear about to be struck by messenger weight; C, the towing bridles released and the net closed by throttling rope.
What has just been said will have been sufficient to have corrected that very common misconception that the plankton exists almost entirely near the surface of the sea. Some people seem to have thought of it as existing as a kind of scum on the very surface itself; this is no doubt due to a misunderstanding of the expression often used that the plankton is the ‘floating life’ of the sea. The plants, as we have already seen, do in fact only flourish for a little way below the surface; but animal members may be found at all depths. Later on—in Chapter 12 (#litres_trial_promo)—we shall describe the plankton and nekton that is to be found at various levels in the ocean between the surface and the bottom, thousands of fathoms deep beyond the continental shelf. There is another erroneous impression about the plankton that is frequently held: the idea that it is more or less evenly distributed over quite wide stretches of the sea; it is often thought that if we used a tow-net in one place and another two or three miles away on the same day, the two samples would be almost exactly alike. This indeed may occur, but it is by no means always so.
A great many surveys have been made in the past, often in relation to some fishery problem, attempting to give some idea of the varying quantities of the major plankton organisms over a particular area. I have already described how a research ship will proceed in such a survey to traverse the area, stopping or slowing down to take tow-net samples at regular intervals. If the area to be covered is a big one, the stations—as the different points of observation are termed—cannot be very close together or the survey would take much too long; they are frequently spaced twenty miles apart. It has usually been assumed that a sample at one point, will, within a reasonable range of error, give a fair representation of the plankton in the area for ten miles around it; thus it has been felt that a series of such stations twenty miles apart will give an adequate quantitative survey. Some plankton organisms are much more patchy in their distribution than others; for some kinds such a method may give quite an adequate picture, but for others it may be hopelessly misleading. Very early in my career as a marine naturalist I had an experience which I will recall because it so well illustrates this very point; it was an episode which had a marked effect on much of my later work. In 1921, soon after leaving the University, I was appointed as Assistant Naturalist on the staff of the Fisheries Laboratory of the Ministry of Agriculture and Fisheries at Lowestoft and was delighted to be allowed to study the plankton in relation to herring. In March of the following year, through the illness of a senior, I found myself, at the last moment of sailing, as naturalist in charge of a cruise on that grand old research trawler the George Bligh.
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Alister Hardy
The New Naturalist editors believe this to be the greatest general work on the subject ever written.Professor Alistair Hardy is truly obsessed by animals of the sea – devotedly enthusiastic about the nature of their adaptations and life histories, brilliantly critical in the examination of their mysteries, acutely lucid (and at the same time highly artistic) in his descriptions of them in his arresting plates.To describe the relatively unknown and mysterious world of plankton is a task that the greatest of marine zoologists might boggle at. Yet the plankton is to the sea what vegetation is to the land. The study of plankton is a complex discipline which few amateur naturalists have had the privilege to enjoy. Never before has such a synthesis of knowledge been attempted in a community of animals so mysterious, yet so important. Professor Hardy has grasped this problem in a new and exciting way; and at least the common reader can discern the pattern of life that dominates two-thirds of the world’s surface.
Collins New Naturalist Library
34
The Open Sea – Its Natural History:
The World of Plankton
Alister C. Hardy
EDITORS: (#ulink_e6e54ff0-6414-5b73-9650-9f68a5f32df3)
JAMES FISHER, M.A.
JOHN GILMOUR, M.A.
JULIAN HUXLEY, M.A., D.SC, F.R.S.
L. DUDLEY STAMP, G.B.E., D.LITT., D.SC.
PHOTOGRAPHIC EDITOR:
ERIC HOSKING, F.R.P.S.
The aim of this series is to interest the general reader in the wild life of Britain by recapturing the inquiring spirit of the old naturalists. The Editors believe that the natural pride of the British public in the native fauna and flora, to which must be added concern for their conservation, is best fostered by maintaining a high standard of accuracy combined with clarity of exposition in presenting the results of modern scientific research. The plants and animals are described in relation to their homes and habitats and are portrayed in the full beauty of their natural colours.
Table of Contents
Cover (#u82c38694-9eb0-577d-9fa4-3ab644beba53)
Title Page (#ua3d0bf02-d61d-5172-9d69-2d2f0063da7b)
Editors (#u33b5931d-34cc-591f-8e03-74f51ec6208b)
Editors’ Preface (#u15a5dc3c-e074-5bd3-b86f-385281f19702)
Author’s Preface (#ucb71242a-0b06-57a9-80b3-2ac33fa25869)
CHAPTER 1 (#ubcb89914-6a12-5f9a-961d-a32f7a64e31a)
INTRODUCTION
CHAPTER 2 (#u3397ab9e-de11-5382-badf-93137316dee4)
THE MOVEMENT OF THE WATERS
CHAPTER 3 (#uf28d2542-dfd2-59a4-8652-470397ba0dae)
PLANTS OF THE PLANKTON
CHAPTER 4 (#uf9e2ac0e-843a-5efb-963b-16fe015b8f29)
SEASONS IN THE SEA
CHAPTER 5 (#ub9d1ef9b-f5a9-51ce-96c7-37d3c4e2a8de)
INTRODUCING THE ZOOPLANKTON
CHAPTER 6 (#litres_trial_promo)
LITTLE JELLY-FISH AND LESSER FORMS OF LIFE
CHAPTER 7 (#litres_trial_promo)
SIPHONOPHORES AND THE LARGER JELLY-FISH
CHAPTER 8 (#litres_trial_promo)
MORE ANIMALS OF THE PLANKTON—BUT NOT THE CRUSTACEANS
CHAPTER 9 (#litres_trial_promo)
THE PLANKTONIC CRUSTACEA
CHAPTER 10 (#litres_trial_promo)
PELAGIC LARVAL FORMS
CHAPTER 11 (#litres_trial_promo)
THE PUZZLE OF VERTICAL MIGRATION
CHAPTER 12 (#litres_trial_promo)
LIFE IN THE DEPTHS
CHAPTER 13 (#litres_trial_promo)
PHOSPHORESCENCE AND PHOTOPHORES
CHAPTER 14 (#litres_trial_promo)
SQUIDS, CUTTLEFISH AND KIN
CHAPTER 15 (#litres_trial_promo)
PLANKTON AND THE FISHERIES
Glossary (#litres_trial_promo)
Bibliography (#litres_trial_promo)
Index (#litres_trial_promo)
Colour Plates (#litres_trial_promo)
Plates in Black and White (#litres_trial_promo)
Copyright (#litres_trial_promo)
About Publisher Page (#litres_trial_promo)
EDITORS’ PREFACE (#ulink_472ac1a3-e0a2-5cc4-97be-95d34706cd70)
PROFESSOR HARDY began his marine biologist’s life over a third of a century ago on his return from service in the first world war. After Oxford and a scholarship to the Stazione Zoologica at Naples, he soon became a member of the Fisheries Department in the Ministry of Agriculture and Fisheries; and, in the middle ‘twenties, served as Chief Zoologist to the R.R.S. Discovery expedition, to the Antarctic seas, making a special study of plankton. His subsequent professorships—first at University College (now the University of) Kingston-upon-Hull; next at Aberdeen; and since 1945 at the University of Oxford—have brought him the highest academic status and honours, but have not kept him away from his beloved sea. In the closing stages of the writing of this volume, as the editors well remember, he was correcting the typescript, and completing his unique and wonderful colour illustrations, on the deck of the latest Royal Research Ship, Discovery II, scanning the contents of each netting or dredging, sketching new or rare creatures of the sea before their colour faded, applying himself to his research with an enthusiasm excelling that of most naturalists of half his age.
If the editorial board were asked to select from Professor Hardy’s many scientific qualities that which has contributed most to the creation of this extraordinary book, they would perhaps settle for enthusiasm. Throughout The Open Sea it is quite apparent that he is devotedly obsessed by, and interested in, animals; he is eternally curious about the nature of their adaptations and lives, brilliantly critical in the examination of their mysteries, acutely lucid and at the same time highly artistic in his depiction of them in his remarkable plates. It was a welcome burst of enthusiasm that caused Professor Hardy to write so much and so well of the life of the sea that he has written us two books instead of one. It is the first of these, concerned with the general natural history of the open sea and the world of its plankton, that we here welcome. The second part of The Open Sea concerns the open sea’s fish and fisheries, and will be published some time in 1957 or early 1958; like the present book, it will be illustrated by Professor Hardy’s own colour paintings, which represent what no colour-camera has yet been able to catch, and by black and white photographs by that most distinguished marine biologist and skilful photographer, Douglas P. Wilson.
To most readers the subject of this first of Professor Hardy’s two contributions to our series—the world of Plankton—will be relatively unknown and mysterious; but here the enlightened amateur naturalist is shown how, with modest equipment he may investigate it himself. The world of plankton is a world of complex anatomy, much of which can be understood only with the lens of the microscope. The life-histories of the animals are also complicated; some of them are extraordinary. To describe the plankton of our seas, and to set it in its pattern of community, climate, sea-scene and season is a major task. Professor Hardy has brought vast knowledge and experience and scholarship to a synthesis never before attempted.
THE EDITORS
AUTHOR’S PREFACE (#ulink_e7956feb-8941-5258-be07-4c5f44a5e981)
ORIGINALLY it had been intended that the whole natural history of the sea, apart from that of the sea-shore and of the sea-birds already dealt with in the New Naturalist series, should be treated in one general volume. As the writing proceeded, however, it became clear that to do justice to the subject it would be impossible to include all its different elements within a single cover. There is the life of the plankton in almost endless variety; there are the many kinds of fish, both surface and bottom living; there are the hosts of different invertebrate creatures on the sea-floor; and there are those almost grotesque forms of pelagic life in the oceans depths. Then there are the squids and cuttlefish, and the porpoises, dolphins and great whales. In addition man’s fisheries now play such an important part in the ecology of our waters that they also must form a part of any general natural history of the sea.
Certainly there is too much material to go into one volume. There occurs, however, a fairly natural division between the teeming planktonic world and the other categories of life it supports: the fish, the whales and the animals of the sea-bed. This first book on the open sea deals mainly with the plankton; it aims at giving the general reader a non-technical account, save for the necessary scientific names, of its many remarkable animals and showing how, with only a little trouble, quite a lot of them may be seen and studied alive. Perhaps to some it may introduce a new world of life—a world so unusual that few of its inhabitants have homely English names at all. It is hoped, too, that it may be a guide to the plankton for university students who are beginning their studies in marine biology. The book also deals with the water-movements and the seasons in the sea; and it contains an account of the squids and cuttlefish, and of those queer creatures, including the deep-water (and often luminous) fish, swimming in the great depths only a little way beyond our western coasts. It will conclude by showing how a study of the plankton is helping us to have a better understanding of the lives of our commercially important fish. Later, and before very long, will come the sequel: a separate volume devoted mainly to fish and fisheries, but also including whales, turtles and other marine animals which are likewise, directly or ultimately, dependent on the plankton for food.
Before going any further I must thank the publishers and editors, not only for all the trouble they have taken over the production of this book, but also for the patience they have kindly shown over my delay in its completion. I accepted their invitation to write it in August 1943, some twelve years ago; it has, however, meant more than the writing. My excuse for its late arrival will be offered after I have made my main acknowledgment.
The value to the book of the remarkable collection of photographs by Dr. D. P. Wilson of the Plymouth Laboratory will be clear to all, but just how wonderful they are and consequently how lucky I am to have them as illustrations, may not at once be fully appreciated by those who are not yet familiar with the living plankton of the sea. Douglas Wilson has long been recognised as the leading photographer of marine life and his beautiful pictures in black-and-white and in colour which graced Professor C. M. Yonge’s The Sea Shore in this series of volumes will, I am sure, have been seen and admired by most of my readers. I, too, am showing some of his studies of the larger forms of life, such as those of cuttlefish or his unusual view of that strange jelly-fish, the Portuguese-man-of-war, taking a meal; it is, however, his photographs of the tiny plankton animals to which I particularly wish to draw attention here. Though they are taken through a microscope, they are photographs of creatures swimming naturally, very much alive and certainly kicking. Never before has such a series of photomicrographs of living members of the plankton been published; they are unique and will, I believe, be of immense value not only to marine naturalists but to all students of invertebrate zoology. They are the fortunate result of a remarkable combination; Dr. Wilson has brought his skill and artistry to work with that very modern invention the electronic flash. For the first time this device has made possible such instantaneous pictures at a very high magnification. It is not only that invention, however, which makes these pictures unique; while others will follow him, Dr. Wilson’s photographs will always have a quality of their own, because he is an artist as well as a scientist. He is not satisfied until he has produced a photograph that has an appeal on the score of composition as well as on that of scientific value. All his photographs except two (the stranded jelly-fish and squid) are of living animals. A few excellent black-and-white pictures by other photographers are included in some of the plates and these are acknowledged in the captions or the text.
It was my hope, and that of the editors, that in addition to his black-and-whites Dr. Wilson would have been able to contribute a series of colour photographs of the living plankton and especially of the richly pigmented animals from the ocean depths. At that time the electronic flash was only just being developed and he felt unable to attempt them. The movement of the ship at sea, he said, would prohibit the use of a long enough exposure to enable the deep-water animals to be photographed in colour by ordinary means; they quickly die and fade, and so must be taken as soon as they are brought to the surface. I had already had some experience in making water-colour drawings of living plankton animals on the old Discovery during the Antarctic expedition of 1925–27; the editors kindly allowed me to undertake a series of such studies to form the accompanying twenty-four colour plates. To obtain and make drawings of the full range of animals which I felt to be desirable, meant a considerable delay and this was added to an earlier postponement of my start on the book caused by my being appointed to the chair of Zoology at Oxford soon after I had accepted the invitation to write it. For several years the work of my new department and research to which I was already committed took all my attention.
All save seven of the 142 drawings in the plates were made from living examples or, in a few cases, from those taken freshly from the net when some deep-water fish and plankton animals were dead on reaching the surface. The seven exceptions, which are noted where they occur, were drawn from preserved specimens but with memories or colour-notes from having seen them alive; I should like to have drawn these too from life, but I could delay the book no longer. It may be of interest to record how the drawings were made. All the animals, except the larger squids and jelly-fish, were drawn either swimming in flat glass dishes placed on a background of millimetre squared paper where they were viewed with a simple dissecting lens, or on a slide under a compound microscope provided with a squared micrometer eyepiece; in either case the drawings were first made in outline on paper which had been ruled with faint pencil lines into squares which corresponded to those against which the specimen was viewed. In this way the shape and relative proportions of the parts could be drawn in pencil and checked and rechecked with the animal until it was quite certain that they were correct. The outline was then gone over with the finest brush to replace the pencil by a permanent and more expressive water-colour line; next all the pencil lines, including the background squares, were rubbed out and the full colouring of the drawing proceeded with. If rough weather at sea made such a course impossible, the living animal would be sketched in pencil, and painted, in perhaps one or two different positions, to give life-like attitudes and colouration without attempting to get the detailed proportions exactly right; it would then be preserved in formalin for accurate redrawing by the squared-background system when calmer conditions returned. The animals I have selected for illustration are mainly either those which are not included in the black-and-white photographs or those for which colour can add supplementary information. I have, for example, drawn some of the transparent but iridescent comb-jellies, but not the transparent and colourless arrow-worms or salps. I am most grateful to the Sun Engraving Company, who made the blocks for the colour plates, for the great care they have taken in making such excellent reproductions.
I must now make special acknowledgments in regard to these drawings. First I must record my thanks to Dr. N. A. Mackintosh, the Deputy Director in charge of the biological research of the National Institute of Oceanography, for kindly allowing me to accompany the R.R.S. Discovery II on two of her biological cruises in the Atlantic in the summers of 1952 and 1954. It is to the Discovery, with all her equipment of deep-water nets, powerful winches and laboratory accommodation, that 71 of these drawings are due, including all those representing the remarkable animals which live in the great depths over the edge of the continental shelf to the southwest of Britain. Without such facilities they could never have been made; actually three of them date back to earlier days when I had the honour of sailing south in the old Discovery in 1925. Next I must thank a number of kind helpers who have sent me living specimens of plankton in specially protected Thermos flasks from many parts of the coast: Mr. J. Bossanyi from Cullercoats, Dr. E. W. Knight-Jones from Bangor, Dr. Richard Pike from Millport, Professor J. E. G. Raymont from Southampton and Mr. R. S. Wimpenny from Lowestoft. Although I made many visits to different places to draw my specimens, there were still a number I could not get myself in the time available; these were supplied by these kind friends who were on the lookout for what I wanted at widely separated points. I am most grateful to Dr. Marie Lebour and to the Council of the Marine Biological Association of the United Kingdom for kind permission to reproduce some of her beautiful drawings of living plankton animals capturing their prey; these, which form my text-figures 26, 27, 35, 40 and 41, were originally published in her papers in the Association’s Journal in the years 1922 and ’23. Then I must thank Sir Gavin de Beer and members of his staff at the British Museum (Natural History), particularly Dr. W. J. Rees and Mr. N. B. Marshall, for kindly allowing me to make many of the black and white drawings in the text from specimens in the museum collections. I am similarly indebted to Dr. J. H. Fraser of the Marine (Fisheries) Laboratory at Aberdeen who has let me draw some of the beautiful plankton animals he has caught to the north and west of Scotland; and to Dr. Helene Bargmann and Mr. Peter David who have also kindly given me much help on looking out specimens from the Discovery collections for me.
With no less gratitude, I must make acknowledgments regarding the text. Apart from the more normal editorial comments and suggestions I particularly want to say how much I am indebted to my old friend—and once Oxford tutor—Dr. Julian Huxley, who read the whole book with the greatest care and made many valuable suggestions for its improvement. My typescript—how reminiscent of my undergraduate essays of 1919 and ’20!—was covered with his pencilled scribblings in the margin: “Surely you should refer to—, this might be made more emphatic” and the like; not all were adopted, but certainly a great many. The chapter on water movements was read by Dr. G. E. R. Deacon, and that on squids and cuttlefish by Dr. W.J. Rees; I am indeed grateful to them for a number of helpful suggestions they kindly made.
Finally I wish to draw the attention of those who are not scientists to a glossary at the end of the book giving a simple explanation of the few technical terms which have been unavoidably used; and for the zoologist I would point out that the authorities for the different specific names will be found quoted after these names in the index and not in the text where they are left out to avoid undue elaboration.
A.C.H.
CHAPTER 1 (#ulink_79bd5715-643a-5479-aa34-2f4332f09b89) INTRODUCTION
THERE is a very simple fact about the sea which makes its inhabitants seem even more remote from us than can entirely be accounted for by their being largely out of sight. To make my point allow me to imagine a world just a little different from our own.
Suppose for a moment that we live in a country which is bounded on one side by a permanent bank of fog. It is a grey-green vapour, denser even than that often known as a London particular, and it has a boundary as definite as the surface of a cloud so that it is like a curtain hanging from the sky to meet the ground; we cannot enter it without special aids except for a momentary plunge and as quickly out again for breath. We can see into it for only a very little way, but what we do see is all the more tantalizing because we know it must be just a glimpse—a tiny fraction—of all that lies beyond. We find it has life in it as abundant as that of our own country-side, but so different that it might be life from another world. No insects dwell beyond the barrier, but other jointed-legged creatures take their place. Unfamiliar floating forms, like living parachutes with trailing tentacles, show their beauty and all too quickly fade from view; then sometimes at night the darkness may be spangled with moving points of light—living sparks that dart and dance before our eyes. Occasionally gigantic monsters, equal in size to several elephants rolled into one, blunder through the curtain and lie dying on our land.
To make a reality of this little flight of fancy all we have to do is to swing this barrier through a right-angle so that it becomes the surface of the sea. How much more curious about its unfamiliar creatures many of us might be if the sea were in fact separated from us by a vertical screen—over the garden wall as it were—instead of lying beneath us under a watery floor. Who as a child has not envied the Israelites as they passed through the Red Sea as if marching through a continuous aquarium: “and the waters were a wall unto them on their right hand, and on their left.”? What might they not have seen? Because normally our line of vision stretches out across the sea to the skyline and carries our thoughts to other lands beyond, many of us tend to overlook this perhaps more wonderful realm beneath us, or we seem to think it must be too difficult of access ever to become a field for our exploration or delight.
The aim of this book is to give the general reader an account of the natural history of the open sea around our islands and at the same time show how he may, with only modest equipment, see something of this strange world for himself. The amateur naturalist afloat—whether on a yachting cruise, on a fishing vessel, or just out in a rowing boat—may see much if he has the right kind of quite simple gear and knows how to use it; he may perhaps also be lucky and make original observations which will be a contribution towards finding an answer to one of the many unsolved riddles of the sea. The book will also give a sketch of some of the factors upon which the success of our great sea fisheries depend. The lives of the different fish are like threads woven in a web of life—a network of inter-relationships between many various creatures large and small, as complex as any on the land. The story of fishery research, which belongs mainly to our subsequent volume, is so closely linked with this unseen web, that it is hoped an account of these less familiar animals may be as interesting to the fishermen as to the naturalist; indeed many fishermen are naturalists and have much of importance to tell the scientist.
As our title indicates, the book will deal with the open sea—the sea beyond the coastal waters. The life of the intertidal zone has already been beautifully treated in this series of volumes by my friend Maurice (C. M.) Yonge (1949). The sea-shore can be studied by direct observation as the tide recedes and has long been a happy hunting-ground for the naturalist; he can lift up the fronds of seaweed, turn over stones, probe into rock-pools and dig into the sand and mud. Our methods of studying the life of the open sea must be very different; it is far from ‘open’ to the investigator, being in fact a hidden world, but this makes its exploration all the more exciting. Deep-sea photographic and television cameras are important new developments which promise much for the future; they, however, as also submarine observation chambers like the bathysphere, must for some time to come be regarded as very costly and specialist equipment giving us here and there direct confirmation of what we usually have to find out by other means. The diving helmet and the aqualung may help us to see something of this enchanting world in shallow water, but for the discovery of what is happening over wide stretches of the underwaters of the open sea we must devise more indirect methods.
The fact that we can see only a very little way below the surface indicates a property of water, and particularly of the sea, which is of fundamental importance to the life it contains. Held up in a glass, water appears so very transparent that we are at first surprised to find how quickly light is absorbed in the sea itself and what a little distance its rays will penetrate. Measurements made in the English Channel off Plymouth show that at a depth of five metres (just over 16 feet) the intensity of light is less than half that just below the surface, while at 25 metres it is only an insignificant fraction, varying between 1½ and 3 per cent. This at once tells us that the green plants, which must have sunlight in order to live, will only be found in the upper layers of the water.
The one real difference, of course, between animals and plants is a matter of their mode of feeding. We know that an animal of any kind, whether mammal, fish, shrimp, or worm, must have what we call organic food: proteins, carbohydrates (sugars, starches and the like) and fats, which have been built up in the bodies of other animals or plants. One animal may feed upon another kind of animal which in turn may have lived upon other kinds, and perhaps these upon yet others, but always these food-chains, long or short, must begin with animals feeding upon plants. Only the green plants, with that remarkable substance chlorophyll acting as an agent, can build themselves up from the simple inorganic substances by their power of using the energy of sunlight (photosynthesis); they split up the molecules of carbon dioxide, liberate the oxygen, and combine the carbon with the oxygen and hydrogen of water to form simple carbohydrates, which are then elaborated into more complex compounds by being combined with various minerals in solution. On the land we are all familiar with this elementary fact of natural history; my reason for recalling it is to emphasise that it is of universal application. The plants are the producers and the animals the consumers as much in the sea as on the land. Indeed ‘all flesh is grass’.
Where then in the sea, we may ask ourselves, are all the plants upon which the hordes of animals must depend? They cannot grow in the darkness or dim light of the sea-floor, and the seaweeds, forming but a shallow fringe along the coasts, are of no real importance in the economy of the open sea. From the deck of a ship, or even from a rowing boat, we can see no plant-life floating near the surface; yet we know it must be there. Another little flight of imagination will, I think, help us to get some idea of the extent of this elusive vegetation.
Let us suppose for a moment that the herring is not a fish, but a land animal. We know that some three thousand million herring are landed every year at ports in the British Isles; these, together with all those landed in other countries, must be only a small fraction of their total number, for we also know that herring are the food of so many other abundant animals of the sea. For simplicity let us consider them to be feeding directly upon plants—and let us imagine them in their unnumbered millions sweeping across the continent. If we do this it needs no imagination to see that the countryside would be stripped of vegetation as if by locusts. Now let us think of the other fish in the sea besides the herring: the cod, haddock, plaice, skate and such that fill our trawlers (as distinct from the herring-drifters) to the extent of more than a million tons a year; then also think of the crowded invertebrate life of the sea-bottom. If all these animals were on the land as well, what an immense crop of plants it would take to keep them supplied with food! There are indeed such luxuriant pastures in the sea but they are not obvious because the individual plants composing them are so small as to be invisible to the unaided eye; we can only see them through a microscope. Their vast numbers make up for their small size.
As an introduction to all that follows let us consider the natural economy of the sea in its simplest terms. We have the sun shining down, its rays penetrating the upper layers of the water; we have the gases, oxygen and carbon dioxide, dissolved in it from the atmosphere; we have also the various mineral salts—notably phosphates and nitrates and iron compounds—continually being brought in by the erosion of the land, and there are minute traces of some essential vitamin-like substances. These are ideal conditions for plant growth. Just as these are spread through the water, so is the plant life itself scattered as a fine aquatic ‘dust’ of living microscopic specks in untold billions. In a shaft of sunlight slanting into a shaded room we have all watched the usually invisible motes floating in the air, floating because they are so small and light; these tiny plants remain suspended in the water in just the same way. Many of them are provided with fine projections like those of thistledown to assist in their suspension.
Feeding upon these tiny floating plants, and also like them scattered through the sea in teeming millions, are little animals. Crustacea, little shrimp-like creatures of many different kinds, predominate; mostly they range in size from a pin’s head to a grain of rice, but some are larger. There are hosts of other animals as well: small worm-like forms, miniature snails with flapping fins to keep them up, little jellyfish, and many other kinds which surprise us with their unexpected shapes and delicate beauty when first we see them through the microscope.
All these creatures, both animals and plants, which float and drift with the flow of tides and ocean currents are called by the general name of plankton. It is one of the most expressive technical terms used in science and is taken directly from the Greek πλavktov. It is often translated as if it meant just ‘wandering’, but really the Greek is more subtle than this and tells us in one word what we in English have to say in several; it has a distinctly passive sense meaning ‘that which is made to wander or drift’ i.e. drifting beyond its own control—unable to stop if it wanted to. It is most useful to have one word to distinguish all this passively drifting life from the creatures such as fish and whales which are strong enough to swim and migrate at will through the moving waters: these in contrast are spoken of as the nekton (Gk nektos, swimming). Actually when they are very young, the baby fish are strictly speaking part of the plankton too, for they are also carried along at the mercy of the currents until they are strong enough to swim against them. Photographs taken through a microscope of some typical planktonic plants and animals are shown in Plate I (#litres_trial_promo) and Plate II (#litres_trial_promo); they have been caught by drawing a net of fine silk gauze through the water. Their natural history will be dealt with in later chapters.
The simple sketch in Fig. 1 (#litres_trial_promo) shows this general economy of the sea in diagram form. A number of fish, including the herring, pilchard, sprat, mackerel and the huge basking shark, feed directly upon the little plankton animals; and so also, curiously enough, do the great whalebone whales, the largest animals that have ever lived. From this world of planktonic life, dead and dying remains are continually sinking towards the bottom and on the way may feed other plankton animals living in the deeper layers. For this reason the zoöplankton (animal plankton—Gk zoön, an animal) is not confined to the upper sunlit layers as is the phytoplankton (plant plankton—Gk phuton, a plant). On the sea-bed we find a profusion of animals equipped with all manner of devices for collecting this falling rain of food. Some, rooted to the bottom, spread out their branch-like arms in umbrella fashion and so look like plants; others, such as many shellfish, have remarkable sieving devices for trapping their finely scattered diet. Feeding upon these are hosts of voracious crawling animals. These and their prey together—worms, starfish, sea-urchins, crustaceans, molluscs and many other less familiar creatures—in turn form the food of the fish such as cod, haddock and plaice which roam the sea-floor in search of them. Finally comes man: catching the herring and mackerel with his fleets of drift-nets near the surface, hunting the great whales with explosive harpoons, and sweeping the sea-bed with his trawls for the bottom-living fish.
FIG. 1
A diagrammatic sketch illustrating the general economy of the sea.
We see how all-important the plankton is. All the life of the open sea depends for its basic supply of food upon the sunlit ‘pastures’ of floating microscopic plants.
Our knowledge of life in the sea has been built up step by step by many pioneer naturalists. Oceanography is still quite a young science; its beginnings were made only a little over a hundred years ago. It is worth while looking back.
The vast community of planktonic animals and plants was unsuspected till it was discovered by the use of a very simple device, the tow-net: a small conical bag of fine silk gauze or muslin, usually with a little collecting jar at its end, towed on a line behind a boat. In nearly all the text-books of oceanography it is stated that the tow-net was first used in 1844 by the German naturalist Johannes Müller, and I have myself been guilty of repeating this error. It is certain that Müller’s researches excited the scientific world and led many others to follow him; but our own great amateur naturalist J. Vaughan Thompson, when serving as an army surgeon in Ireland, was using a tow-net to collect plankton from the sea off Cork as early as 1828. It was there that he first described the zoëa, the young planktonic stage of the crab. A little later, 1833, he discovered the true nature of the barnacles and so solved an age-long puzzle. These enigmatic creatures, fixed to rocks or the bottoms of ships, had been thought to be aberrant molluscs. Thompson caught little undoubted crustaceans in his tow-net and found that they settled down to be transformed into barnacles. His classical discoveries were described in privately printed memoirs which he published in Cork; they are among the rarest items of biological literature. He showed that the plankton consisted not only of little creatures permanently afloat, but also of the young stages—larvae, as the scientist calls them—of many bottom-living animals; these latter more sedentary forms throw up their young in clouds to be distributed far and wide by the ocean currents, just as many plants scatter their seeds in the wind for the same purpose. Charles Darwin also used a tow-net before Müller, on his famous voyage in the Beagle; in his Journal of Researches (1845) under the date of 6 December 1833 he writes: “During our different passages south of the Plata I often towed astern a net made of bunting and thus caught many curious animals.” Today many forget that our famous T. H. Huxley, champion of Darwinism, began his career as did Darwin before him, as a great field naturalist; in 1846 he sailed for the South Seas as surgeon in H.M.S. Rattlesnake and by his use of the tow-net laid the foundations of our knowledge of those remarkable composite jellyfish-like animals, the siphonophores, which we will later discuss (see here (#litres_trial_promo)).
Another simple device, the naturalists’ dredge—a coarse netting bag on a rectangular iron frame—dropped and dragged along the bottom of the sea revealed another new world of life. It was first used by two Italian zoologists, Marsigli and Donati, in the middle of the eighteenth century, but it was another of our own great marine naturalists, Edward Forbes, who became the leading pioneer in this work; he began his dredging in 1840, both in British waters and in the Aegean Sea.
How deep in the sea can life exist? This became the subject of much controversy among scientists following the discoveries made by the use of an ingenious device invented by just a boy—a brilliant young midshipman in the U.S. Navy—J. M. Brook. He hit on the idea of attaching a quill to the sounding lead used in plumbing the ocean depths and so bringing to light a sample of the ooze from the bottom into which it had penetrated. It gave only a tiny sample—but how exciting! That was in 1854, and soon from all over the Atlantic basin, from any depth over 1000 fathoms, came samples of oozy sediment containing minute calcareous shells. These were shells of animals belonging to the group of the Protozoa (single-celled animals) known as Foraminifera and nearly all belonging to one genus, called Globigerina on account of the spherical form of their shells. This form of deposit has consequently become known as Globigerina ooze. Did the creatures which made the shells actually live at these great depths, or did the shells fall from near the surface when their floating owners died? That was the problem. It is amusing for us now to recall that most of those who held the latter and correct view did so on quite false grounds: they believed that it would be quite impossible for life to exist at these great depths and that therefore the shells must have fallen from above. A drawing of a living Globigerina is shown in Plate 2 (#litres_trial_promo).
Edward Forbes had considered there was what he called a zero of life at about 300 fathoms—a boundary below which no life could stand the great pressure of the depths. This fallacy was soon to be exposed. The laying of submarine cables was just beginning. In the Mediterranean one of these after a little use had parted and was hauled up for repair in 1858; it came up encrusted with bottom-living animals, some of them at points on the cable which must have lain at a depth of over 1000 fathoms. Once it is pointed out, the truth of the matter seems obvious: an aquatic animal should feel no ill effects of pressure provided it has no spaces or bubbles filled with air or gas inside it. All liquids are only very slightly compressible. A body made up of fluid or semi-fluid protoplasm, and covered with a flexible or elastic skin, will contract only very slightly even under the greatest pressure; its contents too will be of course at the same pressure as the surrounding water. With the stresses inside and outside the body perfectly balanced in this way, the animal can have a most delicate structure and make the finest movements just as well in the great depths as can one living nearer the surface. Even the seemingly rigid armour-platings of such animals as crabs are in fact made up of a number of parts separated from one another by thinner flexible joints, so that changes of pressure are equalised inside and out; the same applies to the starfish and sea-urchins, whose armour is actually not strictly on the outside of the body, but just below the skin.
Simple as the explanation seems to us now, the discovery of these animals living under great pressure came as a real surprise to most people. This was all the more extraordinary, for actually there was in existence a thoroughly attested instance of a remarkable starfishlike animal (one of the Gorgonocephalidae with branching arms) being brought to the surface from a depth of 800 fathoms; it came up entangled round a sounding line on Sir John Ross’s expedition to Baffin Bay in 1818. It had been forgotten or overlooked by the naturalists of a later generation, who also did not appreciate the significance of the dredgings reported by his even more famous nephew Sir James Clark Ross. Accompanied by the young Joseph Hooker, he had made a number of rich hauls from depths down to 400 fathoms during those great south polar voyages in the Erebus and Terror from 1839 to 1843. Unfortunately these important deep-sea collections, which contained marine invertebrate animals in great variety, were subsequently lost to science.
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The waters immediately to the north and west of the British Isles may perhaps be regarded as the cradle of oceanography; they became the scene of the pioneer deep-sea dredging expeditions in the naval surveying ships Porcupine and Lightning led by Dr. W. B. Carpenter and Professor (later Sir) Wyville Thomson. During the summers of 1868–70 they made nearly 200 dredge hauls over a wide area and reached a depth of 2,435 fathoms; as far down as they went they revealed a wealth of life and opened up a new world to the naturalist. Thomson’s great book The Depths of the Sea (1873) is still fascinating reading. It was their remarkable discoveries, together with the interest taken in the new venture of laying transoceanic cables and the consequent need for a more accurate knowledge of the ocean floor, that led in 1872 to the dispatch by the British Government of H.M.S. Challenger on her famous expedition; under the leadership of Sir Wyville Thomson she sailed on a three and a half years’ voyage to explore all the oceans of the world. The results of this magnificent venture filled more than 50 large volumes with a wealth of information not only of the life of the ocean and of the nature of the sea-floor as revealed by tow-net and dredge, but also about the physics and chemistry of the sea at different depths. Oceanography as an organised branch of science had come into being. Other nations followed the example of the Challenger and sent out similar expeditions.
Having mentioned The Depths of the Sea I must also refer to another great book of similar title which I believe will always be a classic in the literature of Oceanography: The Depths of the Ocean by Sir John Murray and Professor Johan Hjort, published in 1912. Murray was on the Challenger with Wyville Thomson and later, when Thomson’s health failed, directed the Challenger Office, seeing to the completion of all the work and the editing of the great series of Reports; Hjort, who died as recently as 1948, was the great Norwegian marine biologist and Director of his country’s fisheries research. I shall be referring to this book again, particularly in Chapter 12 (#litres_trial_promo), for it contains the results of a very successful expedition which the two authors made in 1910 in the Norwegian research ship Michael Sars to study the deepwater life of the North Atlantic. I draw attention to it here, however, because it is also a splendid introduction to our science in general, with a valuable chapter on the early history.
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Like the correction of the idea of a zero of life, the destruction of another early myth, the mystery of the ‘Bathybius’, is also amusing history. In 1857 H.M.S. Cyclops had made a line of soundings across the Atlantic with an improved modification of Brook’s apparatus for collecting samples of the sea-bed. The deposits of ooze were found to contain a strange gelatinous substance which was supposed to be a primitive form of life. Carpenter and Wyville Thomson again found it in their deep-sea dredgings. To give an idea of the universal interest it aroused at the time I will quote from ‘The Depths of the Sea’; Wyville Thomson is here referring to his deepest haul from 2,435 fathoms.
“In this dredging, as in most others in the bed of the Atlantic, there was evidence of a considerable quantity of soft gelatinous organic matter, enough to give a slight viscosity to the mud of the surface layer. If the mud be shaken with weak spirit of wine, fine flakes separate like coagulated mucus; and if a little of the mud in which this viscid condition is most marked be placed in a drop of sea-water under the microscope, we can usually see, after a time, an irregular network of matter resembling white of egg, distinguishable by its maintaining its outline and not mixing with the water. This network may be seen gradually altering in form, and entangled granules and foreign bodies change their relative positions. The gelatinous matter is therefore capable of a certain amount of movement, and there can be no doubt that it manifests the phenomena of a very simple form of life.”
“To this organism, if a being can be so called which shows no trace of differentiation of organs, consisting apparently of an amorphous sheet of a protein compound, irritable to a low degree and capable of assimilating food, Professor Huxley has given the name of Bathybius haeckelii. If this has a claim to be recognised as a distinct living entity, exhibiting its mature and final form, it must be referred to the simplest division of the shell-less rhizopoda, or if we adopt the class proposed by Professor Haeckel, to the monera. The circumstance which gives its special interest to Bathybius is its enormous extent: whether it be continuous in one vast sheet, or broken up into circumscribed individual particles, it appears to extend over a large part of the bed of the ocean …”
The ‘Bathybius’ however came to an inglorious end. It was shown by the naturalists of the Challenger to be a precipitate thrown down from the sea-water associated with the deposits by the alcohol used in their preservation, and T. H. Huxley made a public retraction of his earlier ideas.
Towards the end of the century came the founding of the famous marine stations, the first at Naples in 1872, and then those at Plymouth and Millport in this country and Woods Hole in America; in these laboratories and many more to be founded later, researches into the structure, development, physiology, life and habits of marine creatures of all kinds have been continued to the present day.
It then began to be realised that progress in oceanography was essential to a better understanding of fishery problems and to the development of a more rational exploitation of the sea. The rapid development of trawling, with the introduction of steam power and the replacement of the old beam-trawl by the much larger and more efficient otter-trawl, gave rise to some concern as to the possible depletion of the stocks of fish; this led a number of nations, our own included, to set up fishery investigations. As we shall see, those fears were indeed well founded. In 1899 King Oscar II of Sweden invited all the nations of Europe interested in sea fishing to send representatives to a conference in Stockholm; the discussions which took place led to the foundation in 1901 of the International Council for the Exploration of the Sea. The different nations began a series of investigations to form part of one great plan. In spite of the temporary suspension of activities in two world wars the work of the Council still goes on.
The scientists of the various fishery departments are not only enquiring into the natural history of the fish themselves, their life-histories, food and feeding habits, migrations, growth, birth-rates and so forth; but with continually improved equipment they are studying the distribution of the different planktonic forms upon which they depend, the conditions under which they live, the flow of the ocean currents, the physics and chemistry of the sea and the varying nature of the sea-bottom and its life. It was wisely realised from the start that in order to provide answers to such questions as: ‘Why are fish sometimes plentiful and sometimes scarce?’; ‘Can the future of a fishery be forecast?’; ‘Is this or that area being overfished?’ and so on, the natural history of the sea must be investigated in all its different aspects.
Everything the naturalist wants to find out about the conditions under which fish live he must grope for in this unseen world, often below a storm-tossed surface; he must always remember too that the sea is a very big place—he must work with a sense of proportion and perspective. He stops his research ship at intervals to let down his instruments on wires and ropes: some to take the sea’s varying temperature and to collect samples of water from different depths for analysis, others to measure the amount of light reaching different levels, and others again to record the speed and direction of ocean currents. He samples its life with all kinds of nets to estimate not only the varying quantities of the plankton but of the eggs and fry of the fish themselves. Building up a picture of life in the sea is like putting together a huge jig-saw puzzle made up of tiny pieces, but much more difficult. Not only have we a very imperfect idea of what kind of picture will emerge, but all the pieces to be fitted together are not on the table before us; they are lying about somewhere underneath it and we must feel about for them in the darkness. It is certainly a fascinating pursuit, but full of disappointments. Some bits of the puzzle—perhaps a stage in a life-history or some evidence of a migtation—can only be picked up during a short period of the year; before the missing pieces can be found, stormy weather may intervene and we must wait a whole year before we can try again.
In spite of these obstacles the picture of life in the sea is continually growing: the chapters which follow will endeavour to sketch an outline of what has been achieved. The amateur naturalist should not be discouraged by these difficulties for it is because of them that there are so many gaps in the story yet to be filled in. There are still many original discoveries to be made. And the difficulties add spice to the game; a golf-course would indeed be a dull one if there were no bunkers on it.
(#ulink_9afc2b6b-46b2-5c7f-b0d9-1ec96f0deda4) In the “Summary of the Scientific Results of the Challenger” Part 1, p. 79, Sir John Murray refers to the deep dredging of the Erebus and Terror. He says: “Sir James Ross was an indefatigable zoological collector, but it is to be regretted that the large collections of deep-sea animals, which he retained in his own possession after the return of the expedition, were found to be totally destroyed at the time of his death. Had they been carefully described during the cruise or on the return of the expedition to England, the gain to Science would have been immense, for not only would many new species and genera have been discovered, but the facts would have been recorded in journals usually consulted by zoologists instead of being lost sight of as was the case.”
(#ulink_cc99d3a0-0fef-5460-97aa-c036db547ab3) Sir John Murray also wrote an excellent little introduction to oceanography: The Ocean (1913). For those who wish to make a fuller study of the development of our science, and particularly of the early plankton investigations by the German Hensen school, there is J. Johnstone’s important book on Conditions of Life in the Sea (1908). Two other valuable introductions should be mentioned: Fowler and Allen’s Science of the Sea (2nd edition, 1928) which gives much practical advice on collecting specimens and the working of gear, and Russell and Yonge’s charming general natural history of The Seas (1928) which deals with the life of tropical waters as well as of our own.
CHAPTER 2 (#ulink_418798ab-0498-50b2-954e-b824ce9838fe) THE MOVEMENT OF THE WATERS
ONE OF THE MOST important features in the world of a marine animal is the movement of the sea itself. Apart from the wave-action near the surface and the to-and-fro tidal streams in shallow coastal areas, the whole water-mass is in continual flow as part of a greater system of oceanic circulation. Carried with the moving waters go the floating animals and plants. The main surface currents in the North Atlantic are shown in Fig. 2 (#litres_trial_promo).
In some places the sea may be richer in plankton and in others poorer—like contrasting regions of luxuriance and barrenness on the land; unlike them, however, these areas in the sea are day by day changing their positions in relation to the coasts and the sea-bed. Not only is the basic food-supply on the move, but the delicate and helpless young of many fish and bottom-living animals are carried in one direction or another. Clearly these water movements are profound in their effect. To understand the lives of the inhabitants of our seas we must first have a knowledge of the main average pattern of the ocean currents flowing round our islands. They are by no means fixed in an unalterable course: variations are continually occurring. Sometimes these changes have striking effects; occasionally some exotic creatures from warmer seas, such as the beautiful Portuguese Man-of-war (Physalia) with its iridescent float and long trailing tentacles, or the smaller blue Velella riding the surface with a little ‘sail’, is driven ashore upon the coasts of Devon and Cornwall. (see here (#litres_trial_promo) and Plate 5 (#litres_trial_promo)). Normally we find typical Atlantic water outside the western entrance to the English Channel, but at times there may be a marked incursion of water from the Bay of Biscay carrying with it these and other planktonic visitors from further south. It is probable that long periods of strong south-westerly winds may contribute to this movement.
FIG. 2
The surface currents of the North Atlantic Ocean. Drawn, with kind permission, from Admiralty Chart No. 5310 (1949), omitting some of the detail.
The general systems of the surface currents of the world have been worked out largely from the innumerable observations of navigators. Suppose a ship steers from point A on a particular course by compass which should carry it to a point B in, say, a day’s time; if at the end of the 24 hours the navigator finds by reference to the sun or stars that he has instead reached a point five miles to the south-east of it, then if there is no wind to account for his drift he knows that the surface water must be flowing in a south-easterly direction at a speed of five miles a day. The great pioneer in organising a systematic recording of winds and ocean currents throughout the world was Lieut, (later Admiral) M. F. Maury of the United States Navy who wrote the first text book of oceanography, The Physical Geography of the Sea in 1855, to be followed by many editions. The charts he produced of the wind and current systems of the world rendered immense service to commerce; before their production the average time of sailing between England and Australia was 124 days, but with their use it was reduced to 97, or the average passage to California (presumably from New York) was 183 days and this was reduced to 135 (from Maury, loc. cit.).
FIG. 3
Types of drift-bottle for investigating ocean currents, a, surface drifter, old type; b, bottom drifter; c, surface drifter, modern type; for further explanation see text.
Observations on the movements of ships are suitable for indicating the surface drift only in the wide open oceans. For working out the current systems in more detail in more confined areas science has made use of that romantic object, the shipwrecked sailor’s bottle with a message to his home. Thousands and thousands of these drift bottles have been cast into the sea at many different points. Each bottle displays through its glass sides, a notice in several different languages asking the finder to break it and read the instructions within; these tell him that if he will fill in and send off an enclosed postcard giving particulars of the place and date of recovery, he will receive a small reward. Many hundreds of these cards have been returned from various points along the coasts of different lands. In this way Dr. T. W. Fulton of the Scottish Fishery Board first charted the main surface movements of the North Sea at the beginning of the century. Later Dr. G. P. Bidder introduced bottom-drifting bottles, each provided with a trailing wire and weighted so as to drift along with this just touching the sea-bed; these are designed to be recovered by the many trawlers dragging their nets along the bottom of the North Sea and English Channel. Thus knowledge has been gained not only of the surface currents but also of the movements of the lower layers of water, which are often very different. Objects floating on the very surface of the sea may be driven by the wind much faster than will the true upper layer of water; surface drift bottles have now been improved by Dr. J. N. Carruthers (1928) who has provided them with little weighted ‘sea anchors’ to keep them drifting with the main body of water. Sketches of these different bottles are shown in Fig. 3 (#litres_trial_promo) and an example of their use in studying the drift of plaice eggs is shown in Fig. 5 (#litres_trial_promo). Quite recently oceanographers have started using water-proof plastic envelopes instead of bottles.
FIG. 4
Diagrammatic sketches showing the working of the Ekmann current-metre.
For the more exact measurement of the speed and direction of current flow at different depths special instruments have been invented to be used from anchored ships. One of the most successful is that of Dr. Ekmann. It is such a beautiful and ingenious device that it is worth examining in some detail to see how it works. It is suspended in the sea on a wire, as shown in Fig. 4 (#litres_trial_promo), and, being provided with a vane like a weather-cock, always points head on into the current. It also has a little ‘propeller’ which will be turned by the water flowing past it; this however is held by a catch until the measurement of flow is about to begin. On the underside of the apparatus, and held horizontally, is a circular box, like a large and rather deep pill-box. The lower half of this is divided into a number of compartments by partitions radiating from the centre like the spokes of a wheel; swinging above these compartments, and pivotting upon a spike in the centre, is a stout compass needle having a slightly sloping groove on its upper surface running from its mid-point to the end pointing to the north. In addition the compass box has a small hole in the centre of the lid and immediately above it is another little box containing a supply of bronze shot. To take a current measurement the instrument is lowered to the desired depth—perhaps 10, 50 or 100 metres; then a small brass weight, called a messenger, is threaded on to the suspending wire and sent sliding down to release the catch and set the propeller free to turn. After one, two or more hours as may be desired, the recording is ended by the dispatch of a second ‘messenger’, which moves the catch further over to stop the propeller once more, and the machine is hauled up to the surface. The number of turns the propeller has made in the interval of time is recorded on a series of little dials and so enables the speed of the current to be estimated; also after every so many revolutions a little valve opens and allows a shot to drop into the box below and be guided by the grooved compass-needle into one of its compartments. The compass-box, being fixed to the instrument, is always orientated in relation to the direction of the current; thus as the instrument swings in response to changes in current flow, each shot is dropped into whatever compartment happens to lie below the north-pointing end of its compass needle. By counting the shot found in each of the compartments at the end of a recording we know to what extent the current has varied during the time of observation and also the average direction of its flow. The direction is, of course, given by the angle between the mid-line of the box and that of the compartment which most often pointed north, as revealed by the shot in it. Such observations are made at several depths to show how the speed and direction of the water flow varies at different levels.
The upper layers of sea are nearly always travelling faster than the lower ones. The surface layer is affected by the wind; apart from this, however, the lower layers will be retarded by the frictional resistance of the sea-bed, just as in a river the water in the middle travels faster than that near the bank. We shall later see (see here (#litres_trial_promo)) that these differences in speed may have a marked effect on the distribution of plankton animals which make extensive day and night migrations between the upper and lower layers. In deep ocean basins we may even find currents at different levels flowing in opposite directions.
FIG. 5
Showing where drift-bottles liberated at A and B were recovered. A map reproduced from one in the museum of the Fisheries Laboratory at Lowestoft showing the results of one of many experiments made by the late Mr. J. O. Borley when investigating the drift of plaice eggs and larvae from the spawning areas to the coastal nursery grounds. The numerals show the number of bottles picked up at each point.
There are several different kinds of current-meter but most of them are similar in general principle to that described. Dr. Carruthers (1926) has devised a much more robust machine which is used on a number of lightships anchored round our coasts to record automatically the main drift of water for periods of a month at a time. It records the to-and-fro tidal stream as well as the residual current-drift. These instruments have given much valuable information on the variations in the flow of water through the English Channel into the southern North Sea (Carruthers, 1930). It is in the Flemish Bight that so many plaice congregate in winter to lay their floating eggs at the point where the Channel water enters; these eggs, and later the hatched-out fry, are carried by the current and so bring the new generation to settle down as small flat-fish on the nursery feeding grounds in the shallow waters off the Dutch and Danish coasts. Herring fry in vast numbers are also carried by the same current into the North Sea from one of the largest spawning grounds off Cape Gris Nez. The spawning migrations of many fish have been evolved in relation to the prevailing current systems; selection has naturally acted to preserve the offspring of those parents who migrate to lay their eggs at the point up-stream most favourable for their survival; eggs spawned elsewhere are less likely to be successful because they are carried to less suitable nursery grounds. Fig. 5 (#litres_trial_promo) shows the results of two of many experiments made with drift-bottles during the English fishery investigations into the life-history of the plaice. Those liberated at a point in the main spawning area in the Southern Bight have all been picked up on the Dutch coast, whereas those from the lesser spawning area off the Yorkshire coast are carried into the Wash, which is another smaller ‘nursery’ ground; the number against each point on the coast indicates the number of bottles found there. In some years a marked variation from a normal current-flow may well have a considerable effect on the relative success of a particular brood of fry; if the flow is weaker than usual they may not reach the best ground, if it is too strong they may be carried beyond it. The speed of flow of Channel water into the North Sea may be affected by the wind at a critical time apart from any variation in the fundamental current system; a prolonged westerly wind may accelerate the flow, and an easterly one may have the reverse effect. These are just some of the many factors we must take into account in puzzling out the possible causes for this or that unusual event in the fisheries; it is so often that such factors have an effect which is most marked in the fishery several years later, i.e. when the young fish of that brood will have grown to maturity—or failed to.
Without the use of drift bottles or current-meters some indication of the direction of current-flow may also be got by mapping the contours of varying salinity (measurements of salt-content) obtained by the analysis of water-samples collected at a number of different points in the area; for instance, a tongue of very salt water projecting into a less saline area might indicate a flow of ocean water into a more coastal region where the salt water has been diluted by drainage from the land. Indeed modern oceanography has developed an elaborate mathematical system for estimating the direction and relative speeds of water movements from a knowledge of the varying densities of the water at a number of different points. We shall later see how at times certain planktonic animals and plants characteristic of one particular type of water may be used as indicators of the incursion of such water into other areas: in fact one such example, that of the tropical Physalia and Velella reaching our coasts, has already been mentioned, and we shall discuss others at the end of the chapter. We are apt to think of the use of plankton animals as current indicators as rather a modern idea; I was interested to find that Alexander Agassiz in 1883 was emphasising the importance in this respect of the animals just mentioned. “This group of Hydrozoa,” he wrote “is eminently characteristic of the Gulf Stream, and wherever its influence extends these Velellae and Physaliae have been found. In fact these surface animals are excellent guides to the course of the current of the Gulf Stream—natural current bottles, as it were.”
In the Department of Natural History in the University of Aberdeen there is a cabinet containing a remarkable collection of South American and West Indian seeds picked up on the shores of the Outer Hebrides. They were gathered from 1908 to 1919 by William L. MacGillivray who was a nephew of a former Regius Professor; most of them he found on the West Sand of Eoligarry, Barra, where he lived, but some came from Lewis and the Island of Fudag. There are Brazil nuts, the seeds of a leguminous liana Diodea, the Virgin Mary nut, palm seed of different kinds, the pecan nut, the Calabar bean, nutmeg, the seeds of the Central American soapberry tree and many others. Altogether seventeen tropical species are represented. Just as these seeds are drifted to our shores, so also are the baby eels carried round by the ocean circulation from where they were spawned—from a small area situated between Bermuda and the Leeward Islands—and eventually scattered by the Gulf Stream to enter the rivers along the whole seaboard of Europe. This was the amazing discovery made by the famous Danish oceanographer Professor Johannes Schmidt. On many special voyages he plotted the distribution of the tiny eel fry all over the Atlantic until at last he could show that there is only one limited area where the very smallest and newly hatched young are to be found—a breeding ground some 3,000 miles from the rivers in which they grow to maturity. In Fig. 6 (#litres_trial_promo), I reproduce his map showing the spread of the fry of different sizes. Their drift round the ocean to Europe takes from 2 to 2½ years, and during this phase they are little flat and quite transparent creatures having the shape of a willow leaf. They used to be thought to be a separate species of fish, called Leptocephalus brevirostris, until the Italian naturalists Grassi and Calandruccio kept some in an aquarium and were surprised to find they turned into the common elvers, as the young freshwater eels are called when they ascend the rivers from the sea. The story is now very well known and I only recall it here because it is, to use Agassiz’s simile, nature’s greatest drift-bottle experiment and demonstrates so clearly the constancy of this vast current system; year after year, for many millions of years, the eel-fry must have been transported in this way with never a break in the sequence. How the adult eels navigate back to breed in this one place is one of the most profound mysteries of the sea; a discussion of this, however, belongs to a chapter on fish and must await the subsequent volume.
FIG. 6
The distribution of the common European Eel (Anguilla vulgaris) during its various stages of development. The contoured areas represent those in which the larvae of various sizes, 10, 15, 25 and 45 mm. are found; the line ul represents the limit of occurrence of unmetamorphosed larvae; the black bands along the coasts indicate the countries where the adult is found in fresh water (after Schmidt).
The causes of the great circulations of water—as distinct from mere tidal streams—are of three kinds; oceanographers, however, are still not fully agreed as to which is the most important: indeed all three play their part together. Primarily there are the effects of the prevailing winds over wide stretches of ocean, particularly the northeast and south-east trade winds blowing obliquely towards the equator from north and south respectively. These certainly take a great part in driving the equatorial water towards Central America, so that from the Gulf of Florida emerges the powerful warm Gulf Stream to flow across the North Atlantic and give to our islands and the north of Europe so temperate a climate compared with that of the corresponding latitudes of North America. The latter are cooled by the Arctic Stream of the Labrador Current. The Gulf Stream, or the North Atlantic Drift as it is more correctly called on this side of the ocean, has a profound effect upon our waters.
Although marine physicists are beginning to believe that the stress of the wind on the sea surface, together with the effect of the earth’s rotation, can give rise to slow movements of water in the deep layers of the ocean as well as near the surface, they have to consider a second kind of cause: the action of what are often termed Archimedian forces. These are the forces due to internal changes in the water-mass causing alterations in its density. Such changes may be due to the expansion or contraction of the water on being warmed or cooled; they may also be due to an increase in the salt-content caused by excessive evaporation of water at the surface, as in the tropics, or to a decrease in saltness caused by large additions of fresh water from melting ice or excessive rainfall. Whether these causes actually produce the deep water movements, or do no more than make the water take the path of least resistance, they have far-reaching effects, particularly in giving rise to vertical as well as horizontal differences in the great ocean basins. One of the most surprising discoveries of worldwide oceanography was made between the two world wars—indeed through the work of our own Discovery Expeditions and the German Meteor Expedition, in the Antartic and Atlantic Oceans; it is the fact that the heavy snow or rainfall and the melting ice in the Antarctic seas has an immense influence that extends across the equator into the northern hemisphere. It is so striking an example of these Archimedian forces that I cannot resist using it as an illustration.
FIG. 7
A section through the Atlantic Ocean, from latitude 55°S to 15°N along the meridian 30°W, showing the water of varying saltness (34–00/00 to 37–0 0/00) and the directions of the main water movements at different depths. It shows how the great Antarctic icecap extends its influence into the northern hemisphere. Redrawn in diagrammatic form from Deacon (1933).
The great ice-cap at the south pole dominates the oceans of the world. The Antarctic continent rising in a plateau to elevations of some 8,000 feet is covered with a sheet of ice many hundreds of feet thick; this is continually being added to by the frequent heavy falls of snow, and is constantly and slowly moving as a vast glacier to the coast and beyond into the ocean where it juts out as the floating ice barrier. This is shown on the left of Fig. 7 (#litres_trial_promo). At its edge this barrier from time to time breaks up into the massive tabular icebergs so characteristic of the south polar seas. This is freshwater ice, which on melting, helps to form a cold but light surface layer; in spite of being colder it is lighter than the normal sea-water because its salt content is reduced by the addition of the fresh-water. All round the pole this cold surface layer flows away to the north. Below this is water that is heavier because it is just cooled and not diluted with fresh-water; this sinks and forms a cold current, also flowing north but over the ocean floor. To take the place of these two streams of water flowing away from the pole, a mass of warmer water flows southwards and wells up against the ice, to be itself diluted, cooled and turned north again. The surface current thus formed continues till it meets warmer water which, although more saline, is lighter because it is so much warmer; the cold current now dips below this warmer water but still travels northwards and can be traced to a point some 30° of latitude north of the equator; it then sinks and joins in the return flow going south again to complete the circulation.
Here we have a striking case of waters at different levels travelling in opposite directions: a layer going south flows in between two layers coming north. We shall see in a later chapter how the behaviour of some plankton animals is adapted in a most remarkable manner to take advantage of this fact.
Dr. G. E. R. Deacon, who has done so much to increase our knowledge of this remarkable system (1933 and 1937), while on one of the Discovery expeditions took water-samples all the way along the path of this northward-flowing current after it had dipped below the surface; when he had analysed them he found something very extraordinary. We have said that this water was of low salt-content because of the melted ice; it also has a high oxygen-content because of a great production of planktonic plants in the polar surface waters (due to the rich nutrient salts and to the long hours of daylight in high latitudes). Now as this water travels north the salt-content increases by diffusion from the surrounding layers and the oxygen-content is lowered by the respiratory requirements of animals. As he went along the path of the current Dr. Deacon obtained a clear indication that the salt and oxygen values did not increase and decrease respectively in a perfectly steady manner as one might have expected, but in a series of waves. As far could be judged from the graph of the increase in saltness there were seven undulations in the curve from south to north; likewise in a graph of oxygen-decrease there were also seven undulations. Now the crests of the undulations of one curve corresponded in position with the troughs of the undulations of the other curve; in other words as we passed along the stream of water, regions of higher oxygen-content and lower salinity alternated with regions of lower oxygen-content and higher salinity. This water had come originally from the Antarctic surface layer in which during the summer more ice melts and also more plants are produced than in the winter. More ice melting means a lowering of salinity and more plants mean a greater production of oxygen. Clearly these regions of lower salinity and higher oxygen-content alternating with regions of higher salinity and lower oxygen-content represent the water which left the antarctic surface layers in past summers and winters respectively. Dr. Deacon tells me that he now fears that there are not sufficient observations along the path of the current to make their number quite certain; but since the indicated rates of water movement agree very well with most other estimates, he feels that they give us a fairly reliable time scale for this great circulating system. This water takes at least seven years on its journey from the antarctic to the northern hemisphere!
There is a somewhat similar system of a cold and less saline surface current flowing away from the north polar basin due to melting ice and the fall of snow and rain, but it is not so far-reaching as that from the south because this precipitation is less and in addition the Arctic Ocean is almost entirely enclosed by submarine ridges; this cold stream dips below the warmer water at the northern boundary of the Gulf Stream. It is partly to replace this cold water stream that the extension of the Gulf Stream—the North Atlantic Drift—is carried so far to the northward of our islands and up the northern coasts of Scandinavia. Here we see how these Archimedian forces may contribute to the North Atlantic system.
The third factor affecting ocean-currents is a much more subtle one, due directly to the actual spin of our planet. This deflecting force of the earth’s rotation is sometimes called Corioli’s force, after the French physicist, though in fact it was carefully worked out by his countryman Laplace 60 years before; it applies to the atmosphere as well as the sea. It is not a cause of the initial motion of the water but a cause of its deflection. A body of water moving in any direction is deflected to the right in the northern hemisphere and to the left in the southern hemisphere. The effect is greater towards the poles and reduced towards the equator; on the actual equator itself there is no such effect at all. It applies not only to water but to any moving object; we can perhaps understand it best by considering the effect upon a swinging pendulum. Let us suppose we could hang a fairly heavy weight, say of some 20 lbs., on a long string from a 100 foot tall gallows-like structure at the north pole; now if we set the weight swinging to-and-fro in the same direction as a straight line drawn in the snow beneath it, we should soon observe that its line of swing would deviate from the line in the snow. Its swing would be deflected in a regular fashion in a clockwise direction; even in ten minutes its path would be deflected 2½. Unperceived by us the earth and the gallows would be rotating in an anti-clockwise direction, but the heavy pendulum weight is swinging free and its path is not affected although the string at the top will twist. If we watched it for a full twenty-four hours we should see the path of the pendulum complete a deflection of 360° and once more for a moment swing directly above the line in the snow. If we repeated this experiment at the equator—drawing our line in the sand—we should see no such effect; if the pendulum was set swinging say north and south it would continue to swing thus as it would also continue to swing in any other direction in which we might choose to start it; here the earth makes no turning motion in relation to the swing; for the line in the sand and the line of swing are carried on together by the earth’s motion round its axis. At the south pole we should of course get a similar effect to that at the north pole except that the pendulum would appear to be deflected in an anti-clockwise direction. Swinging such a pendulum at places in different degrees of latitude will give a different amount of deflection. At a place situated in latitude 30° north the pendulum will swing through 180° in the 24 hours, for, speaking mathematically, the effect depends on the sine of the angle of latitude; in London at 51.5° latitude it will swing through 281°. The effect was shown very clearly by Foucault, the French physicist and inventor of the gyroscope, by swinging a hundred-foot pendulum at the Great Exhibition of 1851. This demonstration which is to be seen in the Science Museum in London and in a number of provincial museums is not difficult to set up in any building with a high roof or in any house that has a fairly wide staircase well above the entrance hall; it is an impressive sight to see in the matter of a few minutes the apparent change of motion of the pendulum, which really indicates the rotation of the hall itself or, indeed, the earth.
FIG. 8
The varying saltness (31.0 0/00 to 35.4 0/00) of the surface waters and the main circulation typical of the North Sea and English Channel in winter. Drawn from a chart kindly provided by Commander J. R. Lumby, of the Fisheries Laboratory, Lowestoft.
Just as the pendulum is deflected in relation to the objects in the hall, so any body of water in motion tends to be deflected to the right in the northern hemisphere and to the left in the south in relation to the surrounding land masses and the ocean floor; account has to be taken of it in every practical treatment of tides, wind drifts and ocean currents. Whenever a water-mass meets an obstruction, either a mass of land or an opposing water-mass, it will, other things being equal, turn to the right in the north rather than to the left, and vice versa in the south. There is another important effect. We have seen how through differences in temperature and saltness the water varies in density; the lighter water will naturally be on top. In a current system the water of a particular density—say the lightest water at the top—is not lying in a layer of uniform depth; owing to the earth’s rotation the lighter water is pushed more to the right-hand side of the current stream than the heavier water, so that imaginary surfaces separating waters of different density are not horizontal but tilted. It is from a consideration of the deflection of waters of different densities that the speed and direction of ocean currents can be mathematically worked out as mentioned earlier in the chapter.
With this slight introduction, intended merely to give an idea of the kind offerees at work to produce the circulatory ocean-systems, we may now briefly review the main streams of water in the seas around our coasts. Fig. 8 (#litres_trial_promo), is based on the account by Comd. J. R. Lumby (1932), hydrologist at the Fisheries Laboratory, Lowestoft, with a small revision which he has kindly made in the drawing for this figure. The water in the North Sea and English Channel is slightly less salt than the Atlantic Ocean water; it is typically coastal water diluted by freshwater drainage from the land. The Baltic has a much lower salinity still. A stream of Atlantic water flows into the North Sea from the north, mainly passing round to the east of the Shetland Islands to flow due south and not usually entering between the Orkneys and the Shetlands as was originally thought; a less powerful stream flows up the English Channel and enters it from the south. The northern influx is generally thought to flow on a broad front down the middle of the North Sea forming, as it goes, swirls off the coast of Scotland especially in the Moray Firth and in the region of the Firth of Forth. Dr. J. B. Tait of the Scottish Fishery Department has in recent years, however, put forward the view (1952) that the main streams are much narrower than formerly supposed—more like rivers flowing in the sea. Which is the correct view is at present by no means certain; some evidence from plankton distribution appears to support one view and some the other. Just before reaching the Dogger Bank the main stream, whether broad or narrow, appears generally to divide into three branches: one running south-westerly, another south-easterly and a third turning east to enter the Skagerrak. The south-westerly and south-easterly branches form large swirls in the southern North Sea as they meet the stream of water entering from the Channel. Another smaller swirl is formed outside the Skagerrak as the stream entering on the southern side meets the stream flowing out of the Baltic on the northern side.
(#ulink_f48d51e1-8bc0-56e7-b606-d6579a8e8558) The stream entering the North Sea from the Channel flows north-eastwards past the Dutch and Danish coast and some of it joins the stream going into the Baltic. Most of the North Sea is shallow, but there is a deep hollow running up the western coast of Norway to the north; it is along this Norwegian trough that the water leaves the North Sea—the less saline water from the Baltic on the top and the bulk of the North Sea water proper in the deep channel below.
The extent of the inflow of Atlantic water varies from year to year; such variations affect the distribution of the plankton and are likely to influence the distribution of the herring shoals which depend upon the plankton for food. In some years of exceptional influx numbers of plankton animals usually only found in the more open ocean make their appearance in the northern North Sea. There is some evidence to support the view that it is the pressure of this water from the north (produced by the main wind systems) which, apart from the occasional effects of local winds already referred to, controls the inflow of Channel water into the southern North Sea. If the pressure from the north is high it seems that the Channel flow is reduced; if it is weak then a larger influx from the Channel seems to take place. It is in the study of this inflow into the northern North Sea that the charting of the relative movements of certain Atlantic plankton animals in different years can be most helpful. We shall see in a later chapter (see here (#litres_trial_promo)) how, by the use of plankton-recording machines towed at monthly intervals by commercial steamships on regular routes, we can compare the areas of invasion of these more oceanic forms in different months and years. We shall find that not only does the extent of the Atlantic inflow vary from year to year, but the time of the advance of typical invading organisms will vary: in some years it may be a month earlier or later than in other years. There is an interesting suggestion now being investigated that the time of the appearance of the shoals of herring at different points down the east coast of Scotland and England, and consequently the time of the different fisheries, may be earlier or later in different years depending on whether this Atlantic inflow is earlier or later.
(#ulink_dc23d649-8c64-57d0-aa00-db1867ee1350) Whether this indication—it is no more at the moment—will be proved correct or not, there can be no doubt that the fluctuations that are found to occur in the water movements round our islands must have a profound effect upon the fish and other life inhabiting our seas.
A more definitely established connection between water changes and fisheries has been demonstrated at the western entrance to the English Channel. The water of the greater part of the Channel is like that of the southern North Sea—coastal water which is less saline and less rich in plankton than the Atlantic water that flows into it. This more coastal water can readily be distinguished from the more oceanic water by the presence of certain of these indicator plankton species—particularly two species of Sagitta, the slender transparent arrow worm shown in Plate IX (#litres_trial_promo); Sagitta setosa being found in the coastal water and Sagitta elegans in the more oceanic water.
(#ulink_adddf64c-ed23-545c-9c18-ed66cb5c3d2b) The boundary between the two waters formerly used to lie somewhere in the region of Plymouth where sometimes the plankton would have elegans predominating in it and sometimes setosa) during the investigation up to 1929 it was more usually elegans, indicating Atlantic water richer in phosphates and other nutrient salts. The importance of these salts in the economy of the sea will be discussed in Chapter 4 (#uf9e2ac0e-843a-5efb-963b-16fe015b8f29). Since 1929 the boundary between elegans and setosa water has lain much further to the west so that the water off Plymouth has been of the coastal type and much poorer in plankton. Since this date there has also been a marked reduction in the number of young fish of many kinds present in the plankton as well as a change in the herring fishing; since that time the herring which used to visit the Plymouth area around Christmas have not turned up in their usual numbers so that this winter fishery, once quite a prosperous one, now no longer takes place. An excellent account of this trend was given by the late Dr. Stanley Kemp in his presidential address to the Zoology Section of the British Association in 1938. More recently some other interesting differences between the elegans and setosa water have been discovered; these will be referred to later when the various chemical constituents of the water are being considered (see here (#ulink_e4b329bb-1a47-559c-8b6e-e322d668184d)).
FIG. 9
Map showing the distribution of three kinds of water round Great Britain each characterised by a different species of the arrow-worm Sagitta: serratodentata in open ocean water, setosa in coastal water and elegans in oceanic water mixing with the coastal water. The conditions are those which might be expected in the autumn of a year with a strong Atlantic influx into the North Sea from the north. From Russell (1939), but modified in the north in the light of more recent surveys and with some other details omitted.
Mr. F. S. Russell, the present Director of the Plymouth Laboratory, who carried out these studies on Sagitta (1935, 1936) and young fish (1940), made cruises to trace the boundaries between the different kinds of plankton. He has told me how very abruptly one type of water may give place to another. On one occasion he has said it was even possible to place the ship across the very margin between them, so that elegans water could be sampled from the bows and setosa water from the stern! Fig. 9 (#litres_trial_promo), shows the general distribution of the elegans and setosa water round the British Isles as it might be expected in the autumn of a year in which there is a strong influx of Atlantic water into the North Sea from the north; it is taken from another of Mr. Russell’s papers (1939). These different waters may also be sometimes discernible by a difference in their colour, a contrast of shades of blue and green making a line across the sea. In 1923, when on the staff of the Fisheries Laboratory at Lowestoft, I acted as observer in some attempts to locate shoals of herring and mackerel from the air. In flying from Plymouth to the western mackerel grounds we passed over a sharp line separating the green water of the Channel from the deep blue of the Atlantic; it ran on a slightly irregular course from the Lizard to the south-west as far as we could see to the distant horizon. Then while circling over the mackerel area we saw another equally definite boundary running from Land’s End towards the Scilly Isles separating the deep blue water from a more brown-green area lying to the north. At that time I could not interpret that striking pattern of colour contrasts; now on looking at Mr. Russell’s maps I have little doubt that the blue area I saw was oceanic elegans water lying between the setosa water of the English Channel and that of the Irish Sea. Fig. 10 (#litres_trial_promo) shows a comparison between my sketch of these colour boundaries, which was published in the official report (Hardy, 1924a) and Mr. Russell’s maps of the distribution of the setosa and elegans water in the same area but in different years (Russell, 1935, 1936). If these marked colour-changes can be correctly interpreted we may in the future find aircraft being used to make rapid surveys of the surface conditions in relation to the fisheries. The actual experiments in spotting shoals offish were not successful in these waters; in the southern North Sea the water was too opaque with the large amount of sediment constantly stirred up by tidal currents running over sand and mud banks; at the western entrance to the Channel the ocean surface was too much broken up by waves into light and shade to allow of any observations below it.
FIG. 10
Well defined areas of blue and green water (A) seen from the air during mackerel spotting tests off Cornwall in 1923 drawn from the chart by Hardy (1924) and compared (B and G) with the distribution of western and Channel water as indicated by the arrow-worms Sagitta, elegans and setosa, charted by Russell (1935 and 1936).
In the shallower waters—especially in the southern North Sea—we must not forget the influence of the tidal streams just mentioned; they may have a most profound effect in modifying the action of the main currents, especially when they vary so enormously in their force between spring and neap tides. At spring tides in certain places a mass of water may be moved for some thirty or forty miles in each direction.
The movements of water in the Irish Sea are also dominated by tidal currents; these flow into it from both ends and follow the general direction of the coast lines. Professor K. F. Bowden, who has given us such an excellent account of these tidal streams (1953), writes, “Knowledge of the non-tidal drift, however, is much less certain and is based on indirect evidence. It was recognised at an early date that the distribution of salinity indicated a north-going drift and in 1907 Knudsen estimated that the rate of flow was such that the water in the Irish Sea would be completely renewed in a year.” After saying that “this implies a flow through the Dublin-Holyhead channel at an average rate of just over a kilometre a day,” he later stresses that, although there seems little doubt about this average northward movement, “its magnitude, its variations and the degree of its dependence on the wind are still uncertain.”
We have now dealt with the main water movements round our islands; later in the book we shall see instances of more local effects and how upwellings and the mixing of waters may be important in producing a richer plankton. I will end the chapter by referring to some surprising and significant plankton records being made by Dr. J. H. Fraser (1952e, 1955) of the Scottish Fishery Laboratory at Aberdeen. In some years, over a wide area to the north of Scotland, he finds plankton animals which we should more usually associate with the latitudes of the Mediterranean; they indeed indicate a very unexpected movement of water. It now appears that some of them may in fact actually have come from the Mediterranean Sea itself.
FIG. 11.
A chart showing the northward flow of the ‘Lusitanian’ plankton; kindly provided by its discoverer Dr. J. H. Fraser of the Scottish Fishery Laboratory, Aberdeen.
It has long been known that a surface stream of Atlantic water flows eastwards through the Straits of Gibraltar and that this influx is balanced by an outpouring (at a lower level) of Mediterranean water of very high salinity; this spreads out from the Gulf of Gibraltar underneath the North Atlantic water and some of it is carried north up the edge of the European continental shelf. This movement is well summarised in Sverdrup, Johnson and Fleming’s important book The Oceans (1942, pp. 646, 685–6) which, for the serious student, gives such an excellent account of the main results of modern oceanography. Dr. L. H. N. Cooper of the Plymouth Laboratory has recently (1952) made a study of the distribution of this water to the west of the British Isles as it continues northward below the Atlantic water at a depth of some 600 to 1,200 metres. How far north it goes seems to vary greatly in different years; in some it appears to go no further than the west of Ireland, but in other years it flows onwards to upwell and spread over the continental shelf. Its course has been followed by Dr. Fraser by finding its typical but exotic fauna in his plankton nets; to the west he finds it deep down—but let me quote from his recent paper.
“It apparently follows the edge of the Hebridean Continental shelf, mixing on its western edge with open oceanic water, and upwells somewhat on the east side to overflow and mix with coastal water on the shelf. In some years this current may not reach Scotland or is too weak to be recognised, but on occasions it is sufficiently strong to continue into the South side of the Faroe Channel, though it only rarely penetrates beyond the north of Shetland. Frequently, however, it mixes with the coastal water on the shelf and the resulting mixture floods the area to the west of Orkney and often passes through the Fair Isle-Orkney Passage into the Moray Firth area.”
How lucky we are to have such a remarkable current carrying its rich and southern life far below the surface and then spreading it out, as it were, on our northern doorstep for our examination. Fraser calls this a planktonic ‘Lusitanian fauna’ and lists no fewer than 43 species characteristic of it; he has kindly prepared for me a chart of its typical distribution which is reproduced in Fig. 11 (#litres_trial_promo). Through his recent publications I have been able to add to my account some very interesting animals which I shall be describing in Chapter 7 (#litres_trial_promo) and Chapter 8 (#litres_trial_promo) and which hitherto I should never have dared to include as inhabitants of British waters. He defines his Lusitanian fauna as that which “originating in the outflow from the Mediterranean, has become modified by admixture with fauna from the area between the Azores and Bay of Biscay.” This work is an outstanding example of the importance of natural history in helping us to have a better understanding of the physics of the sea. I will give a final quotation from his work:
“The whole of this oceanic system to the north and west of Scotland overlies a south tending mass of artic or boreal water. The main flow of this water mass is to the west of Faroe from whence it thrusts southwards in deep water (below about 1,000m.), but part also penetrates the Faroe Channel where it is checked by the Wyville Thomson Ridge.
(#ulink_f5885b56-36c1-5d71-ad68-bb7fbd6399fb) Although this water affects the inflowing system where it mixes at its interface it is not of such importance as are the more massive cold water currents on the other side of the Atlantic.
“Each of the above water masses has a typical plankton fauna (see Russell 1939, and earlier works), which varies within certain limits, in the abundance and in the proportions of its constituent species from year to year. As these organisms are transported further from their natural habitat they gradually die as their limit of tolerance is reached, and they are replaced by other species through mixing either with other oceanic streams or with coastal water. The fauna of an incoming water mass thus gradually changes along its length; for example, few of the oceanic species noted off Scotland normally reach north-western Norway (Wiborg 1954). The degree of survival of the original fauna gives a measure of the purity of the inflow, and the relative life of the species less tolerant to various factors may give an indication of the type of dilution or change involved.”
(#ulink_534c11bf-fb7f-5157-a059-80d613c3cc0e) A valuable review of the changes in the southern North Sea, clue to variations in the influence of the low salinity water from the Baltic and that of the higher salinity water from the Channel, has been made by Lucas and Rae (1946).
(#ulink_bd866a47-5ab5-5497-aaf0-f45fbb2d3351) This will be referred to again in chapter 15, see here (#litres_trial_promo).
(#ulink_86f4bd14-62a8-5c14-b5bd-cb3514905b99) The difference between the two is shown in Fig. 42 (#litres_trial_promo).
(#ulink_067bab02-ceff-5f3b-bf76-329fc94211cc)see here (#litres_trial_promo).
CHAPTER 3 (#ulink_b58f47bc-0a94-5b5f-8c94-7f01869e6090) PLANTS OF THE PLANKTON
HAVING DISCUSSED the movement of the waters it might perhaps seem more logical to pass on at once to consider other physical characters of the sea and something of its chemistry before proceeding to deal with any of the life within it. On the other hand, since the plants of the open sea are so intimately dependent upon their physical and chemical background it will be more interesting if we know what kind of plants we are dealing with before we actually discuss the conditions which are most favourable for their growth.
The vegetation of the open sea must be floating freely in the water in order to be sufficiently near the surface to get enough light; the great difference between it and that of the coasts or land, is that it consists entirely of plants of microscopic size. They are, as we have already seen, part of the plankton: the phytoplankton. Each is composed of just one unit of life—a single cell—instead of being made up, as are the larger plants, of a vast number of such units. Instead of having various kinds of cells specialised to perform different functions in organs such as roots, stems, leaves and reproductive bodies, all these activities of life are carried out by just one highly organised unit. It does at first sight seem strange that there should not be even a few larger plants adapted for such a floating existence. There is the famous Gulf-weed Sargassum which, buoyed up by the little floats upon its fronds, is found in masses drifting round that great eddy of the tropical Atlantic—the Sargasso Sea; this however is not a true open-ocean plant for it has been broken away by wave action from the coasts of Central America and the West Indies. It continues to grow for a long time but never produces reproductive organs as it does when attached to its native rocks. In our own waters we may sometimes meet with patches of bladder-wrack, Fucus vesiculosus, torn from the sea-shore by storms and floating in the same way. They are but mutilated stragglers, out of place and lost in the open sea.
The microscopic plants must have some great advantage over larger plants in this floating drifting life. The smaller an object is the larger is its surface in relation to its volume. If we increase the size of an object—keeping its shape in the same form—the volume increases by the cube of linear measurement but the surface does so only by the square. This elementary fact is so important in the present discussion that it may be well to emphasise it by a simple concrete example. If we have eight small cubes of soap of the same size and press them together to form one big cube, the volume of this new cube will then be eight times that of one of the smaller ones, whereas its surface will be only four times as large. We have of course lost all the surfaces that were pressed and fused together. Inversely, the more we cut up our soap into smaller and smaller cubes, the more surface will each new cube have in proportion to its volume. A cube the size of one of our little plants will have a surface-volume ratio many hundreds of times as large as that of a cube with a side no more than an inch or two. A large surface-volume ratio is a great advantage to our little plants in at least two important respects. Firstly, the larger the surface in relation to mass the greater will be the frictional resistance to the water which will retard its sinking and so enable it to remain more easily in the upper sunlit layers. Secondly, since absorption must take place through the surface, the larger its surface in proportion to its volume the more readily will it be able to take up for its needs enough of the necessary mineral salts which may be present in the water in only very small amounts. This indeed may be the cardinal factor which has prohibited the development of larger plants in the plankton; but for this they might well have evolved bladder-like floats to support their larger mass, as some animals have done. Each tiny plant, as a single cell, can also take better advantage of the scattered sunlight than can a number of such cells massed together.
It is at first difficult to believe that these finely scattered and microscopic plants can really form a vegetation which has sufficient bulk to support all the teeming animal life of the sea: the dense populations of planktonic crustaceans, the vast shoals offish and all the invertebrate animals on the sea-bed. Yet we know this must be so. Some estimates of the actual quantities of plants present in a cubic metre of sea-water will be given later in the chapter; here, in passing, we will only note that, given suitable conditions, the amount of plant life produced under a given area of sea may well exceed that produced for the same area in a tropical forest. Just as our coal supplies are giving us the energy stored up in the great primaeval forests of some hundred million years ago—so is the energy in the petrol, which drives our motor and flying age, derived from that originally trapped from the sunlight by the tiny planktonic plants in the seas of long ago. According to current geological theory, the great supplies of mineral oil have been formed, in the course of ages, from the remains of marine organisms buried in sedementation under specially favourable conditions which are not yet fully understood. It is most likely that the planktonic crustaceans, whose modern representatives are so rich in oil, would in the past be the main contributors to the supplies of petrol we are burning up today; those crustaceans, of course, derived their energy either directly or indirectly from such tiny plants as we are now considering.
A microscope of sufficient power to enable us to see a great deal of this world of planktonic plants and animals need not be an elaborate one, nor need it cost much more than a good pair of field-glasses. We shall want some glass slides and coverslips, small dishes (such as watch-glasses), pipettes, i.e. old-fashioned fountain-pen fillers, for picking up very small plankton animals, and some glass jam jars; apart from that, all we need is a tow-net and line with which to collect the plankton from a rowing boat or any larger vessel that can be made to go slowly enough.
FIG. 12
A simple form of the plankton collecting tow-net.
A tow-net can be bought from the laboratory of the Marine Biological Association at Plymouth (address: The Laboratory, Citadel Hill, Plymouth) or it can be home-made. It consists essentially of three parts: in front is a hoop made either of light galvanised iron or strong cane and provided with three bridles of cord which will come together at a small ring or shackle for attaching to the towing rope; next comes the actual net, a conical bag made of a fabric which will act as a fine sieve; lastly at the end of the net is a small collecting jar, either a glass honey-jar or one made of zinc or copper with a slight lip. Such a simple tow-net is shown in Fig. 12 (#litres_trial_promo). For ordinary collecting purposes a hoop of 18 inches diameter will be quite sufficient. The net is best made of the silk ‘bolting cloth’ used by millers for sieving flour, but a good quality muslin will do if this cannot be obtained; with a mouth of 18 inches it should be almost five feet in length. If it is to be homemade great care should be taken in cutting out the material in order to ensure that a perfect cone is formed; if lop-sided it will not fish properly. It is a good plan to pin together a paper model to serve as a pattern; this will also enable one to see how best to use the material with as little waste as possible. Round its wide mouth a canvas or calico band is sewn for attachment to the hoop; it may either be provided with a series of eyes for lashing it on or it may be folded over the hoop and sewn to. enclose it, leaving gaps where the towing bridles are secured. At the hind end is sewn another canvas band to form a small cylinder, say 2½ inches in diameter, which will slip closely over the mouth of the collecting jar and be firmly held in position by a tightly tied tape. It is well to be provided with two such tow-nets; one made of the very finest material for the collection of the small plants—the finest bolting-cloth has 200 threads to the linear inch—and one of coarser material, having about 60 threads to the inch, for the capture of the somewhat larger animals. The coarser net lets most of the plants go through its mesh but filters a very much larger quantity of water more quickly and so captures the larger more active animals which are only rarely taken in the finer net.
To collect the phytoplankton the fine net should be towed just a little way below the surface. A weight, say a 71b. lead, is slung at the end of the rope and the net attached a little way above it. The essence of successful tow-netting is to tow very slowly, never at more than 1½ knots. If it is towed faster the water will not be filtered quick enough; the net will just push a mass of water in front of it which will prevent any more water entering it. A ten minutes’ tow may give quite a large enough sample. Most of the plankton will have passed down into the jar at the end as it is towed; a number of specimens, however, may still be sticking to the inside of the net as it is taken from the water, so that it should be carefully washed down from the outside with water from a bucket, to flush them into the jar.
Our sample will contain a vast number of both plants and animals. In this chapter we will concern ourselves only with the former, which are so small that they must be looked for with the compound microscope. After bringing our sample home and letting it stand for a little we should take only a few drops at a time with a fine pipette from near the bottom and place them on a slide under a coverslip; now we shall hunt with the low-power lens and then turn on the high-power to examine each new specimen we find. We shall not, of course, expect to find examples of all the different kinds in one sample but there may well be representatives of several of the more important groups. The most prominent members of the phytoplankton are the diatoms. They are unicellular algae differing from all other algae in having a cell wall which forms a siliceous external skeleton enclosing the cell like a glass box. The pigment bodies, or chloroplasts, which enable the plant to make use of the energy of sunlight are not the usual bright green of chlorophyll but a brown or brownish-green pigment closely allied to it. The siliceous skeleton is in two parts which fit together like the top and bottom of a pill-box; indeed some of the diatoms are just like a pill-box in form, but many others are drawn out into all manner of fantastic shapes. When first we see a sample of plankton rich in diatoms under the high power of the microscope it is like looking at a group of crystal caskets filled with jewels as the strands of sparkling protoplasm and groups of amber chloroplasts catch the light. Every plant or animal cell consists of a mass of protoplasm with a more or less central body, the nucleus, which appears to govern its life; it is characteristic of the diatoms that, in addition, the protoplasm usually has large cavities in it containing clear fluid. The nucleus is usually central and surrounded by a mass of protoplasm; radiating from this and forming an irregular network are protoplasmic strands stretching across the cavities like the spokes of a wheel to join up with a layer of protoplasm which lines the inner surface of the boxlike covering. More rarely, in some forms, the nucleus may be in the layer of protoplasm at the side. The pigment granules usually lie more or less regularly spaced against the cell-wall, where they are exposed to the light; if, however, the light is too intense, they come close together either down the strands to the centre or to some other part of the cell where they can partly screen one another from the harmful effects of the rays.
The top, bottom and sides of the glass-like box are not made of just plain sheets of silica; their surfaces are sculptured with all manner of striations, pits and perforations forming intricate patterns peculiar to the different species. This detail of design has always made the diatoms favourite specimens with microscopists, not only on account of their beauty, but because they are such excellent objects with which to test and display the quality of their instruments in the higher ranges of magnification. They are now being put under the electron microscope which can give a micrograph with a magnification of up to 100,000 times; this has at once revealed an arrangement of structure far more elaborate than that seen with the highest powers available in the optical systems (Hendy, Cushing and Ripley, 1954). Instead of there being just one system of pits or perforations in their walls, some forms are shown to have smaller and yet smaller ones, secondary and tertiary systems, on inner layers of silica; in others the wall is more like a basket of spiral threads intricately woven together. Some of the perforations measured were less than a ten-thousandth of a millimetre in diameter and the surrounding walls were of equal thickness. These delicate lattice systems have at least two important qualities for floating plants: they give strength with lightness and at the same time provide a framework for presenting a greatly increased surface area of protoplasm to the surrounding water.
FIG. 13
Diagrams showing the division of a simple pillbox-like type of diatom. a, a sketch of the cell before division has begun, b to d sections through the diatom after cell division to show stages in the formation of the new skeletal cell walls. Note that the upper cell in d is smaller than the original cell b.
Diatoms normally reproduce by simply dividing in two. The nucleus divides first and then the protoplasm becomes separated into two masses, each containing a nucleus, one at either end of the box; each mass of protoplasm now forms, between it and the other mass, a new valve as the halves of the pill-box are called. These new valves each fit closely their own part of the old box; we have in fact two pillboxes now instead of one, as is shown above in Fig. 13 (#litres_trial_promo). They may separate entirely, or in some species they may remain attached to form long chains. It will be realised that in this process of repeated division by forming new half-boxes within the old, the average size of the diatoms so produced will tend to get smaller and smaller; at each division, as shown in the drawing, one of the new boxes will be the same size as the old one but the other must be smaller. Thus we find a considerable range in the size of diatoms of the same species, but there must be a limit to this reduction. After a certain number of such divisions there is formed what is called an auxospore, by which the original size is recovered; throwing off the old valves the cell becomes a bladder-like mass of protoplasm within which new valves are formed two or three times the size of the old discarded ones. Some diatoms seem to do this at definite seasons whereas others do so only at intervals of two or three years. By taking sample measurements of the diatoms forming some of the dense concentrations which are carried by currents about the North Sea, planktologists have been able to identify individual patches in their wanderings by the regular decrease in the size of their component cells (Wimpenny 1936; Lucas and Stubbings 1948). Some of these concentrations, as we shall see in Chapter 15 (#litres_trial_promo) have been thought to have a marked effect upon the herring fisheries; their wanderings may therefore be of economic interest.
In addition to forming auxospores, diatoms may produce what are called resting spores when conditions become adverse. The contents of the cell become concentrated in a central mass which forms a new thick wall of a different but characteristic shape and the old cell-wall is discarded. They now either sink into the deeper water layers or right to the bottom where they will remain till more suitable conditions return; thus some may pass the winter in a resting state and then come up to start active life again in the spring. Planktonic diatoms are not definitely known to have any sexual phase, but in some a number of smaller spores (microspores) have occasionally been observed to be formed within the cell-wall and it has been thought that these may be gametes (sex cells), but this is not yet established.
Not all diatoms are planktonic; in the shallow coastal regions there are numbers living on the bottom where sufficient light reaches it; these have much thicker shells than the more delicate floating forms and are more uniform in character. What makes the planktonic diatoms so interesting is the variety of devices that have been evolved to assist in their flotation. It is not the purpose of this book to attempt a systematic treatment of the groups of animals and plants. Here we shall just refer to some of the more important kinds of diatoms in relation to their mode of suspension. Dr. Marie Lebour’s excellent book (1930) on the planktonic forms should be studied for a full account.
Although so very small they have a considerable range in size; a few exceptionally large ones may be over one millimetre in diameter and the smallest may be but a few thousandths of this. Sketches of examples of the different genera to be mentioned are shown in Fig. 14 (#litres_trial_promo) and Plate 1 (#litres_trial_promo), and some are also included in the photographs in Plate I (#litres_trial_promo). A number have the typical pill-box form, such as members of the genus Coscinodiscus and some of these are of comparatively large size; a few like C. concinnus may be just visible to the naked eye. These kinds have large vacuoles and would seem to be buoyed up by globules of oil. Members of this genus live singly, but others of allied genera may remain after division attached together to form long chains; the cells of Paralia and Guinardia are linked rigidly together by their valve surfaces, those of Thalassiosira form flexible chains as they are strung together by fine threads of protoplasm, and others like Lauderia are still more loosely held together by irregular strands of slime. The cells of Thalassiosira also produce such slime-strands but for a different purpose—around the margin of each valve (i.e. each ‘pill-box lid’) are a number of small hollow spines from which can be extruded long slender threads of slime so that they radiate on all sides like the strands of thistle-down and indeed act in the same way to assist in parachute-like support.
The remainder of the planktonic diatoms, while essentially built on this pillbox plan, have each valve or half-box modified into all sorts of shapes which increase their surface area in relation to their volume and so give greater frictional resistance to sinking. Some are flattened like thin sheets of paper and often twisted to some extent; these usually remain attached together to form long ribbons: such are Bellarochia, Eucampia and Streptotheca. Others are drawn out either into long thin hair-like forms such as Thalassiothrix longissima or into the more rigid pencil or needle-like members of the genus Rhizosolenia, pointed at each end; in the former the division plane between the two valves runs lengthwise along the thread whereas in the latter it occurs transversely to the long axis. Other forms again, such as Biddulphia and Corethron, increase their surface area by being provided with spines. This last method is developed to a remarkable extent by the many species of the genus Chaetoceros whose cells have four very long hair-like processes (two extending from each valve); long chains of these cells are formed and held together by their curving processes becoming interlocked with those of adjacent cells close against their point of origin.
FIG. 14
Some characteristic plankton diatoms not shown in Plate I (#litres_trial_promo), all magnified ×90 diam. a, Coscinodiscus concinnus; b, Bacillaria paradoxa; c, Thalassiosira gravida; d, Rhizosolenia styliformis; e, Paralia sulcata; f, Bellarochia maleus; g, Thalassiothrix nitzschioides; h, Streptotheca thamensis; i, Rhizosolenia hebetata (form semispina); j, Nitzschia seriata; k, Gyrosigma sp.; l, Chaetoceros curvisetus; m, Ch. convolutus. The actual length represented by the longer side of this figure is 1/20th of an inch.
There are many species of Biddulphia in our waters, but one, B. sinensis (Plate 1 (#litres_trial_promo)) is of special interest; it is now one of our commonest diatoms, often occurring in dense concentrations, yet it was unknown in European waters before 1903 when it was first recorded in the Heligoland Bight. It is a well known inhabitant of the coastal waters of the Indo-Pacific region, extending from the Red Sea to the coasts of China. It seems likely that it must have been brought, perhaps in ballast water, by some ship to the mouth of the Elbe; being tolerant of wide ranges of temperature and salinity, it found our waters congenial and spread rapidly. In the following years it was recorded further and further to the north until it reached a point a long way up the Norwegian coast where its further spread was probably checked by too cold water. More extraordinary, in view of the prevailing currents, was its spread down the Channel and into the Irish Sea; in the same year, 1909, it was reported for the first time both at Plymouth and off Port Erin in the Isle of Man. This would appear to provide evidence of an occasional reversal of the usual current flow up the Channel; indeed such a reversal has been suspected by some oceanographers on other grounds. Or was it transported to the western Channel and Irish Sea in the same way as it had apparently reached the Elbe? We may never know the answer to that. Some have maintained that Biddulphia sinensis must have been a native of our waters all the time and only just noticed at the beginning of the century; this, however, can hardly be so because of extensive collections that were made, particularly by the Kiel planktologists, throughout the eighteen-nineties.
In coastal regions we may find a number of typical bottom-living diatoms carried up into the plankton, such as the boat-shaped members of the genus Navicula. They are capable of a remarkable gliding movement, thought to be produced by a flow of protoplasm passed out through a slit in the wall of the cell. Gyrosigma is another bottom form often met with in the shallow-water plankton. Other related forms such as Bacillaria and Nitzschia are more planktonic but typical of coastal waters. B. paradoxa forms bands of long slender cells held together side by side like the planks in a raft yet each capable of sliding up and down along its neighbours; N. seriata, in contrast, forms long strings of narrow boat-shaped cells end to end with just their tips overlapping and in contact.
It is impossible to mention all the kinds of diatoms of our seas in such a general review, and I shall only refer to one other species, one which may well attract attention: Asterionella japonica (Plate 1 (#litres_trial_promo)). Its cells are rod-like but thickened at one end; by these thickened ends the cells remain attached to one another to form beautiful radiating star-like clusters.
In striking contrast to the brown-green colouring of the diatoms is the brilliant green sphere of Halosphaera viridis which may reach a size of nearly a millimetre in diameter; it and one or two closely allied species are the only representatives in the marine plankton of the Yellow-green Algae or Heterokontae. H. viridis is found over the whole Atlantic from the tropics to the far northern branches of the North Atlantic current off Spitsbergen. In autumn it is often brought into the northern North Sea in large numbers and is usually found floating very near the surface. It is exceptional in its mode of reproduction; it does not divide in two, but when full-grown undergoes multiple fission into a large number of small spores which break out of the surrounding envelope and swim, like the flagellates about to be described, by the use of whip-like locomotory organs. The full life-history has not yet been observed; whether after fusing with others or not, these spores must eventually give rise to the little green spheres which gradually grow to a full size again.
FIG. 15
Some flagellates of the plankton. a–h, Dinoflagellates: a, Ceratium fusus (×180); b, C.macroceros (×200); c, Protoerythropsis vigilans (×320) (note clear spherical lens against dark eye-spot); d, Dinophysis acuta (×400); e, Peridinium granii (×360); f and g, P. ovatum (×320), side and top view; h, Polykrikos schwarzi (×250); i and j, the Silicoflagellate Distephanus speculum living and half of skeleton (×320); k, a very small part of the large gelatinous capsule formed by the tiny cells of Phaeocystis; l and m, Coccolithophores (×1000): Coccolithus huxleyei and Coccosphaera leptopora; n–r, some of the smallest flagellates (×1500): n, Dicrateria inornata; o, Hemiselmis rufescens; p, Isochrysis galbana; q, Pyramimonas grossii; r, Chromulina pleiades. Original drawings except c from Marshall (1925), h from Lebour (1925), m from Murray and Blackman (1898) and n to r from Parke (1949).
All the remainder of the planktonic plants belong to the big assemblage of organisms known as flagellates of which there are many different kinds. A selection of the commoner forms is shown in Fig. 15 (#litres_trial_promo). They are all characterised by possessing at least one, and often two, of the motile whip-like processes termed flagella, with which they draw or propel themselves through the water and are thus able to keep up in the sunlit surface layers. These flagellates are claimed for study by both botanists and zoologists, for among them are indeed both plants and animals—and some which have the characters of both in one. Some possess green pigments allied to chlorophyll or even chorophyll itself, and so feed as true plants; others lack pigment and may feed either by absorbing organic substances through their surface or actually live as animals by capturing particulate food; yet again, others may combine the methods of plant and animal feeding. In this lowly group of organisms the animal kingdom has not yet become fully separated from the plant kingdom. However, most of the planktonic flagellates are in fact plants and most of them have a small red ‘eye-spot’ or stigma which is sensitive to light and so enables them to tell whether they are moving towards or away from the radiant energy necessary to build up their food. If you are able to obtain a plankton sample very rich in these small green flagellates you will be able to see how readily they are attracted upwards towards the light. Fill a tall narrow glass jar with the sample and cover the lower three-quarters with thick brown paper. Now if you leave it for half an hour in the full light of the window you will find on removing the paper that the top quarter of the jar is distinctly greener than the rest; the little flagellates from the whole jar have become concentrated in the sunlit zone. By standing a sample of sea water in the light you may be able to grow a more abundant culture of these little flagellates and so give a more striking demonstration of this experiment. There are some of the Dinoflagellates (members of the family Pouchetiidae) which have a much more elaborate light-sensory organ furnished with both a lens and a pigment-cup; indeed it might almost be called an eye. One of these is shown in Fig. 15c (#litres_trial_promo).
The Dinoflagellates are the most striking members of the phytoplankton after the diatoms and are usually present in large numbers; for a full account of them another excellent volume by Dr. Marie Lebour (1925) should be consulted. They have a cell wall made up of a number of plates of cellulose fitting together to form a mosaic and are characterised by possessing two flagella: one working transversely in a prominent groove which almost completely encircles the body like a girdle, the other projecting behind from out of a small longitudinal groove running backwards from the girdle. This latter groove is often protected by curtain-like membranes so that the flagellum may be withdrawn spirally into a sheath. Typically, as in Peridinium, they have a single spine pointing forward in front and two spines projecting backwards from the half of the body behind the girdle. The flagellum working in the groove sets them waltzing round as they are at the same time driven forwards by the other flagellum behind (Fig. 15 (#litres_trial_promo)); they screw themselves through the water. There are a great many species of Peridinium and closely allied genera with very much the same general appearance (Plate IIIb (#litres_trial_promo)); one related genus, however, Ceratium, is most striking in having the spines drawn out into long horns, with the two posterior ones usually curving forwards to give the whole body the shape of a little anchor. Sometimes, particularly in late summer and autumn, plankton samples may be full of Ceratium tripos or perhaps another of the many species of the genus distinguished by only small differences (Plate Ib (#litres_trial_promo)). Two species which are very common in our waters stand out in contrast to the rest: Ceratium furca (Plate 1 (#litres_trial_promo)) in which the posterior spines are rather short and point straight backwards, and C. fusus (Fig. 15 (#litres_trial_promo)) in which there is only one posterior spine, long and only very slightly curved, just like the anterior one. There is a remarkable range of colour in Ceratium from a bright green to a yellow-brown.
Dinoflagellates, like other flagellates, multiply by simple fission into two and, like diatoms, each daughter-cell retains one half of the old cell-wall and forms another half anew; but unlike the diatoms the new halves are not formed within the old cell wall and so there is no gradual diminution in size. Occasionally recently-divided individuals of Ceratium may remain adhering together to form little chains. The cell-wall grows in thickness but when the little plates of armour become too heavy they fall off to be replaced by new and extremely thin ones. The long horns of Ceratium are certainly organs to assist in suspension. Species in the warmer waters generally have much longer ones than those in colder waters. Warmer water is more fluid—or less viscid—than colder water, for the molecules move over one another more freely with greater heat-motion; in the tropics an object will sink twice as fast as will one of similar density and shape in the polar seas. In general plankton organisms, both plants and animals, are more spiny in the tropics to give them a greater surface resistance. It has been observed that species of Ceratium have the power of adjusting the length of their horns to the varying viscosity of the water; on being carried by currents into warmer water they grow longer horns, conversely if carried into a colder area they can shed parts of them. (Gran 1912).
Several species of Dinoflagellates can produce a brilliant phosphorescence. Many plankton animals are luminous and produce the sparks of light we often see in the water at night. In addition to such displays, however, we may also see a more general ghostly light or sometimes when out in a rowing boat our oar as it cuts the water may leave a trail of blue-green flame behind it; and even from the shore we may see the waves breaking in a flash of light. Such displays are caused by countless millions of dinoflagellates each glowing by an oxidation process as it is agitated in the water. Noctiluca is the most celebrated for this, but although a dinoflagellate it is curiously modified to be entirely animal in its mode of life and so will be described in a later chapter (see here (#litres_trial_promo)); other members of the group, however, particularly Ceratium, give almost as good a display. Once on a fisheries research trawler, having stopped at night to make some observations in the Channel, I looked over the side to see a small shoal of fish, most likely mackerel, lit up by each individual being covered by a coat of fire; they were being chased this way and that by some much larger fish similarly aflame. On putting over a tow-net, which came up brilliantly illuminated, the sea was seen to be full of a very small Peridinium-like dinoflagellate of the genus Goniaulax.
Two other genera of dinoflagellates occuring in our waters, and shown in Fig. 15 (#litres_trial_promo), will just be mentioned. Dinophysis has the part of the body in front of the girdle reduced to a minimum so that the girdle itself, with very pronounced margins to its groove, appears like a band round its very front or top; the posterior part bears a marked keel at one side as if designed to prevent the rotation which is normal to the group. Instead of spinning round it is thought to set up a vortex current by which it draws into the groove still smaller organisms as food. Polykrikos is a remarkable genus having a number of girdles, usually four or eight, placed in regular succession down the body; it is often spoken of as a ‘colonial form’ as if made up of several individuals which have failed to separate on division, but this can hardly be the correct view since the number of nuclei is always smaller than the number of girdles present. It appears to be an individual with a repetition of organs similar to the segments of an animal like an annelid worm. They are also said to feed like animals as well as like plants; they possess remarkable little capsules containing coiled threads which can be shot out like those found in the stinging cells of sea anemones and jelly-fish, and may possibly be used for a similar purpose—the capture of prey. In addition to all these forms with their different characteristic patterns of armour plating and spines, there are a great many so called ‘naked’ dinoflagellates which lack all such coverings; many of these are, for part of their lives, internal parasites in a number of different marine animals.
Among the small shells of Globigerina first brought up from the ooze of the ocean bed were found numbers of still smaller calcareous bodies, little plates, some oval and perforated, others round and bearing stout blunt spines; they were called coccoliths and rabdoliths respectively, and presented naturalists with a puzzle as to what they were. It was Sir John Murray who discovered their real nature by showing them to be plates which had covered the bodies of other little plank-tonic flagellates which were given the name of Coccolithophores.
(#ulink_915325a6-6fdf-5ea6-8c39-8c85b71158f3)Coccosphaera and Coccolithus (Fig. 15 (#litres_trial_promo)) occur in our seas. They are commoner in the tropics, although in the Atlantic water coming into the northern North Sea they may occasionally be so numerous as to give a milky appearance to the water and cause a chalky deposit to be left on the fishing nets as they dry. This is what the herring fishermen call ‘white water’ and generally believe to be a good sign for the presence of herring. A well-known herring skipper, Mr. Ronald Balls, who is also a keen naturalist, has recently written, under the pen-name of “Peko”, an excellent article on this white water in World Fishing (July 1954). He describes how this water gives ‘the queer impression of whiteness coming upwards: as if the light was below the sea instead of above it’. He then refers to recent views that the coccoliths are shields reflecting light from their owners which normally live in tropical seas where the illumination is too strong; ‘and here’, he writes, ‘was the perfect explanation of the fairy glow or white reflection that I had experienced long ago, and wrote about before I knew even that this organism existed’. As with the cell walls of diatoms, the electron microscope is showing that each little plate or coccolith has a much more complicated structure than was originally supposed; its base consists of radiating ribs like the spokes of a wheel and its rim is decorated with a frill like that with which a chef may decorate a ham. There are other similar little creatures, the Silicoflagellates, which form a delicate siliceous skeleton with radiating spines (Fig. 15 (#litres_trial_promo), i and j).
Herring nets, although they hang in the water near the surface, may often come out of it in a very slimy condition; this may be due to an excessive number of diatoms; more usually, however, it is due to globules of jelly large enough to be seen quite easily by the unaided eye. These slimy blobs are produced by aggregates of microscopic flagellates called Phaeocystis which colour the surface of the jelly in green patches (Fig. 15k (#litres_trial_promo)). All the meshes of a tow-net may be blocked with them. Dense concentrations of Phaeocystis, like those of diatoms, which cover wide areas of sea, have also been thought to have a deleterious effect on the shoaling of herring and at times to have led to a poor or delayed fishery. We shall refer to this again in Chapter 15 (#litres_trial_promo).
This completes the review of those planktonic plants we shall mention by name; but we have so far left out of account a vast number of still much smaller flagellates which have escaped capture by passing through the meshes of the finest net we can use. It is only comparatively recently that their influence in the economy of the sea has been realised. Their prominence was first demonstrated by the German naturalist Lohmann who examined the remarkably fine filtering mechanism, far finer than any gauze that man can make, used for their capture by some little plankton animals, the Larvacea, to be described here (#litres_trial_promo). They may, however, be extracted from a sample of sea water by centrifuging
(#ulink_a62ac299-94ae-50d9-a773-49ac31ea8893) small quantities of it in tapering tubes. If after such treatment the greater part of the water is carefully decanted, a drop of the remaining fluid may be taken up in a pipette and examined on a slide under the high power of a microscope; then they will be seen as tiny yellow specks jigging in the water Today a great many of these minute flagellates are being successfully cultured in the laboratory, notably by Dr. Mary Parke at Plymouth (1949). Five examples, sketched in line from her beautiful coloured drawings, are shown in Fig. 15 (#litres_trial_promo)n–r.
Smaller still, of course, are the bacteria which really lie outside the scope of this book; at present, very little is known about their occurrence in the plankton. Dr. H. W. Harvey (1945) states that their population density decreases on passing from inshore waters to the open sea and that in the ocean the greatest numbers are found where phytoplankton is abundant and in the water immediately above the sea-floor. They are found particularly in dense phytoplankton regions because of the undigested organic matter passed out by the animals which are eating more of the plants than they really require.
It is exceedingly difficult to get an accurate measure of the amount of plant life in a given quantity of sea water, even of the larger forms which are captured by a net. Although we can calculate the filtering efficiency of the net and know the quantity of water it should filter, we cannot be sure that it actually does filter this amount; in fact it rarely does, for to a varying extent under different conditions the meshes of the net become clogged by the organisms themselves and the filtering is much reduced. However we can get an approximate idea of the number of larger forms—the diatoms and dinoflagellates—in a given volume of water by using a net. Let us take an example. In 1907 Sir William Herdman and his co-workers began an intensive study of the plankton of Port Erin Bay in the Isle of Man which they continued until the end of 1920. Usually six times a week, every week for fourteen years, two standard nets of coarse and fine mesh were towed in exactly the same way over the same distance—half a mile—across the bay. Johnstone, Scott and Chadwick, who describe the results in their book The Marine Plankton (1924), estimate that for each such double haul “taking the two nets we shall not be very much in error (when all the conditions are considered), in assuming that 8 cubic metres of water were filtered through both nets.” The following figures, taken from their book, give the average number of the principal plant forms taken in such a catch during the month of April, i.e. the average of all the April hauls made over fourteen years:
That in round figures is 727,000 per cubic metre or about 20,000 per cubic foot. The number of plankton animals taken at the same time is given in Chapter 5 (#ub9d1ef9b-f5a9-51ce-96c7-37d3c4e2a8de) where the zooplankton is considered and may be compared here (#litres_trial_promo). The actual numbers present are estimated by using a specially calibrated pipette which takes up a known fraction of the sample; the fraction is spread out on a glass slide ruled in squares so that the number of plant cells can be counted below the microscope just as the corpuscles are counted in a sample of blood. We must remember two important things about the figures just given. Firstly they are for the larger microscopic plants; the very small ones are present in far greater numbers as we shall see in a moment. Secondly they are average figures for April over fourteen years; those for one year may be very different from those of another and the average figures for other months of the year will show still greater differences. There are marked seasonal changes in the plankton; but that is the subject of our next chapter.
The difficulty of knowing exactly how much water is filtered by a net when its meshes are becoming clogged by the organisms sampled, has been got over by an ingenious device invented by Dr. Harvey of Plymouth (1934). At the mouth of the tow-net he has fixed a little propeller which is turned by the water flowing into it; the number of revolutions it makes are recorded on little dials which measure the amount of water actually passed through the net. There is still, however, the difficulty of forming a true quantitative estimate of the plant life present. We can calculate, as we have just seen, the number of plant-cells in the sample; but these vary so enormously in size it is difficult to convert such an estimate into a measure of the total bulk of planktonic vegetation. Measurements of the volume of the sample can be made after all the plants have been killed by the addition of formalin and allowed to settle for several days in the bottom of a measuring jar; but this too is a very misleading estimate, because the various kinds, having different shapes, may pack together very differently: for example spiny forms take up more space than round or flat ones. However, these various methods do enable us to say broadly that one area is relatively so much richer in phytoplankton than another—always excluding the small flagellates which escape the net and must be estimated with the centrifuge. A more recent method of estimating the quantity of plant life caught in a plankton sample is to extract the plant pigment by acetone and measure the quantity present by matching up the samples obtained with a standard colour scale and expressing it in so many pigment units.
The late Dr. E. J. Allen (1919), when Director of the Plymouth Laboratory, made a simple but important experiment that gives us some idea of the vast numbers of little plants there are in the sea which are not caught by our ordinary methods. He had first perfected a method of growing them in bottles in a special culture solution, i.e. in sea water enriched with the addition of certain beneficial chemicals. He then took a sterilised quart-sized bottle and filled it with sea water from just below the surface about half a mile outside the Plymouth breakwater. This water he treated in two ways. The procedure may seem a little involved but it is worth following. Firstly he took four 10 cc samples of it and centrifuged them each twice with the result that he obtained an average of 14.45 organisms per 1 cc of water which gives us an estimate of 14,450 per litre. Secondly he took just ½ cc of the water he had collected and added it to 1,500 cc of his culture solution which he had previously sterilized; then after it had been thoroughly shaken up he divided this between 70 small flasks—a little over 20 cc in each—and placed them against a north window. After 10 days signs of growth were apparent. When they were finally examined there was not a flask that had not had some growth in it. He now recorded the different kinds of organisms in each. In two flasks there was only one species; in all the others there were from two to seven different species present, giving an average of 3.3 different kinds per flask. Thus at least 70 × 3.3 or 231 separate plants must have been taken up in the ½ cc originally added to the culture solution; that makes 464,000 per litre as compared with the 14,450 estimated by using the centrifuge! For comparison with the larger plant forms caught by the net in the former example we must express the number as per cubic metre: i.e. 464 million, or about 12½ million per cubic foot. Now this must be regarded as an absolute minimal estimate, for it is made by assuming that only one individual of each kind of plant recorded in a flask went into that flask at the beginning; this is most unlikely.
We begin to have some idea of the great wealth of plant life there is in the sea. Can we make it still richer by adding fertilizers in the same way as we increase our crops on land? Experiments have been made in that direction, but a discussion of them will come better in the next chapter, where we will deal with the various factors which govern phytoplankton production. For a more detailed and fuller account of the pelagic plants in general I would recommend for further reading the splendid chapter by Professor H. H. Gran in Murray and Hjort’s Depths of the Ocean (1912).
(#ulink_2b4c27ff-fcc8-5b2f-a31d-36f3f0c6dd4b) They were subsequently well described by G. Murray and V. H. Blackman 1898).
(#ulink_3aa26175-8e8a-51fc-a388-e5531838abd4) Subjecting to a force greater than gravity by spinning in a rotary apparatus: the centrifuge.
CHAPTER 4 (#ulink_a3346a5f-9582-5e90-b849-8ef1e018c7ee) SEASONS IN THE SEA
THE NATURALIST with a tow-net, if he can sample the plankton at different times of the year, will find contrasts between spring, summer, autumn and winter in our seas almost as striking as those in the vegetation on the land. These seasonal changes in the plankton have a profound effect on the lives of many fish. Just as we can tell the age of a felled tree by the number of concentric rings in its trunk representing summer and winter growth-zones, so we can tell the age of a herring by similar rings on its scales; these mark summer growth-periods, when its planktonic food was abundant, separated by lines showing where the scale, and the fish, had ceased to grow during winter when the plankton was scarce.
There is not, however, a simple and gradual increase in the plankton as spring advances into summer followed by a gradual decline in the autumn. Our naturalist with a tow-net will find some of the changes very puzzling at first sight. In British waters in the winter there is a general paucity of both animals and plants in the plankton; then as the sunlight grows stronger (the date varying in different years, but usually in March) there is a sudden outburst of plant activity. The little diatoms start dividing at a prodigious rate: in a week they may have multiplied a hundred-fold and by a fortnight perhaps ten-thousand-fold. The meshes of the tow-net are clogged by them and the little jar at its end is filled with a brown-green slime, a slime which under the microscope resolves itself into a myriad forms of beautiful design. Then as spring advances into summer the number of little floating plants steadily declines until by late summer there are surprisingly few. Some reduction in their numbers might indeed be expected, for the little animals in the plankton which feed on them are also multiplying as the season advances and the waters are warming up; with the increasing sunlight, however, we might have thought that the plants’ remarkable power of increase could largely keep pace with the grazing of the animals. Something seems to be preventing the diatoms from keeping up that rapid multiplication. In the autumn comes another surprise. As the days begin to shorten and the sunlight is getting less intense, when in fact we might least expect a renewal of plant activity, there comes a second phytoplankton outburst; it is not as spectacular as the spring maximum and not in every year is it of an equal intensity, but there it is—a definite surging up again of reproductive power. From this second peak of production, as winter approaches, the numbers fall again to the lowest level of the year.
This sequence of events was known for a long time before it was properly understood; it was only after much more had been discovered about the physics and chemistry of the sea that it was possible to see at all clearly the chain of cause and effect throughout the year. So important are these events that we must devote a little space to considering some of the more important elements in the physical and chemical background that will help us to explain them.
Let us first consider some of the physical properties of the water. At the very beginning, in the introductory chapter (#ubcb89914-6a12-5f9a-961d-a32f7a64e31a), we referred to the limited transparency of the sea and some figures were given to show how quickly light is actually absorbed on its passage below the surface. As might be expected absorption of light will be found to vary considerably according to the amount of suspended matter, either sediment or plankton, in the water. Far out from the land the water is usually much clearer than in the shallower regions against the coast where detritus and mud may continually be stirred up by the tides or brought in by drainage from the land. If we compare measurements of light made at different depths below the surface in the waters near Plymouth we find the penetration in inshore waters at Cawsand Bay to be only half what it is at a point some 10 miles S.W. of the Eddystone Lighthouse (Poole and Atkins, 1926). Taking the light entering the sea, i.e. just below the surface, as our standard, we find in Cawsand Bay that half of it has been absorbed at 2 metres depth (i.e. 1 fathom), some 75% at 4½ metres and 90% at about 8 metres depth; whereas at 10 miles out the same percentage reductions in light intensity are found at depths of about 4¼, 9½ and 17 metres respectively. At points in the open sub-tropical or tropical Atlantic, where the phytoplankton is very sparse, as in the Sargasso Sea, the corresponding depths might be increased four or five times. A very simple bit of apparatus known as the Secchi disc, which can easily be home-made, will enable you to compare the transparency of the sea at different points; you may well be surprised at some of the results you will get with it. Take a white painted metal disc, say two feet in diameter, and drill three equidistant holes near its margin; now take three cords each about 6 feet long, tie them to a weight, say a 7–1b lead, and then tie a knot in each at 3 feet from the lead; next pass the cords through the holes in the disc and bring them together as supporting bridles to be tied to a loop or eye at the end of the line which will suspend the whole device in the water as shown in Fig. 16 (#litres_trial_promo). If a metal disc cannot easily be obtained, a white dinner plate, with wire clips behind it to take the cords will serve the purpose quite well. To compare the transparency of the sea all you have to do is to stop your boat and lower the disc over the side at different places and find how deep it must go before you can no longer see it. The weight not only carries the disc down but, if the cords are properly adjusted, ensures that it is always kept horizontal. The line can be knotted at metre intervals to facilitate measuring the depth. It is well to raise and lower it about the disappearance point several times in order to make quite sure just at what depth it goes out of sight; at some places it may vanish in only 5 metres, at others it may be seen for as much as 12 metres or more.
FIG. 16
The Secchi disc for measuring the transparency of the sea.
In passing I may say that the Secchi disc is also valuable in helping us to compare the varying colour of the water. Here, of course, I am not thinking of the often striking and delightful changes of hue which we may see as light of different quality is reflected from a changing sky: when for example dark grey clouds give place to an open space of blue or when cumulus clouds dapple the sea with purple shadows. Reference was made in Chapter 2 (#u3397ab9e-de11-5382-badf-93137316dee4) to the contrast between the green water of the Channel and the deep blue of the Atlantic; such differences are due to the nature of the contents of the sea itself and are examples of what I mean by the varying colour of the water. A great variety of shades and hues may be found at different times; these are mainly due to the presence of different kinds of very small plankton organisms in exceptional numbers. The light reflected back through the water from the white background of the disc enables us to judge and compare these differences more easily than by just looking into the depths. Dense concentrations of diatoms such as Rhizosolenia and Biddulphia, or of the colonial flagellate Phaeocystis, may give a brown appearance to the water over large areas; the North Sea fishermen often call such patches of water “Dutchman’s baccy juice”. Some dinoflagellates may make the water almost red, coccolithophores may give it the white milky appearance referred to in the last chapter, and other small flagellates may occasionally make it a vivid green.
But let us return to the sunlight and these little plants of the sea; in order to flourish and grow they must produce more oxygen in the process of photosynthesis (see here (#ulink_6442f009-7e57-5b3f-89d0-645624049396)) than they use up in respiration. Plants, of course, breathe as well as animals. Some very significant experiments were performed in the Clyde sea-area by Drs. Marshall and Orr (1928) of the marine biological station at Millport on the Island of Cumbrae. They grew cultures of diatoms in glass bottles in the sea at different depths; these they suspended on strings from a long thin rod between two buoys at the surface and so kept them free of shadow. All their bottles were in pairs; one of each pair was exposed to the light and the other covered with a black cloth. In each bottle the oxygen-content of the water was measured at the beginning of the experiment and again at the end of twenty-four hours. An increase in the oxygen in the uncovered bottles showed the amount produced by photosynthesis less that used up in respiration; a fall in oxygen-content in the ‘blacked-out’ bottles measured respiration alone. By adding this oxygen-loss to the oxygen measured in the uncovered bottles the total oxygen-production as a record of photosynthesis could be estimated. The experiments were repeated as the spring passed into summer, and were also made on days which were overcast and on others which were sunny. As the sun went higher in the sky and the light became more intense the depth at which diatoms could produce more oxygen than they used in respiration increased from a depth of less than 10 metres on an overcast day in March to nearly 30 metres on a sunny day at midsummer. By far the greatest photosynthetic activity—on which their growth depends—took place, however, in the top 5 metres. In the waters round Great Britain we may now say that practically all the plant-production that matters takes place in the top 10 or 15 metres. This is one important clue in the puzzle of the seasons; we must now turn to temperature.
FIG. 17
The Nansen-Pettersen water sampling bottle: shown open and closed.
The water round our coasts varies in temperature from about 8°C in winter to sometimes as much as 17°C in the Channel in a warm summer. It is, of course, because the sea loses and gains heat so much more slowly than the land that we in Britain have so equitable a climate compared to that of an area in the middle of a continent. Two methods are used in taking the temperature of the sea. Down to moderate depths, say to 50 metres, the insulated Nansen-Petterson water-bottle, which is shown in Fig. 17 (#litres_trial_promo) is used; it is of metal and is sent down suspended on a wire to obtain samples of water both for chemical analysis and for temperature determination. It goes down with the bottom and top open so that water can circulate through it; then at the required depth a small ‘messenger’ weight is sent sliding down the wire to hit a trigger which releases springs to close it. Projecting through the top, in a protective casing, is the stem of a thermometer whose bulb is in the centre of the sampling bottle; its scale and mercury thread are visible through a slit in the upper casing so that it can be read as soon as the bottle is brought back to the surface. There are actually three walls to the cylindrical bottle, one inside the other, with a little space between; when the top and bottom are firmly closed there are thus two water jackets outside the bottle proper and these act as insulating chambers preventing loss or gain of heat in the water sample while it is coming up and the thermometer is being read. As soon as the temperature has been noted the water is run out from a cock at the bottom to be stored for later analysis and the bottle is opened ready to be sent down to another level. From much greater depths the bottle would take so long being drawn up that the insulation just described would not be adequate to prevent a change of temperature in the process. To get over this, special so-called reversing thermometers and bottles have been devised. The mercury tube of the thermometer, just above the bulb, has a loop and a kink in it, so that when it is swung rapidly upside down the thread of mercury breaks; as soon as this happens all the mercury that before was above the kink now runs to the opposite, and now lower, end of the tube. When it is brought up the height of this inverted column of mercury is seen against a scale which can only be read when the thermometer is upside down; it tells us the temperature that the thermometer was recording at the moment it was turned over. The bottle and thermometers (there are usually two to give check readings) are mounted in a frame which rotates when a trigger is hit by a messenger weight; the bottle, which before was open, is closed as it swings over.
(#ulink_4b8d8474-868f-5ef4-9b22-459132cbd589)
After this digression on thermometers, let us return to consider the temperatures of our seas with the passing of the seasons. Water, above 4°C, expands when warmed and contracts when cooled; so its density is altered: a given volume of cold water weighing more than the same volume of warm water. In winter the atmosphere is colder than the sea so that the surface waters are cooled and therefore sink beneath the warmer and less dense layers which were below; this is repeated again and again until after a time there is an almost uniform low temperature from top to bottom. The winter gales help in the process of mixing up the layers too. The sea, of course, is rarely so cold in winter or so warm in summer as is the atmosphere; as we have already noted, it gains and loses heat much more slowly. As spring passes into summer the air warms up and the radiant heat of the sun gets stronger, so we find the upper layers of the sea becoming warmer too; as they heat up they become increasingly lighter than the layers below and thus tend more and more to remain separated on the top because less and less are they likely to be mixed with the heavier waters beneath. This division between the upper and lower waters is called a discontinuity layer (or thermocline in still more technical language) and is usually set up at a depth of round about 15 metres. Let us take an actual example from the summer temperatures in the English Channel in July as found by the hydrologists of the Plymouth Laboratory. At depths from just below the surface down to 15 metres the temperature only varied from 16.5° to 15.82°C; but at 17½ metres it had dropped to 12.09°C and then, as it was sampled deeper and deeper, it remained practically constant to read 12.03°C at 60 metres. The upper layer was effectively cut off from the lower by this sudden drop in temperature of nearly 4°. A strong summer gale may destroy this discontinuity layer, but if it is not too late in the season it will soon form again. It is in the autumn that the air cools again and so the surface water loses heat; also the equinoctial gales stir up the sea and the more uniform temperatures of winter again become established from top to bottom. It will be noted that this warm summer upper layer corresponds very closely to the region (sometimes called the photic zone) in which the little plants get sufficient light to carry out effective photosynthesis. Two bits of the puzzle seem as if they would fit together; we require, however, yet another piece to go with them before we can see the explanation of the seasonal changes in the plankton. This last link concerns certain salts in the sea, and to them we must now turn.
First we must consider the general saltness of the sea; this, of course, is mainly due to the abundant sodium chloride which accounts for almost 77.8% of the total salt content. However there are many other salt constituents, of which the next more important, in order of descending quantity, are magnesium chloride (10.9%, magnesium sulphate (4.7%), calcium sulphate (3.6%, potassium sulphate (2.5%), calcium carbonate (0.3%) and magnesium bromide (0.2%). These proportions are actually those in which these different salts would be recovered from the sea on evaporation; their molecules as dissolved in the sea, however, would largely—some nine out of ten—be split up into their respective parts or ions: sodium and chlorine or magnesium and sulphate ions as the case may be. It is better to think of the salt constituents of sea water, as they mostly are in the sea itself, in terms of separate ions. We can tabulate the percentage proportions as follows, based upon a mean of 77 samples collected from different localities by the Challenger Expedition:
In addition there are minor constituents, for example iron, strontium, silicates, phosphates and nitrates, which constitute together only 0.06%. The degree of saltness of the sea, or its salinity, is usually expressed in terms of the total weight of salts in grams per thousand (°/oo) grams of sea-water; it varies in the open ocean from 34°/00 in polar waters, where it is low on account of additions of fresh-water from melting ice, to 37°/00 near the equator where it is high because of excessive evaporation of water. The North Atlantic surface water as it flows round our islands has a salinity of about 35°/00, but in the southern North Sea it is diluted to some 34.5°/00 by water-drainage from the land.
Now the important salts for our little plants are those which have only been mentioned among the minor constituents: they are the phosphates and the nitrates. Because they are present in such small quantities, it was a long time before accurate methods for their estimation could be devised; these were developed largely through the work of Drs. Atkins and Harvey at the Plymouth Laboratory just after the first world war. It had been realised that the plants of the sea must be limited, as are the plants of the land, according to Liebeg’s Minimum Law; i.e. so long as any really essential nutritive substance occurs in minimum quantities, plant production will be proportionate to the available quantities of it, even though there is a super-abundance of all other essentials. This seemed obvious enough but could not be proved until we had these more refined methods. It now became possible to measure the amounts of phosphates and nitrates taken up from the water by the little plants; it was shown that in our waters these salts could and did in fact limit their growth. The reproductive rate of these little plants grown in culture solutions was seen to fall off as the phosphates and nitrates were depleted and finally growth would stop altogether when they were entirely used up.
It is now possible to explain the seasonal cycle of events. In the winter, as we have seen, the waters from top to bottom are well mixed and their temperature is almost uniform. As the length of the days and the intensity of the light increases there comes a point at which the little plants can begin to multiply and they find a comparatively rich supply of phosphates available—about 40 milligrams per cubic metre of water. We have seen how rapidly they undergo fission when once they start. They are multiplying only in the upper, well illuminated zone; in the early spring these upper waters are being well mixed up with the lower layers by the equinoctial gales and there is a general reduction of the free phosphates as they pass into the plants. Presently, however, there develops an upper warm layer which becomes more pronounced as spring advances into summer; this is also the photic zone, in which alone the plants can multiply. The phosphates and nitrates are now being used up by the plants in this upper zone and are not being replaced by any mixing with the lower waters because of the difference in density between them. In fact the phosphates and nitrates are continually passing from the upper to the lower layers. The plants, which have taken up the salts, may either eventually form resting spores and sink, or may just die and sink, or more likely be eaten by the animal members of the plankton; these may themselves just die and sink or in turn be eaten by other larger animals and so on. The nutritive salts which were once present in the upper zones are now by late summer reduced to a minimum; they are carried in the falling bodies to the bottom or still locked up in animal life. Actually a good deal may be excreted back into the water by the animals,
(#ulink_01fdcb72-0ed5-537d-abcc-c78b4d4e5927) but as most of the animals only make comparatively short visits from the lower into the upper layers to feed on the plants at night, most of the excreted phosphates and nitrates will pass into the lower waters. Thus we see that so long as the discontinuity layer lasts, the plants, such as have not been eaten by the animals, are cut off from the richer phosphates and nitrates below. That is why their numbers decline so markedly as the summer advances and why they cannot reproduce at a rate sufficient to counterbalance the inroads made upon their population by the grazing animals. Down below, the supplies of phosphates and nitrates are being to some extent built up again by their return from dead animals broken down by bacterial action. Now, as the summer wanes, the upper layers are cooled again and the autumn equinoctial gales assist in a general mixing; the water richer in phosphates and nitrates is brought up from below towards the surface where once again we have a fertile layer while the sunlight is still strong enough to encourage photosynthesis.
Here at last we have the explanation of that autumnal outburst of phytoplankton which had for so long been such a puzzle. The time of its appearance and the quantity produced vary markedly in different years; it is usually not very long-lived and eventually the population of plants dwindles to a winter minimum as the light gets too weak to allow much active reproduction. The winter gales stir up the water and the nutrient salts are once again more or less evenly spread through the different layers of water. The temperature, too, is more or less uniform; the cycle is complete.
This brief account of the events throughout the year has dealt with the phytoplankton as a whole. If it suggests, as well it might, that all the different kinds of little plankton plants are increasing and declining together, as the seasons come and go, it would be giving a very false picture. There is in truth a succession of different forms which wax and wane in turn within this larger framework. As the summer advances and the quantity of the phytoplankton is declining, the dinoflagellates come to occupy a much more prominent part in the community; in late August species of Ceratium and Peridinium may be much more evident in the fine net samples than the diatoms. At the second autumn outburst the diatoms will swing back into prominence again. Then within these spring and autumn periods of production there is usually a fairly definite order of appearance of different species of diatoms as the weeks go by; not that one kind disappears entirely of course, but after a period of abundance the reproduction falls to a low ebb and the stock is maintained by only a few individuals or by the resting spores already referred to. The intensive work, already referred to (see here (#ulink_c8c1a1a8-3a7c-5c0f-9c97-cff4a75db940)), carried on week by week for fourteen years at Port Erin in the Isle of Man, has furnished us with a mine of information about these detailed seasonal changes at one place; and now the monthly plankton recorder surveys which will be described in the last chapter (see here (#litres_trial_promo)) are giving us similar information for a very wide area.
What makes one species give place to another? Why for example should Chaetoceros decipiens give way to Ch. debilis and socialis as the season advances or Rhizosolenia semispina be replaced by Rh. shrubsolei which in turn may leave the stage to Rh. stolterfothii? Whilst the grazing of the little plankton animals coupled with the reduction of phosphates and nitrates in the upper layers is bringing about the general decline in the planktonic vegetation it can hardly be controlling the rise and fall of the different species. Johnstone, Scott and Chadwick (1924) in discussing this seasonal sequence of species which they found in their long series of tow-nettings at Port Erin, made an important suggestion as to its cause.
“It is known that some bacteria are incapable of producing their typical effects (say in fixing elementary nitrogen from its solution in sea water) if they are present in pure culture. In order to function effectively they must be associated with some other organism which, by itself, cannot produce the effect in question. Probably such symbiotic relationships may exist on the great scale in the sea. The work of Allen and Nelson (1910) on the artificial culture of diatoms suggests this. In mixed cultures there is always a certain succession of species, one attaining its maximum when another has ceased actively to reproduce. The succession of diatom species during the period of the spring growth suggests that something of the same kind occurs in the sea.”
A similar effect has, of course, now been demonstrated by Sir Alexander Fleming’s great discovery that moulds such as Penicillium produce substances which inhibit the growth of bacteria. In my hypothesis of animal exclusion (in Hardy and Gunther, 1935), which will be referred to again in a later chapter (see here (#litres_trial_promo)), I have suggested that dense concentrations of planktonic plants may produce an effect in the water which is uncongenial to animal life and so account for the fact that animals are usually scarce in regions of great phytoplankton abundance. Dr. C. E. Lucas, my former pupil and colleague, now Director of the Scottish Fishery Laboratory at Aberdeen, has developed much further the idea of chemical interaction between organisms and stressed the possible importance of various substances given out into the water by different plants and animals as a result of their internal activities. Just as cells inside the body of an animal produce those various substances called hormones (or endocrines) which circulate in the blood stream to have profound effects on other parts of the body, so also may substances (ectocrines) be liberated from the body to have their effects on other organisms in an aquatic environment. The changed conditions set up in the water by one species may perhaps become both injurious to itself and at the same time more suitable to another kind which will follow it. Thus, among other interesting ideas, he gives strong support to this idea of seasonal succession: a chain of action, a conditioning and reconditioning of the water, as the year advances (Lucas, 1938, 1947 and 1956a).
Among the animals of the plankton there are also successive changes; particularly noticeable are the various broods of different species which follow one another, giving us in one month mainly adults and in another the young developing stages. The seasons are marked too by the throwing up into the plankton of the young larval stages of various bottom-living invertebrates, and also by the eggs and fry of different species of fish, all of which have their own distinct breeding times.
To return to more general considerations, it is interesting to compare the conditions as found in our waters at mid-summer with those in the surface waters of the tropics; we have the same heating up of the surface to form a discontinuity layer, but there it tends to be a permanent feature. In the open ocean in the tropics the phosphates and nitrates in the upper layers are thus reduced to a minimum all the year round and as a consequence the plankton of those regions is extraordinarily sparse compared with the more temperate or polar seas. This relative poverty of the tropical seas compared to our own was one of the surprising discoveries made by Victor Hensen’s German Plankton Expedition in 1889 and was at first disbelieved by many who thought, on false a priori grounds, that the warmer tropical seas must be richer in life than our own or the cold polar regions. A tow-netting in the tropical ocean may yield many more species than one in our own waters, but the total quantity of life is very much less; when we glance at a tropical sample at first sight nearly every specimen seems a different kind, whereas in one from our own waters there will be thousands of representatives of the same species. Plankton at certain places in the tropics, however, can be remarkably rich; this happens when deeper water with a supply of nutritive salts comes welling up into the sun-lit zones as against the coast or where a submarine bank comes near the surface and gives rise to disturbed water conditions.
The upwelling of water rich in phosphates and nitrates may well be very important in producing a more prolific phytoplankton in our own waters. There can be no doubt that the abundance of fish-food on the sea-bed which makes the Dogger Bank so renowned as a fishing ground, is due to the heavy rain of plankton showered upon it from above; there can also be little doubt that this rich phytoplankton in the surface layers is in turn produced by the upwelling of the richer phosphates and nitrates from below as the Atlantic inflow into the North Sea meets this large submarine bank set across its path (Graham, 1938).
A prolonged off-shore wind may have the effect of producing a heavy crop of plankton near the coast: it pushes the surface water away from the land so that its place has to be taken by water from below which wells up near the coast and thus again brings the desired nutritive salts up into the sunlit upper layers.
There are indications that at times other minor constituents of sea-water may have an effect upon phytoplankton. There is some experimental evidence that organic salts of iron and manganese will stimulate phytoplankton production; and it is suggested that such salts carried out by the drainage from the land may lead to an earlier outburst of reproductive activity among planktonic diatoms in coastal waters than among those further out to sea. Much information about these minor constituents will be found summarised by Dr. H. W. Harvey (1942, 1955) who has himself done so much of the experimental work.
For a long time there has been evidence that suggests that there is in the sea some trace substance at present unknown which is necessary before life can exist—some substance rather like the vitamins in our diet. Artificial sea-water has been made up to contain precisely the same proportions of chemicals that are known to be present in natural sea-water by the most exact analysis; yet planktonic plants will not grow in it unless about 1% of natural sea-water has been added to it (Allen, 1914). Quite recently the vitamin B
(Cobalamin) has been shown to be necessary for the growth of several marine flagellates and a diatom (Droop, 1954 and ’55) and its presence in natural sea water has now been demonstrated.
In addition to ‘trace substances’ which affect plant production, it now appears that there are some which are essential for the healthy development of delicate young animals. D. P. Wilson (1951) has recently shown a remarkable difference in this respect between the two types of water which we discussed in Chapter. 2 (#u3397ab9e-de11-5382-badf-93137316dee4): the more oceanic water characterised by one species of arrow-worm Sagitta elegans and the coastal water by another species S. setosa. It will be remembered that in the 1920’s there was usually elegans water off Plymouth; at that time Dr. Wilson had no difficulty in rearing the planktonic young of some of the bottom-living worms he was interested in. In the 1930’s, however, when the setosa water was over the area, he experienced much frustration in doing so. Thinking that there might be some subtle difference between the two waters, he made experiments to test his suspicions. He took the fertilised eggs of two different kinds of worm and of a sea urchin, and then divided each lot into a number of smaller batches; some he put into elegans water collected out in the Celtic Sea to the west of the Channel and the others into setosa water taken off Plymouth near the Eddystone. In the former water most of the larvae developed well, but in the latter they were abnormal or in poor health. “The experiments indicated,” writes Wilson, “that the Channel water lacked some unknown constituent, essential for the healthy development of these species, present in the Celtic Sea.”
Not infrequently a particularly rich outburst of phytoplankton is reported at a place where the waters of two current systems meet and mix. I have seen it particularly in the region of South Georgia in the sub-Antarctic where waters from the Weddell Sea and the Belling-hausen Sea meet in eddies on each side of the island. It is said to be a feature too of the boundary separating the North Atlantic current from the arctic water and it is not uncommon generally where oceanic and coastal waters meet and mix. Perhaps, on account of different plankton communities, each water has become deficient in some different but vitally important minor constituent; then on their coming together each will fertilize the other with the missing ingredients and so release an outburst of reproductive activity.
A good deal of interest was aroused in experiments performed during the war by the late Dr. Fabius Gross and his co-workers (1944) to see to what extent the growth offish could be accelerated by increasing the quantity of plankton in an enclosed sea loch by the addition of nutritive salts. The plankton was certainly enriched and an increased growth of the fish was recorded. It was in fact doing in a confined part of the sea what had already been successfully done in fresh-water fish-ponds; it is indeed a practice dating from ancient Chinese days. It has been suggested that it might be possible to add fertilizer to parts of the more open sea to increase the plankton in a limited area to provide a better chance of survival for the hosts of young fish that are expected to be developing there. But the open sea is a very big place and to do anything at all effective would need the provision of fertilizers on a scale perhaps too vast to be contemplated as a feasible proposition. Yet it seems that man does unwittingly influence the production of phytoplankton in the sea and consequently the yield of fish. In the southern North Sea opposite the opening of the Thames estuary there is frequently developed an area of a particularly rich growth of phytoplankton and here Mr. Michael Graham (1938) has shown an abundant source of phosphates and nitrates derived from the sewage of London. Dr. K. Kalle of the Oceanographic Institute at Hamburg has recently written a paper on the influence of this drainage from the Thames upon the fish population of the southern North Sea and this has been conveniently summarised in English by Dr. J. N. Carruthers (1954). He points out that the water from the continental rivers is carried quickly to the north-east by the current from up the channel; whereas that from the Thames is held up, wedged between two streams of oceanic water of higher salt content: i.e. that just mentioned and the Atlantic influx from the north. He estimates that 2,900 tons of phosphorus a year are carried from the rivers and when spread through the 171 cubic miles of English coastal waters south of the Humber amounts to an increase of 4 milligrams (0.004 grams) of phosphorus per cubic metre. Dr. Kalle then shows that the catch of fish in this region is per unit area ‘about double the corresponding catch made in the rest of the North Sea, in the English Channel and in the Kattegat/Skagerak region … and is about 25 times the catch reckoned for the Baltic Sea as a whole.’ He holds that two-thirds of this higher average catch may be attributed to the rich supply of nutrients from the population of our metropolis.
For a comprehensive treatment of the physics and chemistry of the sea in relation to plankton production the important books by Dr. H. W. Harvey (1945 and 1955) should be studied.
(#ulink_9b1c9b25-ca70-5b21-b304-2f2392cfbb39) When taking samples from a series of levels in very deep water several of these reversing bottles are generally used together, one above the other, on one wire at intervals of perhaps a hundred metres or more; as the messenger weight hits the trigger to reverse the first bottle, it also releases from below it another messenger which now slides down the wire to operate the second bottle and this again liberates a third messenger and so on to the bottom.
(#ulink_bc69a687-9dee-5558-8517-219d990aa9f2) See Gardiner (1937).
CHAPTER 5 (#ulink_6520a5dc-2d55-5488-9666-9b2074100333) INTRODUCING THE ZOOPLANKTON
THE ANIMALS of the plankton are by definition those which are passively carried along drifting with the moving waters; those other inhabitants of the open sea which are powerful enough to swim in any direction—the fish, whales, porpoises and the squids or cuttlefish—are in contrast referred to as the nekton (see here (#ulink_90dbbd09-52bf-5298-a614-64d2ae53d4e0)). The vertebrates therefore, except for certain primitive relations, will only be represented in the plankton by the floating eggs offish and the young fish themselves up to the time when they become strong enough to migrate at will instead of being just helplessly transported.
In spite of this limitation, and the absence of insects, I believe it is no exaggeration to say that in the plankton we may find an assemblage of animals more diverse and more comprehensive than is to be seen in any other realm of life. Every major phylum of the animal kingdom is represented, if not as adults, then as larval stages with the partial exception of the sponges; the sponges do indeed send up free-swimming larvae but they are in the plankton for so short a time that they can only be claimed as very temporary components of it. In no other field can a naturalist get so wide a zoological education and in few others will he find a more fascinating array of adaptational devices.
It is this great variety of forms, and the unexpected finds which are always turning up, which make hunting in the plankton such an exciting occupation. Except for the jelly-fish and some of the larger crustacea, it is of course hunting with a lens. Nearly every member of the zoo-plankton can be seen with a ×6 hand-lens or a simple dissecting microscope, and the most effective searching can be done with these. Before transferring any specimen to a slide for examination under the more powerful compound microscope, it is well to watch it for a time swimming in its own characteristic way in a small glass dish under the simple magnifier. To anyone who has never seen this life before it is difficult to convey in words a picture of the delights in store for him. I am indeed lucky to have the privilege of having my account illustrated and enriched by the beautiful photographs of living plankton animals taken through the microscope by my friend Dr. Douglas Wilson of the Plymouth Laboratory; they are quite unique and many of them have been taken by that remarkable new device, the electronic flash, which has for the first time made the photomicrography of such small and rapidly moving creatures possible. The naturalist will soon forget the absence of the insects in the wealth of variously shaped and often beautifully coloured crustaceans which are to be seen swimming rapidly in all directions. Tiny pulsating medusae—miniature jellyfish—swim into view; and here and there can be seen the transparent arrow-worms Sagitta which remain poised motionless for a time and then dart forward at lightning speed to capture some small crustacean. Then there may be delicate comb-jellies propelling themselves by rows of beating iridiscent comb-like plates and trailing long tentacles behind them. These comb-jellies and the arrow-worms belong to two phyla—i.e. major groups of the animal kingdom—which are found nowhere else but in the marine plankton. There are many different kinds of Protozoa, among which one order (the Radiolaria) is also entirely planktonic. The segmented worms may be represented by beautiful pelagic polychaetes and the molluscs by the so-called sea-butterflies (pteropods) which are really small snail-like animals with the foot drawn out into wing-like extensions to assist in their swimming and support. Most of these animals are permanent members of the plankton, spending all the stages of their life-histories drifting in the open sea; in addition there are, however, a vast number of the young, or larvae, of the bottom-living invertebrates which ascend to live for a time in the plankton and so distribute the species far and wide. These temporary members present us with some of the most striking adaptations to this floating life. Some of them are nearly always to be found in a tow-net sample from our surrounding seas which have such a rich fauna on their floor. Group by group—flatworms, segmented worms, different kinds of polyzoa, starfish, sea-urchins and, of course, the bottom-living crustacea—each has its own characteristic way of solving the problems of pelagic life. The plankton indeed presents a paradise for the student of invertebrate development; we shall devote a special chapter (Chapter 10 (#litres_trial_promo)) to a consideration of these larval forms.
Hitherto only a small minority of amateur naturalists have shared the delights of exploring the living plankton. Preserved samples, such as are often obtainable from marine laboratories for examination, are certainly full of interest; they can, however, never give the observer the same satisfaction as seeing this teeming world all alive. The professional marine biologist, engaged in investigating the relationship between plankton distribution and the fisheries, finds it very tantalizing to be able only very rarely to find time to stop and look at his captures before he must kill them; he travels to and fro across the sea taking as many samples at intervals as he can, in order to get the most comprehensive picture of conditions in the time available. Usually he only just has time to deal with the concentration, labelling and preservation of one set of collections before the ship arrives at the next position where another set must be taken; for the sake of understanding the fisheries he must always hurry on. In the past the amateur has often had an even more disappointing experience: having obtained a tow-net and hired a boat to take him out in the bay, he has returned home only to find that the wonderful sample of plankton he collected is now just a mass of dead or dying creatures crowded together at the bottom of the jar. Two modern inventions have altered all this: the Thermos flask and the refrigerator. If you have a Thermos flask, or preferably two, you can go to the sea, travel back by train for several hours and still have your plankton alive; if you have a refrigerator, or know a kind neighbour who will allow you to keep one or two 4 lb or 7 lb preserving jars in his, then you can keep your animals healthy for several days to be studied at your leisure.
I believe there are a great many people—and not only those who would call themselves naturalists—who would like to see something of this strange planktonic world, or show it to their children, if only they knew how. Anyone who goes to the sea can catch plankton quite simply. Those who can take a yachting cruise are particularly fortunate; they can study the changes in the plankton as they move from one area to another, can see the difference between the animals at the surface at night and in the daytime, and can try and find out just what organisms are making the flashing lights around their vessel in the darkness. Those, however, who can only take out a rowing boat may make very good collections, especially if there is water from the open ocean bathing their coast. If there is a pier sticking out into the sea and sufficient tidal current, as there usually is at some time of the day, quite good samples may be obtained by streaming out a net on a line and allowing it to fish for a quarter of an hour or so. Some may even think this preferable to a boat if the sea is a bit choppy! If you can only collect from a pier, or from a confined area in a rowing boat, you need not be too envious of your friends in the yacht, for fortunately the water is always on the move; a sample taken at the pier today may be very different from one taken only a few days ago and quite different again from one you may get next week. I have taken very good samples from some of the many piers built out to receive the steamers plying in the Firth of Clyde area.
To help those who do not know how to proceed I will give a few instructions. It is a good thing to have at least two Thermos flasks, so that you can keep at least two different plankton samples separate from one another. If you can manage it, it will be an advantage to start out with one of your flasks filled with sea-water that has stood in a jar in the refrigerator over night. Half of this you can pour into the other thermos just before you add the plankton sample collected. Thus in each flask the animals will be added to sea-water that has been chilled; it will keep them cool, inactive and in good condition whilst they are brought home. Details of how to make and use a tow-net have already been given in Chapter 3 (#uf28d2542-dfd2-59a4-8652-470397ba0dae). The net of very fine gauze suitable for collecting the small plants will also at the same time catch the very small animals, particularly the protozoa and small larval forms. For the capture of most of the zooplankton a coarser net having some 60 meshes to the inch is the most useful. If more of some of the larger animals are required, for example the larger crustacea and medusae, a still wider mesh net, say 25 meshes to the inch, should be used; this will filter much more water but let nearly all the smaller animals escape. The three nets of 200, 60 and 25 meshes to the inch will provide a very good equipment. Remember, as stressed in Chapter 3 (#uf28d2542-dfd2-59a4-8652-470397ba0dae), to tow slowly, at a speed of not more than 1½ knots. It is best to tow only for short periods—not more than five minutes at a time—which can be repeated if too small a sample has been collected. If the plankton is very abundant a longer haul will give you much too much so that all the little animals will be far too crowded together to live healthily for more than a very short time. If you have too thick a sample, pour a lot of it away and only take home in your flask a small part of it, diluted as much as possible with more sea-water. It seems hard to pour most of it back, but you will be sure to have sufficient of the commoner kinds and a few kept in good shape will be better than a great many in poor condition.
I must now give some idea of the actual numbers of animals you may expect to get. Here (#litres_trial_promo) I gave the figures for the diatoms and dinoflagellates taken in two 14-inch diameter tow-nets hauled for half a mile across the bay at Port Erin in the Isle of Man; they were averages for several hauls a week during the month of April over a period of fourteen years. For comparison I now give in the accompanying table the corresponding figures from the same source (Johnston, Scott and Chadwick, 1924) for the more important elements of the zooplankton in the same series of hauls.
The corresponding average totals for the months of June, August and October were 39,105, 38,812 and 35,631 respectively. Since it was calculated that approximately 8 cubic metres of water were filtered by the nets during towing, this gives an average of about 4,500 animals per cubic metre or some 120 per cubic foot of sea-water during the summer months. It must be remembered that these figures are averages and that individual samples may vary enormously from week to week. For comparison it may be interesting to give the average figures for the total plants of the plankton—the diatoms and dinoflagellates—recorded from the same series of net hauls for the four months April, June, August and October; they are in round figures 5,815,000, 6,674,000, 107,000 and 485,000 respectively. It must be remembered, however, that there will have been much larger numbers of the still smaller plants, the tiny flagellates referred to here (#ulink_21cacdb4-e2f9-576c-86e5-ce66f10ad48e), which will have passed through the meshes of the net and so not been recorded. To give the number per cubic metre we must again divide by 8.
If you have time, and the sea is calm enough, you should pour your plankton haul into a dish and examine it with a pocket lens as soon as it comes up; then with a wide-mouthed pipette you can pick out from it into another jar some of the rarer animals that you particularly want to study. After that you can more light-heartedly pour away most of the sample before putting the remainder, together with the rarities you have picked out, into your Thermos for transport home. The most useful dish from which to pick out specimens is one of the large oblong photographic dishes made of white porcelain and used for washing whole-plate negatives; half of the bottom of this can be covered with black paper so that you have a contrast of backgrounds to enable you to see both the darker and lighter forms more easily. All the jars, dishes and pipettes you use for living plankton must be kept thoroughly clean and never be used for samples that have been preserved with formalin or other chemical fixatives. These small animals are delicate in constitution as well as in form.
The majority of plankton animals tend to come up towards the surface at night and sink down into the deeper layers during the day (Chapter 11 (#litres_trial_promo)). Very rich samples of plankton may be collected by simply towing the net just below the surface at night; in the daytime, however, if you are over deep water you may have to send your net down to 15 to 20 fathoms to get a good haul. To reach this depth you will require a good length of line—50 to 60 fathoms—and you will also require a much heavier weight, say a 20 lb lead, to take your net and all this line down. Care must be taken, of course, to know just how deep the sea is at the point where you are working so as not to run the risk of trawling the bottom with your net and either bursting it by filling it with mud or tearing it to ribbons by dragging it over a rough bottom. If you have not a chart you can consult, you should take a sounding with your lead and line before starting.
It is often very interesting to take a series of samples from different depths at the same place as near together in time as possible to enable you to study the depth distribution of the various animals; if you repeat the series again at night you may be very surprised at the different results you will get. As you let the net run out on its line to a deeper level it will fish very little on its way down, for it is moving backwards with the water as it runs out and sinks; when you haul it up, however, at the end of a tow, it will of course fish all the way up. This difficulty is got over in modern oceanographic practice by having in front of the net a special closing mechanism which is operated by a brass messenger weight sent sliding down the cable; this releases the bridles when a trigger is struck and the net falls back to be closed and held by a throttling noose which passes round it behind the mouth. The net is thus hauled up to the surface closed like a sponge-bag with the strings drawn tight and you know that all the animals in it must have been caught at the actual level at which it was fishing. A simple example of this arrangement is shown opposite in Fig. 18 (#litres_trial_promo). These devices, however, are perhaps rather elaborate to be practised by the amateur, especially as a smooth steel cable is required for the messenger and this means the use of a winch; they will not be further described but full information about them will be found in the descriptions of the equipment used on the Discovery Expeditions (Kemp and Hardy, 1929). To minimise the effect of catching whilst hauling up an open net, it is well to make rather longer hauls with it down; the time taken in coming up will then only be a small fraction of that during which the net was fishing at its proper level.
If you are going to take a number of such hauls for study you will soon accumulate far more material than you can hope to keep alive successfully; in this case it will be best to keep only a small part of one or two samples fresh and preserve the rest for study dead. The living plankton will give you the greatest pleasure in studying the swimming movements and behaviour of the animals; the dead samples may nevertheless give you interesting information about the depth distribution of the same animals, which you could not otherwise obtain. The best general preservative for plankton, and the easiest to use, is formalin, i.e. a 40% solution of formaldehyde in water; this, which can easily be obtained at any chemist, can be added to the sample in quite small quantities to give a mixture (about 5% formaldehyde) strong enough to keep it indefinitely in good condition. Remember always to reserve separate jars for preserved samples—never mix them with those used for fresh; a good plan is to stick a red label on them for danger! The dead formalined samples can of course be concentrated into a smaller space; 1lb honey jars with screw-on tops are convenient for their storage. If you are going to keep the samples for any length of time it is well to use what is called neutral formalin, i.e. that to which just sufficient borax—from 5 to 10 grams per litre—has been added to neutralise its acidity; ordinary formalin nearly always contains formic acid which if not so neutralised will very soon dissolve away the calcareous shells and skeletons of many of the animals.
FIG. 18
Sketches of a simple release mechanism for closing the mouth of a tow-net before hauling it to the surface. A, the rig of the net when towed; B, enlarged view of release gear about to be struck by messenger weight; C, the towing bridles released and the net closed by throttling rope.
What has just been said will have been sufficient to have corrected that very common misconception that the plankton exists almost entirely near the surface of the sea. Some people seem to have thought of it as existing as a kind of scum on the very surface itself; this is no doubt due to a misunderstanding of the expression often used that the plankton is the ‘floating life’ of the sea. The plants, as we have already seen, do in fact only flourish for a little way below the surface; but animal members may be found at all depths. Later on—in Chapter 12 (#litres_trial_promo)—we shall describe the plankton and nekton that is to be found at various levels in the ocean between the surface and the bottom, thousands of fathoms deep beyond the continental shelf. There is another erroneous impression about the plankton that is frequently held: the idea that it is more or less evenly distributed over quite wide stretches of the sea; it is often thought that if we used a tow-net in one place and another two or three miles away on the same day, the two samples would be almost exactly alike. This indeed may occur, but it is by no means always so.
A great many surveys have been made in the past, often in relation to some fishery problem, attempting to give some idea of the varying quantities of the major plankton organisms over a particular area. I have already described how a research ship will proceed in such a survey to traverse the area, stopping or slowing down to take tow-net samples at regular intervals. If the area to be covered is a big one, the stations—as the different points of observation are termed—cannot be very close together or the survey would take much too long; they are frequently spaced twenty miles apart. It has usually been assumed that a sample at one point, will, within a reasonable range of error, give a fair representation of the plankton in the area for ten miles around it; thus it has been felt that a series of such stations twenty miles apart will give an adequate quantitative survey. Some plankton organisms are much more patchy in their distribution than others; for some kinds such a method may give quite an adequate picture, but for others it may be hopelessly misleading. Very early in my career as a marine naturalist I had an experience which I will recall because it so well illustrates this very point; it was an episode which had a marked effect on much of my later work. In 1921, soon after leaving the University, I was appointed as Assistant Naturalist on the staff of the Fisheries Laboratory of the Ministry of Agriculture and Fisheries at Lowestoft and was delighted to be allowed to study the plankton in relation to herring. In March of the following year, through the illness of a senior, I found myself, at the last moment of sailing, as naturalist in charge of a cruise on that grand old research trawler the George Bligh.
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Alister Hardy
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Дата публикации: 17.04.2024
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О книге: The New Naturalist editors believe this to be the greatest general work on the subject ever written.Professor Alistair Hardy is truly obsessed by animals of the sea – devotedly enthusiastic about the nature of their adaptations and life histories, brilliantly critical in the examination of their mysteries, acutely lucid (and at the same time highly artistic) in his descriptions of them in his arresting plates.To describe the relatively unknown and mysterious world of plankton is a task that the greatest of marine zoologists might boggle at. Yet the plankton is to the sea what vegetation is to the land. The study of plankton is a complex discipline which few amateur naturalists have had the privilege to enjoy. Never before has such a synthesis of knowledge been attempted in a community of animals so mysterious, yet so important. Professor Hardy has grasped this problem in a new and exciting way; and at least the common reader can discern the pattern of life that dominates two-thirds of the world’s surface.