Collins New Naturalist Library

15

Life in Lakes and Rivers
T. T. Macan and E. B. Worthington

Editors:

Margaret Davies D.Sc.

Sir Julian Huxley M.A. D.Sc. F.R.S.

John Gilmour M.A. V.M.H.

Kenneth Mellanby C.B.E. Sc.D.

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.

Table of Contents

Cover Page

Title Page

Editors

EDITORS’ PREFACE

AUTHORS’ PREFACE

INTRODUCTION

CHAPTER 1 FIRST PRINCIPLES

CHAPTER 2 A TYPICAL LAKE

CHAPTER 3 APPARATUS FOR STUDYING LAKES

CHAPTER 4 DIFFERENT KINDS OF LAKES

CHAPTER 5 RIVERS

CHAPTER 6 ANIMALS AND PLANTS

CHAPTER 7 THE ORGANISM AND ITS ENVIRONMENT

CHAPTER 8 A CLOSER LOOK AT THE ENVIRONMENT

CHAPTER 9 FOOD-CHAINS AND PRODUCTIVITY

CHAPTER 10 LIFE AROUND THE WATER

CHAPTER 11 ANIMAL TRAVELS

CHAPTER 12 STOCK AND CROP

CHAPTER 13 FISH PONDS AND MANURING

CHAPTER 14 IMPURE WATER

CHAPTER 15 PURE WATER

CHAPTER 16 SUMMARY

BIBLIOGRAPHY

INDEX

Plates

Copyright

About the Publisher

EDITORS’ PREFACE

Dr Macan and Dr Worthington, the collaborators in this scholarly and interesting book, have been colleagues for many years. Both are connected with the Fresh-water Biological Association’s laboratory on Windermere in the English lakes.

The subject of life in British lakes and rivers occurred to the Editorial Board early in the planning of the New Naturalist series. No sooner had the idea come to us than we invited Dr E. B. Worthington to be the author of the book. Shortly after he had accepted the invitation, however, he left Windermere to take up an important appointment in East Africa. He suggested, when this happened, that the major task of the book should be taken over by his younger colleague, Dr Macan, and that the final work should be a collaboration of the two of them. The happy result of this suggestion is now before the reader, who will agree that Dr Macan and Dr Worthington have written an admirably lucid and vital book on a somewhat neglected subject.

One of the points made by teachers of ecology in the last twenty years to their students is that the animal and plant community can be readily studied in a pond or a lake. That this is so the authors of this book, which is entirely ecological in its outlook, make quite clear. Moreover, in so far as it can be made simple they make it so. Nevertheless the reader, when he has finished the book, will realize that the solutions of many problems of life in lakes and rivers (solutions which have been often arrived at by workers at Windermere) have only served to create more problems – problems wider and more fundamental than perhaps anybody ever suspected, problems that reach far into the very structure of biology.

First appointed in 1935, Dr Macan returned to Windermere in 1946 after five years as specialist entomologist in the Army. Dr Worthington, well known on account of his explorations of the African lakes, came to Wray Castle as Director in 1937, when that post was first created. Since 1946 he has been Scientific Secretary to the East Africa High Commission, in which position he has been surveying all East Africa in order to ascertain how and where the resources of science might be used to promote prosperity and well-being.

THE EDITORS

AUTHORS’ PREFACE

It is becoming increasingly difficult to write a book which is not out of date in a number of minor, and perhaps a few major, particulars, because advances in every field are continually appearing in print and no-one can hope to keep abreast of all of them. The present authors do not venture to hope that they have not erred, but they have been in what are undoubtedly the most favourable circumstances for writing a book of this kind. Members of the staff of a freshwater biological station, they have been surrounded by colleagues each an expert in one of the fields touched in the following pages. Moreover, these colleagues have been willing to read through chapters on their subjects and draw attention to errors and defects. Mr H. C. Gilson has read chapters 1, 2, and 4; Dr C. H. Mortimer chapters 2, 4, and 9; Dr J. W. G. Lund and Dr Hilda Canter have assisted with the botanical parts of several chapters; Dr W. E. Frost has read chapters 11 and 12; Mr E. D. Le Cren chapters 12 and 13; and Dr C. B. Taylor chapters 14 and 15. Captain C. Diver, C.B.E., Director of the Biological Service, and Mr F. T. K. Pentelow of the Ministry of Agriculture and Fisheries have criticized chapters 8 and 14 respectively. It is appropriate to acknowledge at this point that the editorial board has made many helpful suggestions.

No less important is a lay impression, since it is not for specialists and professional biologists that this book is primarily written. For this we are indebted to Mrs Zaida Macan, who has read through the whole typescript; and to Mr Maurice Illingworth, who has read Chapter 12.

We record all this kindness with gratitude and take the opportunity to thank those who have helped us.

INTRODUCTION

It is interesting to speculate on the contents of an Atlantic Charter drawn up by any species of animal other than man. We may start by comparing the lot of man with that of the rest of the animal kingdom, which is separated by a lower grade of intelligence. People are not often drowned as a result of catastrophic floods, few are blown to destruction by strong winds, and death in a forest or heath fire is a rare calamity; nor do abnormal spells of hot or cold weather claim many victims. The same is true for all large animals. But for innumerable small ones such dangers are great, and the populations of many are seriously depleted at intervals by one or other of these causes. A final calamity, which does not often befall man, though it befalls other animals of all sizes, is death at the hands of some beast of prey.

Medical science has rendered the more advanced sections of the community secure against many of the disease-causing parasites which formerly destroyed them in large numbers; plague (the black death), typhus (gaol-fever), and cholera may be mentioned as diseases that once took a heavy toll of life in the British Isles, but do so no longer. In less advanced parts of the world, disease still brings death to enormous numbers of human beings, and in this respect there is not so much difference between man and the other animals. Indeed on theoretical grounds it can be argued that many other animals are better off than man. Most parasites can survive only in living tissue and therefore it is to their advantage that the host should remain not only alive but unhandicapped in the struggle for existence. Too virulent a strain will be as unsuccessful as the one that is not aggressive enough to gain a foothold in the face of the counter-measures taken by the host. There is continual selection of a strain that can establish itself but will not kill. Selection acts on the host too and specimens lacking resistance to a particular parasite are removed at an early age. The result is a state of tolerance, with the host carrying parasites but not inconvenienced by them. This state is not attained by man for two reasons. First, selection of resistant individuals does not take place because medicine prevents it. Secondly, whereas the total population of most animals is separated into numerous units between which there is little interchange, man travels all over the world and carries strains of parasites from a region where tolerance has developed to one where it has not.

Death from starvation may have two causes: the food of a particular species may fail on account of some climatic irregularity, or may be eaten up by some other animal. We avoid the first of these eventualities by means of a highly organized system of transport; when the harvest fails in one part of the world, food is brought from somewhere else. But the recollection of the Bengal famine will serve as a reminder that this danger is not wholly eliminated. Against animals which eat the same food as himself, man brings to bear all the resources of science, and wages a never-ending war on rabbits, rats, grain-weevils, slugs, caterpillars, and a host of smaller pests. Lower animals cannot attack their competitors on anything like the same scale; until recently it might have been stated that they could not do it at all, but now it is know that certain organisms can produce chemical substances that kill competitors – penicillin produced by the mould Penicillium is the obvious example – and this process is comparable with the steps that man takes to safeguard his food supplies.

Were we writing about philosophy and not natural history, we should of course have to insert a passage about the perils peculiar to civilization – death on the roads from motor vehicles, and other accidents with machines, destruction by high explosives and other weapons of war, and mass annihilation by atomic bombs. However, we are not. The purpose of what has been written is to stress that the life of a small animal – and it is with such that this book is mainly concerned – is a continual struggle of extreme severity. The physical environment, predators, parasites, and competitors all have to be contended with. The response has been steady change and continual modification. Some animals have won and held a place where physical conditions are easiest – but dangers from predators, parasites, and competitors consequently greatest. Others have become adapted to conditions where physical or chemical conditions make life difficult – but where accordingly there will be fewer other organisms to harass them. Fresh water provides some of the most striking examples of the latter.

It might appear at first sight that the gulf between land animals and water animals is great, and that the easiest way into fresh water is from the sea. But, although an animal or plant may pass from marine to freshwater conditions with no alteration of structure, the change confronts it with considerable functional or physiological problems. The concentration of salts is generally much lower in fresh water than in the sea and moreover liable to considerable variation according to rainfall and other factors; from the biological point of view the constancy of the marine environment is one of its most notable features. Further, conditions vary widely from one freshwater locality to another. An animal proceeding up the Hampshire Avon, for example, would find, if it turned aside into one of the tributaries coming from the New Forest, an acid water, poor in dissolved salts, very different from that which would surround it if it followed the main river to a source at the foot of the chalk downs. These chemical conditions have proved a barrier which only a few marine animals have surmounted. Some of the snails, the bivalve molluscs, the freshwater shrimps, and the fishes are the most familiar.

Actually it appears from an examination of the groups to which present-day freshwater plants and animals belong, that it has been easier to invade fresh water from land than from the sea.

The animals of marine origin occupy in fresh water the same sorts of situation that they occupied in the sea, and they have not changed greatly. As a result of isolation they are now quite distinct from their nearest marine relatives, but they present no peculiar freshwater facies. Some of the land-animals, too, have effected the change to fresh water with little alteration; others with no more than some general adaptation such as the conversion of appendages from legs to paddles. But some of the animals from the land, having once established themselves in fresh water, have become considerably modified to live in one particular and difficult part of the underwater world such as a torrent; others have achieved remarkable physiological adaptations, such as the ability to live without oxygen. It is among these specialists that we find the peculiar and characteristic freshwater types.

The main problem confronting an animal taking to the water is how to obtain its oxygen. Often the difficulty is not great, because many land animals live in damp places and have a moist surface. If they are quite small, this surface is all they require for respiratory purposes, and accordingly it does not matter greatly whether air or water is the medium beyond the layer of surface moisture. The problem is not quite as simple as this, but we need go no further for the present.

Other animals, whose land ancestors were probably less dependent on humid conditions, spend their lives in the water, but have developed a variety of methods whereby they can utilize atmospheric oxygen. The familiar water-beetles swim to the surface with the aid of their hind legs (which, with the transfer to water, have been modified into efficient paddles), and take in a bubble of air between their backs and their wing-cases. Some snails come to the surface to fill a lung. Mosquito larvae – the well-known wrigglers of the domestic water-butt – feed at the surface with breathing-tubes penetrating through to the air. The rat-tailed maggot has a remarkable telescopic appendage, so that it can walk on the bottom and keep in contact with the air at the same time. Other animals take in oxygen as a gas but nevertheless live perpetually submerged. Several groups – none of them very familiar to the layman – have a close-set pile of unwettable hairs. Withdrawal of oxygen for use by the body from the gas entrapped in this pile causes a partial vacuum, which is filled by oxygen dissolved in the water. Others tap the gas-filled tubes inside plants by means of some part of the body modified for the purpose.

Fresh water has presented invaders with a variety of problems, and no animal has solved all of them; none is found in all the habitats and most are confined within a rather restricted range.

The surface offers a rich hunting-ground, for many land animals fall into the water and are held there helpless by the force of surface-tension. Two groups of insects have mastered the art of living on the surface film, and are able to prey on these unfortunates. They are, however, confined to small pieces of water or sheltered bays, presumably because the effort of keeping station against the wind in the middle of a big sheet of water is too great. The pond-skaters have developed long legs and proceed somewhat after the manner of long-oared skiffs. The other group, the whirligig beetles, have shortened and flattened their legs till they resemble more the paddles of a canoe, by means of which they move over the surface with great rapidity. The way of life of these surface-dwellers calls to mind that of the wreckers who once gained a livelihood round our coasts, though they are not able to take measures to lure their victims to destruction.

Quiet shallow conditions, where mud settles and rooted plants – all incidentally of terrestrial origin – provide shelter, oxygen, and food, are probably the most easy to colonize; that is, they present the would-be inhabitant with least in the way of physical and chemical difficulties. But life is not easy in this habitat because there are so many different types of organism and competition between them is severe. The water-beetles and water-boatmen pursue their prey through the underwater jungle with the speed and grace of terrestrial felines; dragonfly nymphs lurk concealed like a crocodile in a waterhole; the tiny Hydra trails its tentacles in the water like a fisherman setting out his net. Against these marauders the caddis-larvae seek protection within a cumbersome house of stick or stone, and snails can withdraw into their more neatly made shell. But the snail’s shell, the caddis-larva’s house, the dragon-fly nymph’s protective coloration, and the beetle’s speed may alike prove unavailing when fish come nosing through the undergrowth in search of food. They are not the only enemies, and some birds and the water-shrew also hunt in this territory.

In deeper water, if it is too dark for plants to grow, the mud offers a substratum which is often rich, because the remains of animals and plants rain upon it from above. Here the chief inhabitants are mussels, worms, and midge-larvae, most of which are modified for burrowing and for feeding on minute particles. The diversity of form is not great, though the number of individuals may be colossal.

In yet deeper water conditions may be extremely difficult, because there is no oxygen for part of the year, but some animals, notably midge-larvae, have solved this physiological problem.

In the open water also the variety of form is not great, though numbers may be. Some small animals, such as the water-flea, resemble their counterparts in the sea, but the marine zoologist inspecting a freshwater catch is immediately struck by the lack of diversity. This is partly because many marine animals which lead a fixed or relatively sedentary life have a free-swimming young stage. This presumably serves the end of dispersing the species. In the circumscribed conditions of fresh water such a stage is of less advantage for this purpose, and might indeed prove a danger by being carried away to the sea before it was ready to settle. Freshwater animals which have come in direct from the sea have almost all lost the free-swimming stage, which their nearest marine relatives possess. Another striking feature of the animals of the open water is their small size. This is probably due to the absence of shelter and consequent vulnerability to predation. Some protection is obtained by transparency, a feature seen in both the sea and in fresh water, but the main defence is small size, rapid reproduction while conditions are favourable, and the formation of a resting stage as soon as they cease to be. Large animals could not reproduce fast enough to make good losses due to predation. It may be objected that some animals floating in the open sea attain a large size. In comparison with the sea most bodies of fresh water are very small and fishes feeding on the larger animals on the bottom in shallow water could easily make excursions to prey on any large organism that developed in the open water. Most of the sea is so far from land that this cannot happen, and predators and prey must develop in balance together.

One representative of that enterprising group the insects inhabits open water, and provides one of the most remarkable examples of adaptation that the animal kingdom has to show. It is the larva of a mosquito-like fly, and is known as the phantom larva, on account of the transparency already mentioned. The breathing-tubes, in other insects continuous from back to front, are vestigial except for paired air-sacs fore and aft. These are hydrostatic organs by means of which the animal can maintain itself at any desired depth. The earliest workers supposed that it functioned like a submarine, pumping fluid in or out of ballast tanks as required. Later observers, noting that the bladders never contained fluid, postulated that they worked like the swim-bladders of fishes, and secreted or absorbed oxygen. Finally, when a technique for analysing the minute quantities of gas in the bladders was evolved, it was discovered that the walls of the bladders expand or contract as the result of nervous stimulation, and the gases dissolved in the body-fluid diffuse in or out accordingly.

Rocky shores of lakes are startlingly barren places compared with those of the sea; there is no canopy of weeds nor incrustation of barnacles and other sessile animals. The two are not strictly comparable, because in fresh water there is no tide and the water may sink slowly to a low level and stay there during a long spell of fine calm weather. But even below this level there are not many living organisms. Probably this is due to a poor food-supply, for hard rocks and waters that are not rich in nutrient substances commonly go together. Perhaps this is an environmental niche that has not been completely filled; the fact that neither rocks nor wood are attacked by boring organisms in fresh water, as they are in the sea, suggests that this idea is not as revolutionary as it may seem at first. Further, the zebra mussel, the only freshwater bivalve that can attach itself to a hard substratum as the marine mussels do, has only recently entered fresh-water.

On a rock-face in a lake the only plants are green algae, and the only animal the freshwater limpet. This is actually descended from terrestrial stock, though superficially it resembles the marine limpet closely except in size. Smooth rock in a stream may have some covering of moss, which harbours many animals. If it is bare, it may be covered by great numbers of buffalo-gnat larvae, which spin a web across its surface and attach themselves by means of a circle of hooks on a basal pad. They obtain food by straining the current with hairy mouth-appendages.

Smooth rock is not found very frequently, and, where wave action or running water prevents the settling of finer particles, the bottom usually consists of stones and boulders. Several animals have adapted themselves to these particular conditions. Some mayfly nymphs, such for example as those of the March Brown, have flat bodies and strong claws, and can crawl over the surface of a stone, where they graze on the attached algae, in such a way that the current cannot get beneath them and pluck them off. Certain caddis-larvae spin nets between the stones in a stream and subsist on the debris which these nets strain from the current.

These animals are specialists. They have solved the main problem of life in swiftly flowing water – anchorage; and two, having surmounted this difficulty, have turned the peculiarity of the medium to their own advantage – the constant flow brings them their food. The modifications of the specialists render them unable to compete with other animals except in the habitat to which they are adapted. Except in extreme conditions, specialists and non-specialists are found side by side. In streams, for example, many animals without any particular modifications for life in running water occur beneath the stones. One of them, the larva of the daddy-long-legs, is not greatly different in structure from its relative the leather-jacket, which lives in the soil and damages lawns and pastures by eating the grass-roots. Another, one of the commonest, is the freshwater shrimp (Gammarus pulex), a rather incompetent swimmer which is washed away at once if caught by the current. It is one of the most successful of all freshwater animals. It may be abundant in quiet weed-beds, and sometimes, apparently when fish are very few or absent, it may live in the open water of lakes. But it is not ubiquitous and some of the chemical problems posed by fresh water have proved too much for it; it is not found where the calcium concentration is very low, and it also requires a relatively large amount of oxygen in the water.

There is another difficulty with which fresh water confronts animals and plants that seek to live in it – it may dry up. Many freshwater organisms have, accordingly, developed a resting stage, which is resistant to desiccation and probably plays an important part in dispersal as well as survival. Some animals have specialized in the temporary habit, thereby gaining certain advantages, for they make a quick start when water reappears, and can exploit the resources of the pond before it fills up with competitors coming from permanent pieces of water. Many mosquitoes lay drought-resisting eggs in damp hollows, and no development takes place till the hollows fill with water. That beautiful animal, the fairy shrimp, is found only in temporary pools, and survives the dry periods in the egg-stage. In England it is rare, but in Iraq, for example, where there are innumerable pools that fill when the high river-level causes the water-table to rise in spring, but are dry throughout the rainless summer, it is one of the commonest pond animals.

Nobody in these islands has far to go to find a piece of fresh water, and his search will usually take him into pleasant surroundings. He will not find the gaily coloured, almost gaudy, creatures that the seaside naturalist encounters; most freshwater animals are rather drab. On the other hand neither land nor sea animals can so easily be kept and watched at home. Moreover a day’s pond-hunting is not rendered fruitless by rain, as is an excursion after butterflies and many other land creatures. The fauna is, as we have just seen, the product of a difficult environment, a severe struggle for existence, and the adaptability and plasticity of living organisms. In consequence it shows a fascinating diversity of form and function, which cannot fail to appeal to all who are interested in wild creatures and how they live.

CHAPTER 1

FIRST PRINCIPLES

Life in lakes and rivers is studied by three different sorts of naturalist, whose spheres of interest all too rarely overlap. First there is the naturalist with a pond-net who collects the smaller animals and plants of the water; secondly there is the naturalist with a fishing-rod, who classes organisms according to their relationship to fish; finally there is the naturalist with field-glasses for whom rivers, lakes, and reservoirs are places where interesting birds and mammals may be observed. This book attempts to link these three fields together, to relate them to the geographical background, and to discuss the conflict which is bound to centre round them in a thickly populated and heavily industrialized country.

The fisherman is often less interested in the question of what kinds of animals and plants occur in a given set of conditions than in the question of how many, or rather how much, and this question is also fundamental for naturalists of the other two classes. The answer, as given by the study of productivity, has provided a central connecting theme in the pages that follow. Productivity must ultimately depend on the amount of non-living material brought into a body of water in solution and in suspension, and so the story starts with the geology of the surrounding land. Closely bound up with this is the way in which the body of water was formed. When the primary nutrient materials reach the water they are utilized by plants, and plants always form the first link of the biological food-chain. Succeeding links are provided by various invertebrate animals and finally fish. At all stages living organisms die and decompose, resolving eventually into the simple substances from which they are built up. The mechanism of this process is studied by the bacteriologist. The result presents the chemist with many of his problems, and takes him particularly to the mud over which the water lies. This may be at a considerable depth and far below the point to which light penetrates sufficiently to make plants grow. The return of these substances to the upper layers, obviously of great importance to the biological cycle, involves an incursion into the field of physics.

Fish with predaceous habits are generally the final link of the foodchain within the water itself. But the chain continues on to the land and into the air, for there are piscivorous birds and mammals which the freshwater biologist must study.

Man himself exerts a profound effect on life in fresh water. First, since earliest times he has striven to keep water within defined limits by means of drains and flood banks. More recently he has taken to using rivers as convenient agents for the removal of his waste products. Sewage, in moderate amounts, enriches water as it enriches land, but in excess it uses up all the oxygen in solution, with disastrous effects on most living organisms. In conflict with those who have wastes to dispose of are those who wish to see their waters well stocked with fish. These people, too, have played a big part in altering the conditions of water-life. Finally, man is today faced with an ever-increasing problem of obtaining for domestic use water containing the minimum possible amount of life.

Many insects start life in the water but end it as terrestrial creatures with the power of flight. Their life-histories have fascinated naturalists for a very long time and they attracted the attention of some of the first workers in the field of freshwater biology. But studies involving all the animals, all the plants, the chemical and physical background, and the inter-relationships between them were not made until later. To work of this kind the terms limnology, hydrobiology, or freshwater biology are applied indifferently. The pioneer was Professor F.-A. Forel who, in 1872, settled down to a lifetime’s investigation of Lake Geneva or Le Léman. His main publication runs to three volumes, the first of which appeared in 1892, and considers the lake from thirteen different aspects.

In 1884 scientific investigation of fishery problems with a view to legislation started in Hungary, and in the two succeeding decades research stations with the same object sprang up all over Europe. In 1890 Professor Otto Zacharias started a station for fundamental research at Plön in Schleswig-Hol-stein. It was a private venture but it was supported by the State. With its foundation Germany took the lead in both applied and theoretical research and she retained this position until the recent war. Professor August Thienemann was director at Plön during the period between the wars, and his name is associated with many limnological studies, particularly those relating to the classification of lakes according to oxygen concentration, and according to the species of midge (Chironomidae) found in the bottom mud. Stations for fundamental research were started in other countries in the years which followed, usually in connection with universities. The best known today are: Hillerød in Denmark, opened in 1900 by the University of Copenhagen and made famous by Professor C. Wesenberg-Lund, who devoted the first ten years to plankton problems and has since studied the biology of many invertebrate groups; Lunz, opened in 1905 on an Alpine lake in Austria; Aneboda, started in 1908 by the University of Lund in Sweden and associated particularly with the name of Einar Naumann, who elaborated theories of lake classification; and Tihany on Lake Balaton in Hungary. The Istituto Italiano di Idrobiologia on Lake Maggiore is a later foundation, but no less celebrated than the other stations mentioned, particularly for the study of plankton. Limnology was also studied at universities and at stations devoted to the practical study of the production of fish.

Development in America was similar. The first station was founded in 1894 by the University of Illinois, but the most famous of the early contributions to theoretical studies were made by C. Juday and E. A. Birge at the University of Wisconsin.

Since 1945 expansion has been rapid all over the world, and it is impossible to give any general account of it, one reason being that, whereas before the war few scientific communications were published except in one of the major languages of western Europe, now they appear in a great many. Development in Britain has probably been similar to that in many other countries, and we may therefore pass on to events there.

Great Britain lagged far behind in the early years and it is interesting to quote the words of Professor Charles A. Kofoid (1910) who, in 1908 and 1909, toured the research stations of Europe. He writes: ‘The direct support of biological stations by educational funds of local or state origin, often in connection with universities, so generally prevalent in other European countries, is almost wholly lacking in Great Britain.’

‘The stations have been forced, therefore, to turn to memberships of supporting societies composed to a considerable extent of scientific men themselves, to private benefactors and to the commercial interests of the fisheries for aid. The result has been a relatively meager and fluctuating financial support…and a relatively very large absorption of the funds and activities of the British stations in scientific fisheries work.’

However, in spite of this, or should it be because of this, the ‘meager and fluctuating financial support’ having deterred all but the most determined and enthusiastic from seeking employment of this sort, Kofoid’s opinion is: ‘The scientific fisheries work done by the British stations is unsurpassed in its excellence and effectiveness.’

Marine problems have always taken pride of place in Britain. As befitted the leading maritime power of the day, she was the first to send out a major expedition to explore the depths of the ocean, when it was first realized that life existed there. H.M.S. Challenger set out early in 1873 and was at sea until late in 1876. Soundings, collections of animals and plants, chemical analyses, meteorological records, and other scientific data were obtained in all parts of the world, and the total achievement was considerable. It was the culmination of a collaboration between science and the Royal Navy which had been yielding fruit for a century or more. One of the junior scientists was a certain John Murray, who later, as Sir John Murray, became head of the Challenger Office, and was responsible for seeing the final volumes of the reports through the press, many years after H.M.S. Challenger had been relegated to the scrapyard.

He found time to organize Britain’s first important contribution to limnology – the Survey of the Scottish Freshwater Lochs, carried out between 1897 and 1909. This was a private venture undertaken after he had ascertained that the Lords Commissioners of the Admiralty were not concerned with fresh water, and that the Survey Department of the Office of Works (late Ordnance Survey) was not interested in anything except the surface of bodies of fresh water. Five hundred and sixty-two lochs were surveyed and, though sounding was the principal activity, sufficient observations on temperature were taken to provide the data for a theory about the circulation of water in the deeper parts of lakes – a theory which is still accepted today. Plankton collections were made but other biological observations were few.

In 1901 Mr Eustace Gurney started a station on Sutton Broad in Norfolk, and during the succeeding years a vigorous programme was carried out here under the direction of his brother, Dr Robert Gurney. It was, however, a private venture and it lapsed when the Gurney brothers moved away from the neighbourhood.

The next event of importance in the history of British freshwater biology was the issue in 1915 of the final report of the Royal Commission on Sewage Disposal, appointed in 1898. This Commission had carried out a careful examination of almost all aspects of the problem, even to the extent of inaugurating research to obtain certain information which it deemed essential. That our rivers are still polluted by sewage must be laid at the door of the legislators and not blamed on the Royal Commission.

Between the two wars several rivers were surveyed by staff of the Ministry of Agriculture and Fisheries. The primary object was to discover the effect of pollution on animals and plants, but obviously in order to do this it was necessary to survey unpolluted stretches for comparison, and the result was an important contribution to knowledge about the fauna and flora of uncontaminated rivers. During the same time Dr Kathleen Carpenter of the University College of Wales investigated stream faunas and the effect on them of pollution from lead mines. This work established a tradition of freshwater biology in Wales which has persisted ever since.

In the twenties the foundation of a station for freshwater biological research in Britain was discussed. A number of distinguished men of science came together and worked hard exploring ways and means. The interest of universities, academic societies, fishermen, and waterworks undertakings was aroused and, when, in 1930, subscriptions totalled £575 and promise of a grant of about the same amount had been obtained from the Government, the time to start was deemed to have arrived. Ideas about a new, properly equipped, building had to be abandoned, and search was made for an existing building which could be adapted. It had been decided that Windermere was the most suitable location, and on the banks of this lake the committee found that a place called Wray Castle was only partly occupied. It appeared to be suitable and in October, 1931, work started in a Victorian country house built externally in the style of a medieval castle.

At the beginning there were two naturalists, P. Ullyott and R. S. A. Beauchamp, and one laboratory assistant, George Thompson. The apparatus and general facilities were meagre, as may be appreciated from the amount of money available. Further subscriptions were raised, and by the end of 1935 there were five research workers and three assistants. In the following year a committee from the Development Commission inspected the laboratories and the work in progress, and, as a result of their visit, a bigger annual grant from the Treasury became available. One of their recommendations was the appointment of a full-time director, this office having previously been honorary and filled by Dr W. H. Pearsall (later Prof. W. H. Pearsall, F.R.S.), at that time Reader in Botany at Leeds University.

Expansion continued and in 1947 there were ten research workers, twelve laboratory assistants, and an instrument-maker. Wray Castle was now too small to provide, as it had done hitherto, laboratories, and living accommodation for unmarried members of the staff and visiting research workers, and in 1948 what had been the Ferry Hotel was purchased to take its place. The move was effected in 1950 and now, twenty years later, the staff of nineteen scientific officers and fifty-two supporting staff is once again complaining of lack of space. It is planned to build an annexe. Most of the staff came from Cambridge in the early days, and at that time there were few other universities from which they could have come. If W. H. Pearsall was the father of limnological thinking in Britain, a cofounder of the Freshwater Biological Association, J. T. Saunders, was the father of limnological teaching. Among the students who attended the course he ran, in addition to those mentioned, were F. T. K. Pentelow, B. A. Southgate and C. F. Hickling, three pioneers whose names will be encountered later in this account, and G. E. Hutchinson, who is likely to be the last person to write a comprehensive treatise on limnology single-handed.

Students come to the laboratory of the Freshwater Biological Association every Easter to attend a course. For reasons of accommodation and transport, numbers are limited to about sixteen. During the first few years applications were often fewer than this. After the war freshwater biology became increasingly popular at universities, and demands for places on the course rose, which frequently meant that from five students specializing in freshwater studies two were selected. This situation proved unacceptable to teachers who, one by one, organized their own courses. Today (1970) there are universities where the numbers on the course are well above sixteen. It is the universities and colleges where freshwater biology is not taught that now send most of the students to the Freshwater Biological Association’s course. Cambridge is one of them.

During the decades since the war university expansion has provided posts for freshwater biologists, and some of the leading men have, unfortunately, found that American universities offer better conditions than those in Britain. A station for research on fish was established at Pitlochry by the Scottish Home Office soon after the war, and in 1962 work started at the Freshwater Biological Association’s River Laboratory in the south of England. Increasing numbers of freshwater biologists have also found employment with River Authorities, with which statement we conclude this brief review, for we lack faith in our ability to forecast the future.

As a final introductory topic, physical and chemical properties which affect living organisms demand brief notice. Warm water is lighter than cold water and so floats on it, a phenomenon which leads to temperature-layering of lakes in summer, and thereby exerts a profound effect on the animals and plants. This subject is explored further in later chapters.

In the present chapter we shall notice only some of the properties of water at low temperatures. Water is densest at four degrees above freezing point on the Centigrade scale. This is 4° C., since freezing point is at 0° on this sensible scale, but 39.2° on the Fahrenheit scale, which is still in common use in Britain, and on which 32° is the freezing point of pure water.

As the surface of a lake cools down in the autumn, the upper layers sink and displace warmer water from below. This process goes on till the temperature is uniform at 4° C. from top to bottom. Water colder than 4° C. is less dense and therefore floats at the surface, and, if there is no wind to stir it up and mix it with the water below, this surface layer will be quite thin. Further cooling leads to the formation of ice. There can then be no physical mixing due to wind and, if cold conditions at the surface persist, the effect can only pass through the water by the slow process of conduction. In Britain, therefore, ice never gets very thick.

If water were to become steadily denser until freezing point was reached, a body of water would attain a condition where the temperature was uniformly just above freezing point from top to bottom. Further cooling would presumably cause the whole mass to freeze solid. It has been stated in print that such a state of affairs would mean that nothing could live in fresh waters in temperate latitudes. This is hardly likely to be true because a number of animals can withstand being frozen solid, but it is certainly more convenient, particularly for man, that water is heaviest at 4° C.

For every thirty feet that an object sinks below the surface of the water the pressure upon it increases by one atmosphere. The pressure in deep water has been brought vividly to the notice of many a biologist who has inadvertently lowered a water sampler unopened into deep water, and hauled it up to find quite flat what had been a cylinder. Water itself is almost incompressible, and, if it were quite incompressible, Windermere, which is 219 feet or 67 metres deep at the deepest point, would be only a millimetre or about 1/25th of an inch deeper than it is at present. There is not, therefore, a big increase in density with increasing depth and no grounds whatever for the popular idea that objects thrown overboard in deep water do not go right down to the bottom, but float at a certain depth, light objects reaching a point of equilibrium before heavier objects; anything of higher specific gravity than water will go on sinking till it reaches the bottom. The pressure inside an aquatic organism is approximately the same as the pressure outside, and creatures which live in deep water do not, therefore, possess adaptations to withstand pressure as is sometimes supposed. Rapid progress from deep to shallow water may prove disastrous for any animal, because bubbles of gas appear in the blood on account of the reduced pressure; swim-bladders of fish may burst.

Water is twice as viscous near freezing point as at ordinary summer temperature, and this has an important bearing on the rate at which small bodies sink.

The surface tension of water is a physical factor which looms very large in the lives of animals and plants below a certain size. Some animals such as the water-crickets can support themselves on the surface of the water by it, and snails and flatworms can sometimes be seen crawling along the underside of the surface film. Occasionally aquatic creatures get trapped in the surface film and are unable to get back into the water. Terrestrial animals that alight on the water surface frequently find themselves entrapped, and at certain times of year these unfortunates make quite an important contribution to the food supply of certain predators which dwell in or on the water.

Any natural body of water will contain a certain amount of dissolved matter, the quality and quantity of which will depend on the geology of the land over which or through which the water has flowed. It is possible to recognize certain types, though generalizations are not very profitable because modifying factors are numerous. The main substances in solution in some of the chief types of water are shown in Table 1.

Table 1. The metallic and acidic radicles of the commoner dissolved substances in certain natural waters: figures in parts per million.

Ennerdale is an extreme example of a soft water. Cambridge tapwater is a fairly typical hard water derived from a drainage area in which there are chalk downs. The radicles present in much greater amount in the Cambridge water than in the Ennerdale water are calcium, magnesium, and carbonate. The Burton well-water is included as a curiosity which may be of interest to beer drinkers; it has an unusually large number of radicles present in high or relatively high concentration. The permanent hardness of Burton water is due to gypsum – calcium sulphate. A chloride content higher than usual is commonly due to spray from the sea, to wind-blown sea-sand, or to pollution. In inland areas well away from any maritime influence the chloride content is often examined as a routine part of the test for pollution.

If a water containing calcium carbonate flows through a soil containing sodium, sodium displaces calcium and the calcium goes out of solution. An example of such a water is that from Braintree in Essex shown in column three of Table 1; water draining from a calcareous region passes through the Thanet Sands, which are marine in origin, and emerges with quite a small amount of calcium in solution. This displacement of calcium by sodium is the essence of the ‘Permutit’ process for water-softening. Incidentally hard waters are frequently softened before being supplied to consumers. This is now a practice at Cambridge and its tap-water today contains less calcium than is shown in Table 1, in which the figures are from an analysis made before the softener was installed. Very soft waters, on the other hand, are sometimes treated with lime in the belief that defective teeth in the local children are due to the low calcium content of the water; but no convincing proof that this is so has ever been given. Very soft waters sometimes corrode pipes, owing to the presence of humic acids, and this can be cured by adding lime.

Further figures may be found in Taylor (1958), where there are seventy pages of them, not only from all parts of the British Isles but from other parts of the world as well.

The sea contains the accumulation of salts brought down by fresh waters over a period of aeons. Calcium has been lost from sea-water generally not by precipitation but by incorporation into the skeletons of animals, which have later died and fallen to the bottom of the sea. Small, single-celled animals play a greater part in this process than larger ones; for example, Globigerina ooze, which covers vast areas of the bottom of the ocean, is made up chiefly of the calcareous shells of a small single-celled animal bearing that name. Present-day chalk downs were formed under the sea by the accumulation in this way of the skeletons of myriads of tiny animals.

A similar concentration of salts takes place in lakes occupying areas of inland drainage, where there is no outlet and the water lost by evaporation is equal to the amount flowing in. In some such lakes the process has gone further than in the sea. Common salt or sodium chloride is the most abundant chemical substance in the sea; but the Dead Sea has reached a stage where there is some precipitation of sodium chloride, and this substance is present in smaller amount than the more soluble magnesium chloride. The proportions of these two salts in the River Jordan are the reverse of those in the Dead Sea. But there are no drainage areas in Britain without egress to the sea, and therefore discussion of such places is outside our present scope.

There are many substances present in water in very small quantity. It is known that on land and in the sea some of these so-called trace elements are important biologically and the same is probably true in fresh water.

No mention has been made so far of nitrates and phosphates, which are usually present in fresh water. As will be seen in a later chapter (Fig. 2) they are essential for plant growth, and during the course of it their concentration in the water is reduced. The fluctuation throughout the year is large and a single value for any one piece of water is, therefore, of no great significance.

Finally, of extreme importance to living organisms is the amount of dissolved gases in the water. Under average conditions at 0° C. (32° F.) there will be about 10 cubic centimetres of oxygen and half a cubic centimetre of carbon dioxide dissolved in one litre of water, that is 100 parts and 5 parts per million respectively. The concentration falls with rising temperature and at 20° C. (68° F.) there will be only about 65 parts of oxygen and rather less than 3 parts of carbon dioxide. For certain purposes it is convenient to express the concentration as the percentage of the saturation concentration at the temperature prevailing when the sample was taken. Fifty parts per million of oxygen would be 50% of the saturation value at 0° C. but 77% at 20° C.

Animals use up oxygen and produce carbon dioxide and plants do the same in the dark. While illuminated, the latter do the reverse, absorbing carbon dioxide and producing oxygen. Still water with much vegetation in bright sunlight may for a period have more oxygen in solution than the normal maximum at the temperature prevailing. This condition, which is unstable, is technically known as super-saturation.

Decomposition also uses up oxygen, and serious pollution, by sewage for example, exerts its effect on the fauna by depleting the water of oxygen.

One rather important point is that, if water is quite still, oxygen or any other substance in solution can only pass from a region of higher to a region of lower concentration by diffusion, and this process is extremely slow.

CHAPTER 2

A TYPICAL LAKE

Warm water floats on cold water. If two layers differ markedly in temperature, the difference in density is such that even considerable disturbance will not mix them. However the opening sentence is true only down to 4° C. At lower temperatures cold water floats on warm water. The result of this peculiar property of water is that the lakes of the temperate region, with which we are concerned here, become stratified in winter and in summer. Because of the greater difference in summer it is the stratification during this period that is the more important biologically.

The left-hand side of Figure 1 shows the actual state of affairs in Windermere in February 1948; the temperature is uniform at 4° C. from top to bottom. Incidentally, a fact not always appreciated by dinghy sailors is that these cold conditions may persist well into March. By this time the sun is rising higher each day and shining for longer, and it starts to warm the upper layers – only the upper metre or two because the heating part of its rays is soon absorbed in water. The first fine spell is probably followed by windy weather, and the warm water at the surface is mixed with the colder water below; the lake is once more at a uniform though slightly higher temperature. Sooner or later, however, the two layers are established with such a big temperature difference between them that they remain separate for the rest of the summer.

Fig. 1 Temperature of Windermere at different depths on February 2nd, 1948, and July 8th, 1948 (from data supplied by Dr. C. H. Mortimer)

Once it is firmly demarcated, the warm upper layer of water increases in temperature relatively rapidly, and it also increases in depth because, when disturbed by the wind, it mixes with eddies of cold water from below. The cold lower layer has no source of heat, except perhaps from a small amount of mixing with warm surface water. The right-hand side of Figure 1 shows that by mid-summer the upper layer is many times warmer than in winter, but the lower layer has increased in temperature by no more than two degrees. Between the two there is a short depth of water in which the temperature drops rapidly. The warm upper layer is known as the epilimnion, the cold lower layer as the hypolimnion, and the region of rapidly dropping temperature between them is the thermocline. Greek scholars will have no difficulty with these terms, others may be puzzled to remember which of the first two is which, but there is a simple mnemonic, for epi- and upper begin with vowels and hypo- and lower with consonants.

Wind is the next factor. Blowing over the surface of a body of water it will set up a current which carries water to the leeward side. Obviously there must be a compensatory return current. This will flow along the bottom of the epilimnion where it floats on the hypolimnion, and therefore the wind keeps the epilimnion in constant circulation. There will be some eddying and turbulence and this will keep the water of the epilimnion thoroughly mixed. In Britain totally windless periods seldom last long.

Chemical analysis shows that the water of the hypolimnion is well mixed, for the concentration of dissolved substances is the same at all depths. Dr C. H. Mortimer, F.R.S., made a thorough investigation of this phenomenon in Windermere and eventually provided an explanation. He then devised a model which demonstrated the explanation in a very convincing way. It represented the longitudinal section of a lake enclosed between two sheets of plate glass. Two wires ran the length of the section near the surface, and an electric current passed through them warmed the adjacent water to create an epilimnion. A dye was carefully run into this to mark it. A gale was created by two blowers originally designed to dry ladies’ hair. Water is so heavy that a real gale tilts the water surface so little that only the most sensitive apparatus records a rise in level at the leeward end, but the thermocline tilts considerably. In the model the epilimnion was blown towards the leeward end and the dyed water was displaced into the form of a wedge. If the wind was strong enough the thin end of the wedge did not reach the windward end; in other words the hypolimnion was exposed at the surface here and temperature measurements have shown that this happens in a real lake. Some of this surface hypolimnion water is mixed with the epilimnion which, when the wind ceases, is accordingly deeper and colder. However, the way in which the dyed water in the model retained its identity on top of the clear water was striking. When the wind drops the epilimnion flows back until the thermocline is level once more, but its momentum causes it to overshoot, the epilimnion piles up at the other end and the thermocline is tilted in the opposite sense. This seiche, as it is called, continues for days in a real lake, the angle at which the thermocline tilts decreasing with each oscillation. In the north basin of Windermere an oscillation takes about nineteen hours. On the right-hand side of Figure 1 the epilimnion is of uniform temperature down to a depth of nearly ten metres and this is the condition found after thorough mixing by strong wind. In calm weather the temperature tends to decrease, often in an irregular manner, from the surface to the thermocline. As the hypolimnion flows to and fro with each oscillation, irregularities on the bed of the lake set up eddies, and these produce the mixing of the water which led to the investigation originally.

The rivers and streams flowing into a lake are usually at a temperature well above that of the hypolimnion and accordingly will mix only with the epilimnion.

With the shortening of the days in autumn, particularly if there is a fine spell with cold clear nights, heat is lost by radiation during the hours of darkness. The epilimnion begins to cool down and eventually, sometimes not until December, a gale will obliterate it, mixing it completely with the hypolimnion.

There remains one other factor to mention before anything living comes into the picture. The sun’s rays have been considered so far only from the point of view of their heating properties; for the activities of plants light is more important. Light rays do not penetrate far into even the purest water, and in most waters there is something extraneous to reduce their penetration still more. Any sedimentary matter in suspension, any colouring such as that derived from peat, and living organisms themselves all absorb rays of light. (Fig. 11) shows that in three Lake District lakes light goes farthest into the pure and barren waters of Ennerdale Lake, less far into the richer waters of Windermere, and least far into the peat-stained and rich waters of Bassenthwaite. Since light does not penetrate far into water, plant growth is only possible in the upper layers, and is nearly always confined to the epilimnion.

May we recapitulate here, since so much of what follows depends on the physical conditions which have just been discussed. During the summer months the lake is divided into an upper warm epilimnion and a lower cold hypolimnion, which are to all intents and purposes completely separate (Fig. 1), and all plant growth takes place in the epilimnion.

Algae (minute floating plants) are present in the open water all through the winter but physical factors, notably the short days and the low light intensity, are unfavourable for rapid multiplication. When conditions are right for this there is a rapid and colossal increase in numbers which is checked when the substance in shortest supply is exhausted. Phosphate and nitrate are two important nutrients but in Windermere, the size of the population of Asterionella, the commonest diatom, is limited by the concentration of silica, which the alga requires for its skeleton. Once reproduction is halted, the population declines rapidly (Fig. 2). After the spring outburst various species of algae rise and fall in numbers, but the total attained is much less than that reached in the spring. The zooplankton (small floating animals) reach their maximum abundance a month or two later than the phytoplankton.

Fig. 2 Increase in phytoplankton and decrease in the concentration of certain salts in Windermere in 1936

The animals living in the mud at the bottom of the lake are in perpetual darkness and almost constant temperature. Little is known of their activities in any British lake, but P. M. Jónasson has shown that in the Danish Esrom lake the growth of a chironomid depends on the rain of dead plankton falling from above. This comes to an end in winter and the growth of the larvae stops. It starts again in the spring and proceeds rapidly, but is checked again when the oxygen is used up in the lower layers and the larvae can do little more than survive. They emerge early in the following year. Most larvae take two years to complete development but a few achieve it in one, but their eggs are all eaten by their brothers and sisters who have failed to develop as fast. The result is a big emergence every other year. Nearly all aquatic insects emerge as adults in spring or summer, presumably because of the physiological difficulties of flying in cold weather, and this must impose a seasonal rhythm upon their development.

A ring of green algal growth on the stones in the shallow water of a lake appears in spring, but most of the stoneflies and some of the Ephemeroptera of this region grow during the winter and pass the warm part of the year in the egg stage. This phenomenon will be discussed further when streams are described. One of the commonest animals in the reed-beds is Leptophlebia (Ephemeroptera) and this is a species that grows throughout the winter, but most of the fauna grows during the summer.

These various plants and animals are continually dying and decomposing, broken down by various agencies about which we do not know very much at the present time. Fungi and bacteria set upon their dead bodies and reduce them to fine particles and simple compounds, which serve as food for other organisms, so that there is a constant process of breaking down and building up in the epilimnion. But some of the decaying fragments, with the organisms breaking them down, fall through to the hypolimnion, and we must leave them for the moment to describe what has been happening there. More important perhaps is what has not been happening; there has been no plant growth, because it has been too dark, and therefore no utilization of the dissolved substances for want of which algae have been dying in the layers above. Evidently division into epilimnion and hypolimnion reduces the productivity of a lake.

The decaying matter which falls down to the hypolimnion continues to decay, though at a slower rate on account of the low temperature, and it uses up oxygen. There is no source from which the oxygen in the hypolimnion may be replenished, and consequently the concentration falls steadily all through the summer; it may reach nil if the lake is a productive one and the hypolimnion small – an important point, as will be seen in the next chapter.

The decaying matter may eventually reach the bottom, and here some of it is eaten by the animal inhabitants of the bottom mud, and some of it is broken down into simple substances by bacteria and other agents. Most of the organic matter found deep in the mud, where it must have lain for thousands of years, was washed in from the land. But these simple substances cannot reach the surface layers, where they could be used for building up more living matter, until hypolimnion and epilimnion mix in the autumn. By then biological activity is reduced, and by the time there is a big demand again for dissolved nutrients in the following spring, much of the supply will have been washed out of the lake. On the average, water takes nine months to pass through Windermere, and therefore during the winter there will be considerable depletion of the dissolved substances released from the hypolimnion by the autumn mixing. Again it becomes apparent that the formation of a hypolimnion prevents the development of the full potentialities of a lake.

Large fragments hardly decay at all in the cold mud at the bottom of deep lakes. Wasmund (1935) gives an account, illustrated by gruesome photographs of bodies, including three human ones, that have been brought up, generally in fishermen’s nets, after many years in the water.

Dr C. H. Mortimer (1941–42) has recently shown that, when there is oxygen at the surface of the mud, iron is present in the oxidized ferric state and forms a colloidal complex with various other substances. This colloidal complex tends to hold the simple products of decay, and therefore augments the locking-up process caused by the slow decomposition in the mud. But, if all the oxygen is used up, the ferric iron is reduced to the bivalent ferrous state, which goes into solution with consequent breakdown of the colloid complex. This liberates the other substances, and Mortimer was able to show, both in an artificial experiment in an aquarium and in a lake, that the disappearance of oxygen from the hypolimnion is followed by an increase in the concentration of silicate, phosphate, ammonia, and iron in the water.

The above are factors which affect the plants and animals living out in the open water of a lake and in the mud below it.

A different assemblage of living things inhabits the shallow regions near the shore, and this population too is affected by physical and chemical processes. The most important is wave-action. The effect of this factor depends on the nature of the land on which it acts. Waves beating upon rock will disintegrate the weaker patches and leave the harder ones projecting as ridges but the total effect is small; waves beating upon sand or peat, on the other hand, will erode the shoreline rapidly. Many lakes are surrounded, partly or entirely, by moraine deposits known by various names such as glacial drift, boulder clay, till, or sammel. Waves eroding a shore of this type leave in situ only the larger stones and boulders and carry away the finer particles, which eventually come to rest in deeper water away from the shore, or in some sheltered bay. The coarsest particles will be moved the least, the finest the greatest distance, and there will therefore be a graded series passing into deeper water farther away from the shore. The processes of erosion and deposition result in what is known as a wave-cut platform and are illustrated diagrammatically in Figure 3.

Fig. 3 Diagram of the erosion of a boulder clay shore to give a wave-cut platform

Sometimes the material removed is not carried out at right angles to the shore but at an acute angle so that, when it settles, it forms a spit. Such formations are of importance to animals and plants because they create areas of quiet water which are the resort of certain species unable to tolerate the conditions on a wave-beaten shore.

Deltas are even more important features of the lake shore. They may be no more than bulges in the shoreline, or, at the other extreme, they may cut a lake in two. Good examples of deltas at all stages are to be seen in the Lake District lakes. The delta of the Measand Beck stretched two-thirds of the way across Haweswater, before this lake was dammed in 1941 to provide more water for Manchester. A stage farther can be seen in the valley of Buttermere and Crummock Water, which were left by receding glaciers as one large lake. Since then Sail Beck, flowing in from the east, has cut the original lake into two and its delta now provides the half-mile of flat land in the valley floor between the two lakes. Another pair of lakes, Derwent Water and Bassenthwaite, show a still more advanced stage. Here again the two were formerly one, but the River Greta has poured so much silt and gravel into the original lake that there is now a full two and a half miles of plain separating the north shore of Derwent Water from the south of Bassenthwaite.

Some of these deltas are much too large to have been brought down by the little streams existing today, and much of the material was probably swept down during the last stages of the Ice Age by the bursting of ice dams and other minor cataclysms.

We may pass from generalities to describe a portion of the shoreline of Windermere, for it illustrates several of the points already made, and is referred to later when the fauna is discussed. The shoreline in question is bounded to the north by a ridge of rock jutting into the lake. The sides of this promontory, which is known as Watbarrow point (Fig. 4) are smooth, and run down at a steep angle to a depth of nearly 100 feet. To the south the same kind of rock, Bannisdale slate, is exposed at the edge of the lake, but weathering and wave-action have broken it up considerably, and the products of its disintegration litter the lake floor. They are large flat angular slate-like stones. Moon (1934), who has studied this region of the lake, refers to it as the ‘Bannisdale’ shore and contrasts it with the ‘drift’ shore which lies to the south. The drift shore consists of stones and boulders but these are round, not flat and angular, and there are finer particles between them. This shore has been formed by the erosion of a mound of boulder clay or glacial drift, somewhat after the manner shown in Figure 3. The hinterland of the Bannisdale shore is covered by woodland, but that of the drift shore has been cleared of woodland at some time and is now pasture. This is not coincidence; where the underlying slate is not covered with glacial drift, the topsoil is often so thin, and rocky outcrops are so frequent, that cultivation of the land is not feasible; but where the rock is covered by boulder clay, it has been worthwhile to remove the forest and bring the land into agricultural use.

Fig. 4 Windermere, north end showing reed-beds. Reed-beds are stippled

Figure 4 shows that at the south end of the drift shore there is a bay – High Wray Bay – which is somewhat protected. Only the comparatively rare easterly gales will blow right into it, and the range of direction of wind from which it gains no protection at all is but 30°. High Wray Bay is floored with sand.

Sandy Wyke Bay farther north is more sheltered. The range of direction of wind which will blow straight into it is only 20°. But a glance will show that the amount of exposure is not to be measured entirely by the angles drawn in Figure 4. If a wind blowing in the direction of the more southerly of the two pecked lines bounding the High Wray Bay angle veer slightly, it will still drive waves into part of the bay, and it must shift through nearly another 30° before complete protection is obtained. But if a south-easterly wind that just blows full into Sandy Wyke veer but a few degrees, the projecting coastline will shelter the bay almost completely. Sandy Wyke Bay is also sandy, but there is a big reed-bed growing in it.

Only a north wind will blow right into Pull Wyke South Bay, but it will traverse so short a stretch of water that the waves raised will not be of significant size. This bay is floored with fine mud. The vegetation shows the zonation typical of quiet conditions. In the shallowest water there are various emergent plants such as reeds, rushes, sedges, and horsetail; in deeper water there are plants with leaves floating at the surface, such as water lilies; and beyond them are plants, such as pondweeds, stoneworts, and quillwort, which live totally submerged throughout life.

The phenomena described above are of such general occurence that, in spite of the diversity of lakes, a ‘typical’ lake is a useful concept. There are two main types of lake that may be styled ‘atypical’. Lakes that have a large surface area and little depth do not stratify. Lough Neagh, possibly even Bassen-thwaite and Derwent Water in the Lake District, are examples. Lakes of this type, however, have not been studied thoroughly. and all that can be said at present is that they have been found to be unstratified in summer at a time when epilimnion and hypolimnion are clearly demarcated in other lakes. Of course, any body of water in temperate climates will show some stratification after a hot day; the important point is how long stratification lasts. It is possible that these large shallow lakes may stratify throughout an occasional summer when sunshine is unusually abundant and wind unusually scarce. Information should be available from Lough Neagh soon, as the New University of Ulster has established a station there. It is difficult to make observations sufficiently often unless a laboratory is available, and the ideal, described in the next chapter, is an arrangement of thermometers in the lake connected to a recorder in the laboratory.

The other kind of atypical lake is known technically as meromictic, and its peculiar feature is permanent stratification. The density difference that prevents mixing is due to substances in solution, not to temperature, and is often but not invariably due to peculiar geological conditions. The condition could arise in any lake where production is high and circulation low. Poor circulation occurs in areas where strong wind is rare, and the effect of lack of wind will be enhanced in a lake with a small surface area relative to its depth, and with not much water flowing in. An abrupt transition from winter to summer and from summer to winter is another factor that plays a part. Given these conditions one may postulate that the meromictic condition arose in the following way. If at the end of a summer the hypolimnion is greatly enriched by decomposition of organisms produced in the upper layers, it will be denser than the water from which they have come when both layers are at the same temperature. It is not difficult to suppose a year in which the cycle of events has resulted in both being at 4° C. The epilimnion will float on the hypolimnion. If there is little wind and ice forms soon, this state will endure until the spring. If there is little wind then to upset this delicate state of balance, and plenty of sun to increase the density difference by warming the upper layers, stratification will have lasted a year. By the following autumn the accumulation of two years’ production will have increased the density difference due to solutes between hypolimnion and epilimnion and the chances of their remaining unmixed during the following season are greater. The longer the two remain separate the more the energy required to mix them, and the less likely mixing becomes. It is believed by Professor I. Findenegg, who discovered the condition, that certain lakes in the Carinthian province of Austria became meromictic in some such way.

So far no definition of the word ‘lake’ itself has been attempted. Our colleague, Mr F. J. H. Mackereth, has been heard to say that a lake is no more than a bulge in a river. This idea is more useful to a chemist than to a biologist, but it is salutary that a biologist should remember how much of what takes place in a lake is governed by what is washed in from the drainage area. A lake is a piece of water of a certain size but at what size the word pond becomes applicable is a matter of opinion. It is one of those continuous series, frequently encountered in biology, where the difference between two ends is enormous but any lines drawn in between them to separate categories are arbitrary. One definition maintains that any piece of water which is so shallow that attached plants can grow all over it is a pond. The pedant has no difficulty in picking holes in this definition and pointing out that the depth to which attached plants extend varies very much with the transparency of the water; a cattle pond only a foot deep may be without vegetation in the middle because the light is cut off by innumerable small organisms which live in the open water and batten on the nutrients supplied by the dung. Or the nature of the substratum may be unsuitable for attached vegetation. Another school holds that, if a body of water becomes divided into epilimnion and hypolimnion and remains so divided throughout the summer, it is a lake and not a pond. Stratification, however, depends, not on size, but on the relation of depth to surface area and also to exposure to wind. The latter also determines to some extent whether the edges are eroded by wave action or not, and therefore blurs the definition according to which a lake is large enough for its shores to be eroded and a pond is not. In a restricted area, or for a given purpose, a worker may find a useful distinction between a lake and a pond, but in general no scientific distinction can be made.

CHAPTER 3

APPARATUS FOR STUDYING LAKES

Anyone provided with a stout net, some bottles, and a white dish or sheet can do an immense amount of work in fresh water. He can wade as far as is necessary into many ponds and streams and collect in the shallow water of lakes. He can even collect the plankton from the open water of a lake, if a suitable point of vantage is to be found. However, more serious work on the open water and any kind of work on the fauna of the mud or of the submerged weeds requires more elaborate apparatus. The first necessity is a boat. If work is to be done in deep water a winch is desirable. A very useful type of light winch which can be put on to any row-boat is made by the firm of Friedinger of Lucerne. It has the advantage that the wire is paid out over a pulley block of special circumference to which is attached a cyclometer, so that the depth at which the instrument hangs below the surface is shown accurately to the operator in the boat. With such a winch the different instruments for measuring temperature or light intensity, and for collecting water samples or plankton, can be lowered easily to any depth. It is often necessary to operate an instrument at a considerable depth in the water before hauling it back to the surface, and this may be achieved by despatching a so-called messenger down the wire. The messenger is usually a lump of metal with a hole drilled through it; on reaching the instrument at the bottom of the wire, it strikes some projection which is arranged to release a catch in order to perform the necessary operation.

An example is provided by the reversing thermometer. This thermometer is mounted on a pivot about its middle, and the pivot has a spring which turns the thermometer upside down when the catch at the top of the frame is released by the messenger. This reversal breaks the mercury column, and so, when the thermometer comes to the top, it shows the temperature at the depth at which the messenger struck it; warmer water through which it may pass leaves it unaffected. The ordinary clinical thermometer works on somewhat the same principle.

The thermometer has to be specially built to resist the high pressure which obtains under water and so it is a comparatively large instrument, which will not immediately take up the temperature of the surrounding water. Accordingly it has to be left for a few minutes at each depth from which a reading is desired, and, since, further, it must be hauled to the surface to be read, the taking of a series of observations is a long process. It is still used on expeditions and long excursions, but for regular work it has been obsolete for some years. The popular device at present contains a substance whose electrical resistance changes considerably with a relatively small change of temperature. It requires, therefore, a battery and a galvanometer but, when these are available and transport presents no problems, the apparatus, known as a thermistor, is convenient. Much of Dr Mortimer’s work, described in the preceding chapter, was carried out from a boat, but latterly he had a series of thermistors slung at intervals between the bottom and a buoy moored in the deepest part of the lake. Each was connected to a recorder in the Ferry House, and what amounted to a continuous record was obtained. Dr Mortimer had nothing to do except convert the readings to °C. and work out what was happening. One of the authors was once explaining to a group of visitors what the recorder indicated and had just got to the point where emphasis is laid on the fact that the bottom of the lake is always cold when, by unfortunate coincidence, Mortimer, out on the lake, started to haul his line of thermistors up to the surface.

Apparatus of a somewhat similar kind is used for measuring the amount of light penetrating below the surface, which, we have seen, is so important in determining the depth of plant activity. A photoelectric cell, contained in a pressure casing, is connected to the surface by wires, and a window facing upwards is inserted into the pressure casing so that rays of light penetrating from the surface can strike the cell. They cause a small electric current, varying in amount according to the intensity of the light, and this can be measured in much the same way as with the temperature apparatus by a galvanometer in the boat.

When measuring sub-aqueous light, it is necessary to lower the instrument from a long support projecting sideways from the boat, because otherwise the boat would shade the instrument hanging beneath it. Not only the general intensity of light below the surface, but also the kind of light, is of great importance. This can be determined with the same instrument by covering the window with filters of various colours.

There is a much simpler but useful instrument for giving a rough idea of the clarity of water, known as Secchi’s disc after the scientist who first used it. This consists of a white plate of 20 cm. (8 in.) diameter, which is lowered below the surface to the point at which it becomes invisible to the naked eye. This is, of course, a crude way of measuring how far light can penetrate, but Secchi’s disc is very easy to carry about and use, and is accurate enough to provide comparisons between different types of water.

For most kinds of chemical work on water, and also for studying microscopic life, it is necessary to obtain samples of water from different depths. Here again the simple expedient is adopted of despatching a messenger down the wire to close a water-bottle at the desired depth. A variety of different kinds of water-samplers are used for this purpose. A simple example is a metal cylinder open at both ends so that when it is lowered it will pass through a column of water without disturbing it much. It is halted at the required depth and a messenger is sent down the wire. This releases lids which close over the top and bottom of the cylinder and are kept tightly in place by strong springs. The apparatus is now watertight, and can be hauled to the surface with a sample of water from the depth at which it was closed.

This self-closing metal water-bottle is an excellent instrument for many purposes, but for the study of bacteria, of which very many kinds inhabit fresh water, it is no use. The spores of bacteria are everywhere – in the air, in the water, on one’s fingers – and accordingly a water-sampler for bacteriological investigations has to be arranged so that every part of the instrument which comes in contact with the actual sample of water collected can be sterilized by heat and kept in a sterile condition until the sample enters it. The principle was therefore adopted of using glass sampling vessels of a simple and standard pattern, held in a metal framework fitted with the necessary gadgets to operate an opening and closing device. The bottle is sealed with a bung pierced by two tubes. One is long and runs down to the bottom of the flask, and the other ends flush with the inside of the bung and is bent into an S-shape outside. A U-shaped piece of glass rod fits into a length of rubber tubing attached to each tube. When the bottle is at the required depth, a messenger is sent down to release a strong spring which pulls the glass rod out of the two tubes. Water runs down into the flask through the long tube, driving air out of the other tube until the flask is completely filled. A bubble of air remains in the bent tube so that no mixture can take place between the sample in the flask and the surrounding water during haulage of the whole apparatus to the surface. In practice, a number of flasks, each with its stopper and tubes, are sterilized in the laboratory and then a series of samples for bacteriological examination can be taken at different depths or at different places during the same outing.

Water may be obtained from any depth by lowering a tube and sucking. In the early days of the Freshwater Biological Association, when lack of money placed a premium on ingenuity, Mortimer used a bicycle pump with the washer reversed to obtain samples. If two bottles are connected in series, with the larger nearer the pump, the smaller and the tube will have been sufficiently washed by the time the larger is full. Water can also be raised by a stream of air bubbles emitted from a small tube inside, and extending almost to the lower end of, a larger one.

Plankton is commonly caught by means of a conical net made of material woven in such a way that the holes retain their size. A mesh of 60 meshes to the inch is generally used for animals, one of 180 meshes to the inch for algae. The efficiency of a net falls as the catch blocks the pores and for quantitative work the amount of water that has passed through the mouth must be measured by means of a propeller attached to a recorder. Many methods of catching plankton have been tried, particularly at the station at Pallanza, and the quest continues. One difficulty is that some of the animals swim away from an object they see coming through the water, or away from the pull of a current caused by suction into a pipe.

The easiest medium to sample is the mud on the bottom of a lake, though each sample must be subjected to a tedious process of sieving before the animals can be isolated. Often a simple tube will secure enough animals. If they are scarce, a larger sample may be obtained with a Birge-Ekman grab, which is a metal box open at the bottom and provided with two hinged lids at the top. Two jaws to close the bottom are held along the sides against the pull of strong springs. Going down, the apparatus passes through the water with little disturbance. This is important, for if there is obstruction the apparatus will not pass through the water, but push it aside, and it will also push aside the top layers of the mud if these are fine and fluid. The lids fall when the box sinks into the mud and comes to rest. A messenger trips the bridle that holds the jaws up and the springs then pull them together to close the bottom of the box.

Stones and vegetation are less easy to sample quantitatively. Several workers have found that the number of animals caught in a given time or in a given number of sweeps of a net indicates, sometimes with unexpected accuracy, relative numbers in different places. Numbers per unit area can be calculated if samples with a quantitative sampler are taken in the same part of the lake at the same time. The Danish workers have used a square box open top and bottom to sample stony substrata near lake margins. It is placed over the bottom, stones are removed, and the water inside is baled out and poured through a net. This method cannot be used in running water because the box deflects the current downwards and causes it to scour the area that is to be sampled.

In Windermere H. P. Moon used a square frame on which he could pile stones to represent an area of natural substratum before lowering it onto the bed of the lake and leaving it until it had been colonized. The frame is one-third or one-half of a square metre in area and underneath it is covered with fine gauze to prevent the loss of animals while the frame is being hauled up. Stout wire-netting beneath the gauze adds additional support for the stones.

The Surber sampler is used by some workers to sample the stony substratum of streams and rivers. It consists of two frames, generally about one tenth of a square metre in area. These fold into one plane for transport and open at right angles for use. The horizontal one is placed on the bottom and the vertical one supports a net. Stones are then removed from the bottom inside the horizontal frame, and brushed in the mouth of the net to dislodge animals clinging to them, after which the remaining small stones, gravel and debris are stirred with a stick until it is believed that all living material has been swept into the net. We have not found this a satisfactory instrument because the current is often so swift that when one stone is picked up the stones above it shift to fill the gap. If the current is slow many good swimmers probably swim out of the net, if they are ever carried into it. More satisfactory, though not by any means free of error, is a shovel of some kind which can be pushed into the substratum for a known distance. Designs have ranged from a shovel with high sides with a net at the back, to a cutting edge connected to the handle by two strips which also support the frame of the net. A strong coarse net arrests the stones and a long tapering fine one any animals that have let go. If the stones are tipped into a solution of high specific gravity, calcium chloride or magnesium sulphate are suitable, the animals float to the top.

Weeds in rivers trail downstream and may be severed with shears and caught in a large bag. Another method is to hold a box with sharp edges a known distance above a lid and then bring the two together enclosing and severing the weed in a known volume. Weeds in still water rise vertically, and a device that cuts each leaf or stem as it meets it is preferable to one that pushes them downwards and does not cut them until they are pressed against the bottom. One such instrument consists of two tubes, about 8 cm. across, fitting one within the other. A boss on the inner passes through a slit in the outer and holds it in position, allowing a small amount of rotation to and fro. As the tubes are lowered into a weed-bed, the outer tube is rotated and the vegetation is severed between the sharp teeth which have been cut in the lower end of both tubes. They pass across each other like the teeth of a haycutter.

Incidentally parallel samples with this instrument and a net have shown the latter to be unexpectedly selective. It collects an unduly high proportion of species that tend to flee and an unduly low proportion of those which, like leeches, tend to cling to the substratum.

Larvae of Chironomids and many Trichoptera cannot yet be named, and in order to find out what species are present it is necessary to trap the emerging adults. In still water a box open at the bottom may be floated in a frame. The top should be of some transparent plastic material to keep the rain out, but at least one side should be of gauze to prevent condensation. Dr. J. H. Mundie has devised various modifications for use in both still and running water. In a lake he used conical traps into the top of which a screw-top jar could be screwed. Entrance to it is through a cone which prevents the animals falling back into the water. The whole apparatus can be submerged, an advantage in a lake to which the general public has access. For use in streams he built a heavy trap that could be anchored to the bottom. Triangular in both plan and elevation it offered minimum resistance to the current, which tended to press it downwards. Three legs kept it raised off the bottom and the catch entered a screw-top jar as in the other model.

Much decomposition takes place in the top few centimetres of the mud, and substances diffuse from it into the water. A study of these processes, important to the general economy of the lake, requires a sample disturbed as little as possible. The Birge-Ekman grab does not bring up such a sample and for this purpose the Jenkin surface-mud-sampler was invented. It consists of a large glass tube about 6 cm. in diameter, held by a band about its middle on to a metal frame, standing on four spreading legs. The sampler sinks into the soft mud when lowered to the bottom, but without disturbing it, and then, a messenger sent down the wire having released a catch, two pairs of arms travel forward to place a cap on either end of the glass tube. The speed at which these caps move into position has to be very slow in order not to disturb the mud and water in the tube, and this is effected by means of a pressure chamber of the same kind as that used for preventing doors from slamming. When closed, the glass tube contains a sample of the top few inches of deposit together with the water above, and the caps at either end are held tightly in place by springs. At this point the whole apparatus is hauled to the surface gently to avoid disturbance. The glass tube is detached from the frame, and the sample of bottom deposit, complete with the water immediately above it, just as it was at the bottom of the lake, can be carried into a laboratory for chemical and other tests. The person who devised this most useful apparatus was a retired engineer, Mr B. M. Jenkin, and it is worthy of note that the first experimental model, which he made largely out of meccano, operated so well that it was still in frequent use at the laboratories of the Freshwater Biological Association at Windermere ten years later.

Mr Jenkin was set another and much more complicated problem, namely to devise an instrument capable of extracting cores from the bottom deposits in lakes, if possible to a depth of twenty or thirty feet below the mud surface. A good deal of trial showed that an ordinary open tube or pipe was useless for this purpose because it compresses and disturbs the layers of deposit too much. After some thought, Mr Jenkin hit upon the idea of a sampler which could be thrust into the deposit first and then made to carve out a core by means of a curved cutting blade working on a long pivot. The business end of the instrument, which cuts out the core, is about four feet long, and consists of a tube cut in half lengthways and covered with a metal plate except for a slit down one side. A second half-tube lies within the first, attached on an axis in such a way that it can be rotated out through the slit. When the apparatus has been driven to the required depth, the inner half-tube is rotated, its sharp leading edge passes out of the slit, and, travelling through 180°, comes up against the far side of the plate. Between the inner half-tube and the plate there is now a sample of mud isolated from its surroundings with the minimum of disturbance. The rotation of the inner half-tube is effected by a system of cogs and a driving-wheel worked by a wire from the surface. The cutter can be attached to a series of tubes so that it can be driven to the desired depth in the mud before it is operated. The force required to press the whole instrument into the bottom is provided by a series of heavy lead weights, of which the number is adjusted according to the depth at which the particular sample is required. Thus a complete core, say twenty feet in length, is obtained in a series of overlapping cores each four feet in length.

The method of using this instrument is briefly as follows. First a pontoon with a derrick is firmly anchored over the spot from which the core is to be taken. Next a flat weight on a thin wire is lowered to the bottom to serve as a guide and as an exact measure of the depth to which the main instrument is subsequently lowered. Then the coring machine itself is lowered from the derrick on a stout wire with a pair of arms clutching the aforementioned guiding wire. The machine is allowed to sink into the deposit to the required depth, when a sample is required from near the surface; for a deep core the machine is allowed to sink as far as it will go and driven the rest of the way. Then a messenger is despatched down the guiding wire in order to release the arms, and the guiding wire is hauled up, an action which also operates the machinery for cutting out the core. It remains for the whole machine to be hauled to the surface and laid flat before the half revolution of the cutting blade is reversed and the core is exposed ready for transfer to the laboratory. It will be appreciated that the successful handling of this apparatus is no mean task; in fact it requires a team of three or four operators well trained in the particular functions which each has to perform at the right moment. With its aid, however, a large number of cores, some of them covering twenty-one vertical feet of deposit, have been collected from many parts of Windermere, and these have provided valuable information about the history of lakes since the Ice Age.

Fig. 5 Diagrammatic cross-section of the Mackereth core-sampler (not to scale). The letters are referred to in the text. (Limnol. Oceanogr. 1958)

Mr Jenkins’ apparatus proved excellent for use on Windermere, where the necessary pontoons could be borrowed, but not elsewhere, and accordingly Mr F. J. H. Mackereth devised a portable model (Fig. 5). The problem was to ensure stability while the core was being obtained. This he solved by basing the corer on a large cylinder resembling a dustbin (G). This sinks some way into the mud when the apparatus has been lowered, and is then forced farther in by means of a pump (P) which removes the water from its upper portion. A secure base has now been secured for the rest of the operation. The core is obtained in a long tube (B) housed inside a second tube (A), which is attached to the centre of the top of the anchoring cylinder. The second problem was how to drive the corer into the mud from a small boat that could not easily be kept exactly above the apparatus. Compressed air passing down a flexible tube (O) was the solution. It involved a piston fitting inside the outer tube and closing the top of the inner one (C). Some means of evacuating the inner tube as it moved downwards was essential, for otherwise a solid cylinder rather than a tube was being forced into the mud. This is achieved by a fine central tube (D) which holds in position a piston (F) at the mouth of the inner tube when this is retracted, passes through the upper piston and out through the top of the outer tube (L) to which it is attached. When compressed air admitted to the top of the outer tube forces the inner tube down into the mud, the air in the inner tube escapes through the fine central tube and the corer passes into the mud, causing no more compression than is due to the friction of the walls. When the inner tube is nearly fully extended, the compressed air escapes into a side tube (I), which leads it into the anchoring cylinder. This is forced out of the mud and brings the whole apparatus to the surface. Compressed air passed into the fine inner tube brings the inner tube back into the outer and, at the same time, ejects the core. This apparatus has been carried to tarns in the mountains by a helicopter and successfully used there.

CHAPTER 4

DIFFERENT KINDS OF LAKES

Lakes, geologically speaking, are transitory features of the landscape. The biologist who studies a lake is likely sooner or later to find that the answer to some problem he is seeking to solve is to be sought in past history, and particularly in the way the lake was formed. Lakes and ponds have originated in many different ways and much ingenuity has been devoted to fitting them into schemes of classification. We shall not dwell on the groupings and subgroupings which have been suggested, but, in the first part of this chapter, prefer to take the lakes as they come, starting in the north of Scotland and travelling southwards.

The Great Glen is a tear in the earth’s surface, and Loch Ness, which lies in it, provides an example of a lake associated with faulting. Loch Ness is 21³/₄ miles (35 km.) long and a little under one mile (1.6 km.) in average breadth, so it is a long and narrow lake; its greatest depth is 754 feet (230 m.) and its mean depth 433 feet (130 m.), so it is deep and steep-sided. Its mean depth is greater than that of any other British lake by quite a big margin, though there is one, Loch Morar, which is deeper at the deepest part (1017 feet = 310 metres). All these features are characteristic of tectonic lakes, that is lakes formed originally by movements of the earth’s crust. Also in this class are some of the most striking lakes of the world, such as the Dead Sea and the lakes in the Great Rift Valley of Eastern Africa. These were caused partly by lateral tearing, as is the Great Glen, but there followed a lowering of a strip of the earth’s crust so that what is now the floor of the rift valley was once level with the high land on either side.

Most of the other lochs in the Scottish Highlands owe their present form to the work of ice when the country was covered with it during the Ice Age and so were the llyns of the Welsh mountains and the lakes of the English Lake District. Indeed, nearly all the larger stretches of water in Britain were formed by glaciers, at least in part. In some cases their basins were gouged out by glaciers flowing down mountainsides, usually in valleys cut by a stream in an earlier, more clement period. When the mass of snow and ice and rubble reached the bottom of the slope it dug into the ground and excavated a great trench. This trench became the basin of the lake when the glacier retreated with the onset of warmer conditions. The mass of material which the glacier plucked from the land it passed over was deposited in mounds or moraines at the snout of the glacier, and some glacial lakes are dammed up by a moraine which makes them deeper than if they were contained in the actual excavation alone. Glacial lakes, like rift valley lakes, are usually long and narrow and relatively deep; Windermere, for example, is 10¹/₂ miles long but only half a mile wide on the average, and 219 feet deep at the deepest point. Their sides tend to be parallel and any major irregularity in the shore line is often of more recent age. Lakes of this type were formed only in hard rocks where the relief was rugged, and steep valleys concentrated the glacier and directed its excavating effort to a circumscribed area.

Also of glacial origin are the smaller lochans, tarns, and the small lakes in cirques, corries, or cwms which are often to be found near the tops of mountains. They are frequently circular in outline and they mark the place where the snow or ice piled up and a glacier took its origin.

An ice sheet covering a plain did not excavate because its effort was dispersed and not concentrated, but it did give rise to lakes none the less. As might be expected these are of a different type; Loch Leven is an example and no fisherman requires a biologist or geologist to tell him that there is something fundamentally different between Loch Leven and the Highland lochs. As the ice sheet which covered Scotland began to recede, a large lobe of the glacier flowing down the Forth Valley became isolated in the centre of the Kinross plain. It was surrounded by clay, stones, boulders, and suchlike products of ice erosion washed along in the water from the melting glaciers, much of it coming through a pass in the Ochils from the Tay Valley. A considerable depth of this material was deposited on the plain, but in the middle there was this big block of ice melting slowly because of its large size, like an iceberg in the North Atlantic. When it finally disappeared it left a hollow where it had been sitting and this filled up with water to become Loch Leven. The shape of Loch Leven is quite different from that of either glacier-cut or rift valley lakes: it is not much longer than it is broad, one axis being 3²/₃ miles (5.7 km.) and the other 2²/₃ (4.1 km); its mean depth is only 15 feet (4.5 m.) and its greatest depth only 83 feet (25 m.).

The Cheshire Meres were formed in a similar way to Loch Leven, although subsidence of the land surface also played a part. Outside Britain there are many lakes of the same type: two groups, which are referred to in later chapters because they have been studied in much detail, are the numerous lakes in the Wisconsin area of North America and the Baltic lakes of Denmark, Germany, Poland and U.S.S.R.

Also characteristic of mountainous areas are the much smaller peat pools. These may occupy holes where stone for a wall or a house has been quarried or sometimes a rock basin of natural origin, but most of them are formed by the growth and then the erosion of peat. Some of the largest are to be seen on the Pennines, for the Pennines have flatter tops than the mountains in Scotland, Wales, or elsewhere in Britain, and it is on flat places that these pools develop. Vegetation, of which bog-moss (Sphagnum) is usually an important constituent, dies and accumulates over a long period of years, building up a bed of peat. At a certain stage, for reasons which are not at present understood, the peat becomes unstable, and hollows are eroded by the action of wind and rain. The surface becomes dotted with small pools and, as further erosion takes place, these coalesce. Finally a channel becomes eroded through the rim of the peat bed and all the water runs away. The building-up process then starts again.

Larger bodies of water on the Pennines are few, apart from man-made reservoirs which are now characteristic features of the landscape.

Lough Neagh in Northern Ireland is the largest sheet of fresh water in the British Isles, with a surface area of 153 square miles (393 km.²). It was formed in a way different from that of any of the other lakes so far encountered, and is volcanic in origin. There was no volcanic mountain like Etna or Fujiyama, but basaltic lava welled up from fissures in the ground. It flowed freely over the countryside and eventually solidified as a flat plate-like capping. Later it sagged in the middle and the depression so formed is Lough Neagh. The greatest depth is only 56 feet (17 m.), so it is even shallower than Loch Leven.

There remain to be explored the more recent geological formations of south-east England, and on them there are few large bodies of fresh water, though they are not on that account of any less interest to the freshwater naturalist. The best-known sheets of water are the Broads of East Anglia. The scientific mind, like Nature, abhors a vacuum and there was no dearth of armchair theories about how the Broads had been formed when Joyce Lambert, a botanist, and J. Jennings, a geographer, set out to collect some facts.

Dr Lambert pushed her way through the dense fens along straight lines from edge to edge and took borings at regular intervals. At many places the peat was of a different type at different levels, at others it was uniformly of a type that indicated accumulation at the bottom of standing water. In due course an elaborate and plausible explanation of the origin of the Broads was formulated and it might have remained the accepted one for a very long time, a great deal of hard work having gone into the collection of the evidence. However, Dr Lambert thought it prudent to continue her borings, and turned up evidence which demolished the theory. She found peat composed of different plant associations at different levels and peat that had accumulated uniformly in water so close together that the plane between them must be vertical; indeed there was evidence of columns of the former surrounded by the latter. There could be no explanation of this except excavation by human agency and the research became primarily historical.

No direct evidence has been found but the circumstantial evidence is convincing.

Documents of the thirteenth and earlier centuries refer to turbary rights in the region of the Broads, and there are records of much peat-cutting in parishes where there was no source of peat other than the fens where the Broads now are. There is no reference to water. After about 1350 there are few references in old documents to turbary, but frequent references to fisheries. There is, therefore, good reason to believe that the Broads are old peat-cuttings which became flooded between 1300 and 1350 probably as a result of some change in the relative levels of land and sea (Ellis, 1965).

A river tends to build up a deltaic plain at the end of its course and it inundates this plain every time it rises a little above its normal level. Parts of the plain will be under water only at the height of a flood, parts will be permanently marshy, and parts will be under water all the year round. This is the normal and accepted state of affairs in regions of the world where man has done little towards controlling and taming nature: the Rivers Tigris and Euphrates may be taken as an illustration (Fig. 6). In Britain, however, man has long since decreed that there is a place for everything and the place for water is within well defined banks; any breaking out and overflowing is an irregularity and often a catastrophe, and the victim of a flood is not consoled by the assurance that it is “natural”.

Fig. 6 Lower courses of the Rivers Tigris and Euphrates

The East Anglian fens originated when a flat clay-floored valley opening into the Wash was flooded by the sea after the Ice Age, owing to a slight lowering of the land level. Silt banks deposited by the sea gradually cut it off and it became a great inland lagoon. It was shallow, and rich in nutrient salts. Conditions were, therefore, good for plant growth and the resulting vegetation was luxurious. The dead remains accumulated and formed peat which filled up the lagoon rather rapidly, speaking in geological terms, till open water was left only in a few meres, which must have been very like the Broads today. Man coveted the rich soil and in the seventeenth century he successfully started drainage and reclamation. Now the meres have gone, the natural vegetation is to be found only in a few carefully tended preserves, and the fenland presents to the pond-hunter no more than an endless series of ditches, great and small.

Travelling a little farther south, we come to the chalk region; and a more waterless expanse than a chalk down cannot be found anywhere in the country. But even here there is something to interest the freshwater naturalist. Man has been wont to run stock over the downs for centuries and, in order to provide them with water, he has built ponds which have received the name of dewponds. There are few subjects about which more nonsense has been written. One explanation offered, even by people who should know better, is that dew-ponds are made by a secret process which insulates them from the surrounding land. When heat is lost at night by radiation from the surface, warmth from the lower layers is conducted upwards, and therefore the temperature at the surface does not reach a very low point; but this upward conduction cannot affect the dewpond because it is insulated. Accordingly, it is alleged, the dewpond area gets very cold, the atmosphere above it is chilled and moisture is deposited. The difficulty about this theory is that, if the dewpond were so effectively insulated from the land below, it would get very hot when the sun shone on it by day and much water would be lost by evaporation. Furthermore, considerations of the respective latent heats of water and chalk (that is the amount which a given volume of each would lose in a given time) have been ignored. Several people have examined the problem both experimentally and theoretically and the whole fallacy has been exposed more than once. Mr A. J. Pugsley (1939) has returned to the attack in a small book published by Country Life, but it would be optimistic to expect that the myth has been exploded. There is certainly a secret process in the making of dewponds and it has been handed down from father to son in certain families, but its aim is the construction of a waterproof bottom which will last for many years without cracking.

The dewpond, in effect, is a shallow pan of concrete or clay, and, though sometimes it is situated on top of a hill where it must rely entirely on rain, often it is located to take advantage of storm water, particularly where a road presents an impermeable surface. The belief that dewponds date back to the Neolithic Age is erroneous.

We have now worked our way down to the south of England and come to the coast to study a freshwater lake which owes its origin to sand-banks thrown up by the sea. To the west of Bournemouth lies Poole Harbour, a big enclosed area connected with the open sea by a small entrance, which cuts through a narrow strip of land and so makes two peninsulas. The one which lies to the west is known as South Haven Peninsula, and a conspicuous feature of it is the Little Sea, a shallow lake over 70 acres (28 ha.) in extent. Particular interest lies in the fact that the origin and development of Little Sea can be traced in detail from the information given on old charts. The first of these, dating from the reign of

Fig. 7a Formation of the Little Sea, c. 1600

Henry VIII, is not very accurate, but from it and one or two later charts a fair deduction is that the peninsula then comprised only land of the Bagshot Sand formation, with a small more recent sandspit at the tip. This is shown in Figure 7a, but the sea and the area between tidemarks are omitted from this figure, as any attempt to include them would be based largely on conjecture. A chart of 1721 is remarkably accurate. It shows that a sandbank, thrown up parallel with the land existing in the previous century, has enclosed a lagoon which is apparently a sheet of water at high tide but at low tide an expanse of bare sand, except for water standing in drainage channels. There is a wide beach, shown stippled in Figure 7b, and a detached sandbank lying to the north of the channel draining the lagoon. Rather more than a century later, in 1849, a survey shows considerable development; a second ridge has

Fig. 7b Formation of the Little Sea, c. 1721

been thrown up parallel to the first in the northern half and marshy area indicates the depression between the two; a third is foreshadowed by a long sandbank which now bounds the outflow on the seaward side; it is shown white in Figure 7c, the convention used to denote land above high water, though strictly it should be stippled as, according to the chart, it was covered by the highest tides. The sea runs in and out of the channel between this bank and the second ridge, and water apparently stands in the north and south portions of the lagoon at all stages of the tide. Today (Fig. 7d) there are three dunes, and the Little Sea is an inland lake with water which is actually soft and rather poor in dissolved salts. Other, smaller, bodies of water have come into being and the slacks between the dunes are extensively marshy; man-made cuts traversing them testify to an attempt at some earlier date to drain them, presumably to obtain pasturage.

Fig. 7c Formation of the Little Sea, c. 1849

And so, thanks to the painstaking research of Captain C. Diver (1933), it is possible to reconstruct in detail the changes which brought Little Sea into existence. There are other sheets of fresh water of similar origin, but no one has pursued inquiries into their early history. Some have obviously been formed more simply, and Llyn Maelog and Llyn Coron in Anglesey, for example, lie in long transverse depressions which the sea has blocked at the ends with sand.

Wherever man has had available an impervious soil he has tended to make ponds and lakes, to provide him or his animals with a water supply, for ornament or for sport. A favourite site is a narrow valley which can be flooded by the erection of a dam (Fig. 8), for building a dam is comparatively simple, while sufficient excavation to make a pond of reasonable size is a big and costly undertaking. Where there is hard impervious rock, fish-ponds are sometimes very numerous; for example, in the southern part of the Lake District the staff of the Freshwater

Fig. 7d Formation of the Little Sea, c. 1924

Biological Association have nearly fifty under observation within easy reach of their laboratory.

On heavy clay soil the farmer frequently digs a hole in every field in order to form a pond from which his animals may drink. Many other pieces of water are the by-products of man’s activities. Quarrying for stone, or digging out clay for bricks, produces an impermeable basin which the rain will ultimately fill. Excavating sand and gravel for railway ballast and other purposes may extend down below the water-table so that a pond results. Underground mines and tunnels sometimes cave in and cause at the surface a depression which fills with water.

The prosperity of the fifties and sixties and the boom in aquatic sports such as fishing and boating has meant that many gravel pits that might otherwise have been used for the disposal of rubbish have been saved as lakes. On the other hand, many small ponds are disappearing, because, with state aid to water supplies for farms, they are no longer necessary for watering stock. Indeed their use for this purpose is actively discouraged, since it has been shown that cattle contract Johne’s disease by drinking from fouled ponds.

Fig. 8 Hodson’s Tarn, an artificial moorland fishpond

These are some of the main ways in which bodies of fresh water have originated. There are others, less important in the British Isles, but a catalogue of them would serve no useful purpose here. Our main interest is with the plants and animals of water, and the next stage is to notice how lakes may be classified according to the biological processes going on within them.

A lake receiving the drainage from rich cultivated land will be ‘productive’, because of the nutrient salts it receives, that is, a large quantity of plant and animal material will be produced in the upper layers. Many of these organisms will decay in the lower layer, which, if the lake is stratified, may become depleted of oxygen. A second condition is that the hypolimnion should be relatively small. A combination of good agricultural land and a shallow lake is typical of lowland country, and it is here that lakes with no oxygen in the hypolimnion generally occur. They are known as ‘eutrophic’. An ‘oligotrophic’ lake, that is, one in which the hypolimnion contains oxygen, is typical of mountain conditions where the drainage area is unproductive and lakes often occupy deep basins. For many years the difference was thought to be fundamental, and an elaborate classification arose on a foundation which had been simple originally. As knowledge accumulated, it became evident that the distinction was not as basic as had once been thought, and it is no flight of fancy to say that the edifice of classification was brought crashing about the ears of the assembled company by Professor H.-J. Elster, in a masterly review at the International Congress of Limnology in 1956. Since then the tendency has been to study the primary productivity of a lake, that is the amount of algal material produced in the open water during a year, and to arrange the lakes in a series according to the figure obtained.

Shortly before the First World War, the late Professor W. H. Pearsall started a study of the Lake District lakes the basins of which were all formed in the Ice Age. Whether he was familiar with the continental ideas and ignored them, or whether he was not aware of them, we shall probably never know. Anyhow he arranged the lakes in a series with no attempt to delimit and define categories, although Esthwaite, at the productive end of the series is eutrophic, and Wastwater, Ennerdale, and Buttermere at the other are fine examples of an oligotrophic lake. This concept stimulated a great deal of work, and though Pearsall’s original ideas have been modified, the basic soundness of the idea has been revealed by research in several fields. Pearsall noted that the unproductive lakes lie in the hard Borrowdale volcanic rocks right in the main mountain masses. Consequently the valley sides are steep, the area of flat valley bottom is small (Fig. 9) and rain falling on the drainage area will flow over much bare rock and scree. Consequently it bears little in solution when it enters the lake. The unproductive land supports no more than a farm or two, and few other than farmers have been tempted to settle in the restricted area available. This, however, has also been influenced by the remoteness of the valleys which are distant from the main lines of communication.

Fig. 9 Buttermere, an unproductive Lake District lake

Fig. 10 Esthwaite, a productive Lake District lake

Windermere and Esthwaite Water (Fig. 10) are the two most productive lakes. They lie in the south of the district in a zone of Bannisdale slates, which, though hard rocks, are softer than the Borrowdale Volcanics and have weathered more. Much of the drainage area is floored with the products of weathering and is relatively flat. Obviously rain-water seeping through soil will dissolve out more than water trickling over solid rock, and so the streams and rivers entering Esthwaite and Windermere bring with them a higher concentration of nutrient salts than those flowing into Ennerdale. But the flat land also attracts the farmer and the cultivator who seeks to improve the soil by adding manures to it. Some of these find their way into the lake, and so the difference between the two is enhanced. Within the last century Windermere, particularly, has become a residential resort. The result is that much human sewage enriches its waters and makes still greater its difference from Ennerdale.

Esthwaite Water is a relatively shallow lake and, as already stated, eutrophic.

Position in the series developed by Pearsall was based on three factors, first the percentage of the drainage area which is cultivated, second the percentage of the shallow water region which is rocky, and third the transparency of the water. The first two factors are fundamental; the third is partly fundamental and partly a result, because the transparency of the water depends both on the amount of mineral matter in suspension and on the quantity of life present, provided there are no extraneous factors like staining from peat or pollution by mine washings. In the Lake District none of the larger lakes except Bassenthwaite contain peat-stained water, and pollution from mine washings, though it does occur, is fortunately rare. Table 2 shows the Lake District lakes arranged according to these three factors. The figures in the last column show the depth at which a white disc, 7 cm. in diameter, could just be seen.

On the whole there is a serial increase or decrease in each of the three columns. The most notable anomaly is the low transparency of Bassenthwaite, occasioned by its being the only lake of which the water is stained with peat. The amount of light at different depths in Bassenthwaite, Windermere and Ennerdale is shown in Figure 11, expressed as a percentage of the intensity at the surface.

Table 2. The sequence of Lake District Lakes (Pearsall, 1921)

Fig. 11 Penetration of light into three Lake District lakes

Work on cores, started just before the war, had as its original aim the elucidation of the history of the lakes, but, like many another new line in research, an essentially opportunist activity, it proved most fruitful in a line other than the one aimed at and it revealed more about the land than about the water. However, as a lake is strongly influenced by events in the drainage area, the findings are relevant. Most animals disappear completely, but the shells of some waterfleas (Cladocera) and the heads of some chironomids do not decompose and persist in the cores. Similarly many algae leave no trace, but the siliceous skeletons of diatoms (e.g. Asterionella) that have lain in the mud for thousands of years are still identifiable. In contrast the pollen of nearly every plant that produces any does not decompose and, since that of almost all species is distinct, an examination of cores gives a picture of what the land flora was like when the particular layer of mud under examination was deposited. Research on pollen in cores from bogs and other places where soil has been accumulating since the Ice Age was in vogue all over Europe at the time, which was fortunate, because events in the lake cores could be related to events elsewhere, and some of these had been dated by one means or another. Chemical analysis of cores also yielded a large amount of information about the past.

The lower part of a core from Windermere consists of clay which, on examination, proves to be made up of alternating layers of very fine and coarser particles; it is accordingly referred to as laminated clay by Dr Winifred Pennington, who has described the cores. Above the laminated clay, which is pink, lies a grey layer and above that more pink clay, which may or may not be laminated. On top of this is a thick column of brown mud which extends nearly to the surface; it is capped by a fourth kind of soil, a black deposit which Pennington refers to as ooze.

The laminated clay contains very little organic matter and few remains of plants, and was almost certainly formed towards the end of a glacial period, for similar deposits are being laid down today in certain glacier-fed lakes. During the summer both coarse and very fine particles are washed into the lake. The coarse particles sink almost immediately but the fine particles remain in suspension for a long time. When winter comes the inflowing streams freeze, and so no particulate matter is brought into the lake, but fine particles left over from the summer are still settling. The result is a summer layer of coarse and fine particles and a winter layer of fine particles only.

The low organic content of this deposit and the scarcity of plant remains indicate that there was little life in the lake when the laminated clay was being laid down. In contrast the grey mud contains the remains of animals and plants, and the lake was evidently more densely populated during the period when it was being laid down. These organisms were associated with an improvement in the climate, which is known as the Allerød period, because it was first discovered near the place of that name. It was followed by a return of glacial conditions when the pink clay with few remains of organisms was laid down again. Professor H. Godwin has had dated by means of the C¹⁴ method, a technique which will no doubt be more widely used when facilities for it are more widely and more easily available, a sample from the Allerød layer and found it to be some 12,000 years old. The ooze at the top in which Asterion-ella suddenly becomes common obviously represents enrichment of some kind. Today the effluents from the sewage works are probably the main sources of enrichment, but this is recent. The population, particularly the holiday-making one, has been increasing since the railway came to Windermere in 1847, but the transition from earth closets to running water sanitation has been slower, and it is doubtful if the lake was being seriously enriched from this source a century ago. If mud is being deposited on the bottom of Windermere at a rate that has been constant over the last few hundred years, whatever caused Asterionella to appear happened about two centuries ago. It therefore probably antedates the tourist completely, and is to be sought in improved agricultural practice of which there is some evidence early in the eighteenth century.

Mackereth’s conclusions from chemical analyses upset certain ideas of long standing. The lakes were richest chemically in their earliest days, when the land was covered with rock fragmented by the ice and exposing a great area of surface from which the rain could leach nutrient salts. Esthwaite was eutrophic at an early stage and presumably, therefore, the lakes were richest biologically when they were richest chemically. Some of the algal species identified in the cores support this view. The rocks are hard and when a fresh surface has been leached for a time water dissolves little from it. The lakes slowly became less productive. A climatic change and the arrival of man, who started to fell trees, resulted in more erosion and enriched the lakes, but as more stable conditions were established production fell again. A thousand years ago the Norsemen arrived, and since then man has been the most important agent affecting the lakes. The increased production in some lakes on account of their situation has already been described.

CHAPTER 5

RIVERS

It is possible to entitle a chapter ‘a typical lake’ and to fill it with an account of physical and chemical changes which follow an annual cycle in a great many lakes. Another chapter is entitled ‘different kinds of lakes’, though here a certain degree of accuracy has been sacrificed to obtain a short title, and the account includes pieces of water that are too small to be regarded as lakes in the strict sense. In neither chapter is there much about fauna and flora, the plan being to describe freshwater animals and plants in chapter 6 and then to pass to an account of the various communities found in different biotopes. A biotope is a region in which the conditions are of such uniformity that the plant and animal communities do not vary much; the stony substratum of a lake, weed-beds and the open water are examples of biotopes. The purist would prefer the term biocoenosis for the assemblage of animals and plants that inhabit a biotope, but here the more general term ‘community’ is used.

It is impossible to treat rivers in the same way. Lakes are all recent in geological terms, and many of what were called lakes in the preceding chapter are recent in historical terms, having been made by man. Each lake was formed by one event taking place in a limited area. Water courses are much older and have continued their existence in spite of the events which formed lakes. For example the rivers of the English Lake District tend to rise near the middle of the area and radiate from it. This pattern was presumably established at a time when the mountains exposed today were covered by a dome of younger rocks. The watercourses cut down through this and later eroded the underlying rocks in the same direction, though these originally had the form of a ridge not a dome. The disappearance of the covering layers left the old rocks scarred by valleys whose direction bears no relation to the way in which they were laid down. Each watercourse is, therefore, modified today by many geological strata.

The plan of this chapter is to describe the few rivers about which something is known; how far they are ‘typical’ only further work will show. It has also been found necessary to include some information about the plants, and about human activities. Whereas these have brought whole new bodies of standing water into existence, they have modified rivers. These modifications, however, have been extensive and have affected every British river of large size. Waste disposal, water supply, water power, drainage and navigation have been the main activities through which man has altered water-courses.

The history of investigation of rivers fits more easily into the account of their misuse in Chapter 14. It suffices to record here that important surveys were carried out between the wars by the late Mr F. T. K. Pentelow, Dr R. W. Butcher and colleagues in the Ministry of Agriculture team. Since the war knowledge about small stony streams has advanced greatly, investigations having been made in most of the upland areas of England, Wales and Scotland. One Lake District river has been investigated by Drs R. Kuehne and W. Minshall working in England during the tenure of a year’s fellowship from America. Of recent years the number of biologists on the staffs of the River Authorities has increased considerably. It seems to be envisaged, however, that their task is to analyse as many samples as possible from as many stations as possible in order to keep a check on the condition of the river and its tributaries. For this purpose it is often reckoned that identification to species is not necessary. For basic information about the lower courses of rivers and their inhabitants, we still have to turn to the old surveys.

The property of water important to the study of lakes is its density at different temperatures; for the study of rivers, its flow downhill. This has a direct influence on organisms that live in it and a possibly more important indirect one through its effect on the substratum. Table 3 is taken from Tansley (1939) but its original source is a text-book on river and canal engineering. An engineer contrives even gradients and neatly regulated bends; Nature does not. The irregularity of a natural water-course produces a mozaic that confounds the systematic mind at the start and ensures that any scheme of classification is no more than a rough general guide. What does happen in nature? In attempting to answer that question, we shall take an imaginary river, but admit at once that our imaginations owe much to familiarity with the Lake District. The rocks there are hard, impermeable and often steep. In places water flows a long distance over a flat sheet of rock, but generally it has eroded a gulley of some kind. In this rapids generally alternate with pools in parts of which conditions are surprisingly quiet, especially if the stones and boulders are large. In the rapids large stones and boulders tend to jam in the gulley and hold up a bottom that is far more stable than might be expected on so steep a gradient. Conditions in this zone must obviously depend on the relation between the dip of the strata and the angle at which they are exposed, and on the size and form of the fragments which break off the rock. Where the gradient becomes less steep, moderate-sized stones plucked from the rocks above and washed down begin to come to rest. In the Lake District they tend to be flat and accordingly they have an inherent stability. Further breaking-up is taking place all the time and as the smaller pieces are rolled downstream their edges are rounded. This produces the unstable bottom of round stones that is often found some distance down the valley, if the gradient falls evenly. An outcrop of rock is a fresh source of flat stones, and it, or any other obstruction, produces a striking alteration of the flow pattern, confining swift current to the surface layers. Settling of gravel, sand and finer particles becomes possible, and, as these fill up the interstices between the larger stones, they produce a remarkably stable bottom. This happens also during a spell of low water. Percival and Whitehead refer to it as a ‘cemented’ bottom. It is one which occurs almost everywhere, but generally it is covered by loose stones. Occasionally some new obstruction halts the downward flow of loose stones and then the stream is floored by substratum of this type.

Table 3. Relation of current speed and nature of river bed

Flowing onward, the river generally comes to a plain, often one of its own creating, the gradient approaches nearer and nearer to the horizontal and flow decreases. First gravel, then sand and finally silt settle to the bottom and provide a substratum in which plants can take root.

Probably few except those charged with the task of dredging it realize how much material is being deposited on river beds. It is not a continuous process and varies greatly with intensity of rainfall. At intervals exceptional downpours, often restricted to a comparatively small area of the mountains, make considerable alterations to a river bed, which may remain comparatively unchanged until the next downpour. But change never wholly stops; a boulder may stabilize a stretch for many years but all the time it is being chipped away by the smaller stones washed past it until the day must come when it is no longer large enough to withstand a flood. Away it goes and a considerable section of the adjacent bottom with it until a new pattern is established.

For the biologist the important distinction is between the upland reaches, where erosion is taking place, and only plants, such as mosses, that can attach themselves to flat hard surfaces provide cover for animals, and the lowland reaches where a plain is being built (or would be if the drainage engineers permitted) and rooted vegetation grows. Dudley Stamp (1946) recognizes three zones; mountain, foothill, and plain. Butcher has proposed a classification of rivers according to which of these zones they rise in. In mountain areas with hard rocks the rivers will traverse all three zones, but where there is chalk or other pervious rock the river may spring from the foothill or the plain region. His scheme, however, has not caught on, and most workers agree that an entire river may be so diverse that it will not fit with other rivers into a category. Schemes for recognizing zones within a river have, in contrast, been popular. The best known goes back a long way and has been elaborated in recent years especially by fishery workers. It is based on the species of fish found, which appears to have a fair correlation with the slope. One drawback is that some of the fish do not have a wide geographical distribution. Dr Kathleen Carpenter (1928) has adapted it for British waters:

1. The Headstreams and Highland Brooks are small, often torrential, and without fish. Temperature conditions vary greatly. Low temperature is common, but a stream that arises from shallow soil may be warm, and a slow-flowing stretch may soon reach a high temperature on a sunny day.

2. The Troutbeck is larger and more constant than the headstream. Torrential conditions are typical and the bottom is composed of solid rock, stones, and boulders, with perhaps some gravel. The trout is the only permanent fish of the open water though the miller’s thumb (Cottus gobio) is found sheltering among stones. These first two zones together correspond roughly with Stamp’s upper or mountain course.

3. The Minnow Reach is still fairly swift and patches of silt and mud are only to be found in a few places protected from the current, but higher plants, notably the water crowfoot, Ranunculus fluitans, are able to gain a foothold. This is roughly the middle course of Stamp. It is the Thymallus (grayling) zone of the continental workers, but Carpenter rejects this name because the grayling is not a widespread species in Britain.

4. The Lowland Reach is slow and meandering, with a muddy bottom and plenty of vegetation. Coarse fish are characteristic, and on the continent of Europe it is known as the bream zone.

Tansley classifies rivers into five zones, basing his system very largely on the work of Butcher (1933).

Zone 1 is described as very rapid. Where vegetation is present at all, the important plants are mosses and liverworts; higher plants are often absent altogether and never dominant. This class includes all Carpenter’s headstreams and highland brooks and part, at least, of the troutbeck.

Zone 2 is moderately swift with a bottom of stones and boulders, but with occasional patches of finer material in which a small number of higher plants can gain a foothold. Ranunculus fluitans (or sometimes R. pseudofluitans), the water crowfoot, is the most important.

Zone 3 has a moderate current with a gravelly bed. The list of higher plants is much longer. The water crowfoot is still the most important, others are the simple bur-reed, Sparganium simplex, several species of Potamogeton, and the Canadian pond-weed, Elodea canadensis.

Zones 4 and 5 are medium to slow, and very slow or negligible respectively. The list of higher plants is long and, as it varies a good deal from river to river, confusion rather than clarification would be the result of reproducing it here; but it may be noted the water crowfoot is usually not an important constituent. It is impossible to equate this classification exactly with that of Carpenter, but Zone 2 and part, at least, of Zone 3, correspond with her minnow reach, and Zones 4 and 5 and perhaps part of Zone 3 correspond with her lowland coarse fish reach.

Against this background a few British rivers which have been studied in detail may now be examined. The Lake District, as was described earlier, is drained by rivers which radiate from the centre. Their valleys were enlarged by glaciers during the Ice Age and generally deepened in such a way that a lake was left when the ice retreated. The Duddon is one of the few valleys in which there is no lake. Its highest tributary, Gait-scale Gill, rises at an altitude of 735 m. (2400 ft.) in a flat area covered with bog and small pools of open water. Beyond this it tumbles steeply down the fellside, dropping 300 m. in 900 m. (33%). At the foot of this slope there is a delta of large stones under which the water disappears in dry periods. Exceptional rainfall towards the end of the year during which Kuehne and Minshall were at work enlarged this delta, and incidentally carried away every one of about twelve maximum and minimum thermometers which they had buried in various parts of the system. The analyses made by these two workers showed that the calcium ranged from 0.57 to 1.10 parts per million in Gaitscale Gill, a low value, even for the Lake District, but after the flood the concentration below the delta rose to 3.0 p.p.m. This illustrates the point, stressed earlier, that freshly exposed faces yield more nutrients than those which have been leached for some time. The highest temperature recorded in the gill was 17.2° C., 5.6° C. lower than the highest temperature recorded elsewhere in the system.

Several streams run down the fellside parallel with Gaitscale Gill and feed the main river which here runs roughly westwards. Beside it runs a road, originally made by the Romans, probably as a line of communication through the area to facilitate the subjection of the natives (Rollinson, 1967). It is now used mainly by tourists, for there are no dwellings beside it between Langdale at one end and Eskdale at the other. The river swings round to take a southerly direction in an upper valley with a comparatively slight incline. There were four farms in this valley, but only two are used for farming today. In autumn a few green fields around them stand out among the predominant greyish-yellow of the poorer pastures, probably as a result of liming. Sheep, which range far and wide over the surrounding fells, particularly in summer, are the chief product. A few conifers, planted recently, are the only trees.

The slope steepens to separate the lower from the upper valley. The slope in the lower valley is 1 %, the river is floored with round stones of all sizes, and it flows torrentially down to the estuary. There is no plain region, and only the first two of Carpenter’s four zones and the first of Butcher’s five can be recognized. There are two small villages in the lower valley, residences scattered outside them, and some twelve farms on which cattle as well as sheep are reared. Deciduous woods cover extensive areas of the valley sides. The upper valley comes to an end about 175 m. above sea level, the lower at sea level. The difference in climate between the two is obviously great but no figures are available. The temperature of the swift river gives no indication of it. The valley walls rise steeply on both sides but to the west there is a plateau over which flows the longest tributary.

The River Duddon is about 11 miles (18 km.) long. We pass from it to the River Tees (Butcher, Longwell, and Pentelow, 1937) which is about 100 miles (160 km.) long. It rises in the Pennines and flows to the North Sea. The gathering ground drained by the headstreams is fairly large. It is high above sea level, it receives a relatively heavy rainfall of about 60 inches (1520 mm.) a year, and the rock is impermeable. The result of these four factors is a severe scour in time of flood, and this has carved out a deep bed. Consequently the energy of a flood is not dissipated in inundating the surrounding country and the effect is concentrated on the river-bed. On one occasion some carts were being filled with gravel at the water’s edge when the river rose so suddenly that the carts had to be abandoned and were swept away. This illustrates a most important condition affecting the plants and animals of the river.

The small town of Croft is about 45 miles (70 km.) from the mouth and about 65 miles (100 km.) from the source, travelling by river, and is a convenient dividing point. Above Croft all the river is rapid with little rooted vegetation and few fish except trout, grayling, and minnows. The river downstream is still moderately swift but there is much more rooted vegetation and various coarse fish are plentiful. After flowing for 20 miles (32 km.) from Croft the river reaches the head of the estuary, which is some 25 miles (40 km.) long.

The Tees rises on Cross Fell at a point about 2,500 feet (760 m.) above sea level. Many small tributaries also rise just on the eastern side of the Pennine watershed, and some of them originate in thick peat beds. One, for example, drains the peat pools which were mentioned in Chapter 4. These little streams run down the hillside with a fairly though not extremely rapid flow, because the eastern slopes of the Pennines are not steep. About six miles from the source three main tributaries have coalesced and the river is some 12 metres wide with a fair flow over a bottom of stones and boulders. Then it enters a quiet stretch and for three miles the current is sufficiently slow to allow the deposition of some fine sediment, which provides a foothold for a few higher plants, Potamogeton alpinus, the alpine pond-weed, Callitriche intermedia, the water starwort, and Sparganium simplex, the simple bur-reed. The only other attached plants are the mosses, Fontinalis antipyretica, and Eurhynchium rusciforme, and, at certain seasons, the algae Lemanea fluviatilis and Cladophora glomerata.

This slow stretch provides a pretty example of the sort of exception to the general plan which is to be found in almost any river. It is caused by a stratum of hard rock, and at the end of it there is a fine waterfall. A little farther on, some 16 miles (20 km.) from the source and 1,000 feet (305 m.) above sea level, the river strikes a road and some human habitations, and comes to the end of what may conveniently be taken as the first part of its course. A chemical station was set up just here and the results obtained are shown in Table 4.

Table 4. Some dissolved substances in the Tees near the end of the first part, that is above any pollution

The average amount of calcium is about 12 parts per million, and so the water is soft although the river has been flowing over limestone. But there are big fluctuations in the concentration of all the substances except oxygen, which is plentiful at all seasons and at all times of day and night.

Trout occur in this part of the river and may be taken almost up to the source. They are plentiful but of small size, the average length being but six inches (15 cm.).

The next part of the course extends all the way down to Croft, which is at the point where the river changes in character. The river crosses several geological formations, which affect its nature, but it remains rapid throughout with a bottom of bare rock or stones. There is hardly any rooted vegetation.

The main difference between this and the preceding part of the river is that it receives sewage effluents from towns along its route. The first place of any size is Middleton-in-Teesdale, some twenty-two miles from the source, and the biggest is Barnard Castle, about eight miles farther on. The population connected with the sewage systems of the two places was 1,700 and 5,000 respectively when the survey was made. Below the outfalls there were changes in the flora, and there can be no doubt that these changes were directly attributable to sewage and the products of sewage decomposition. Below some of the larger works the dominant organism on the river bed was the sewage fungus. Father downstream there was a characteristic association of algae, but this finally gave place to the same association as was found in the upper waters, where there was no pollution. Below the smaller sewage outfalls there was no sewage fungus, but there was the characteristic change in the algal community encrusting the stones and boulders. In all this part of the river the amount of sewage was small compared with the volume of water into which it was discharged and pollution was not great. Oxygen concentration in the summer was lower than in the first part of the river (Table 4) but it never reached a seriously low level. Chloride rose, probably as a result of the sewage. The amount of calcium also increased and an average figure just above Croft was 24 parts per million; this rise was probably mainly due to the limestone over which the water had flowed.

Trout occurred throughout this part of the river and reached a greater size than in the first part, ¹/₄ to ³/₄ lb. (120–360 g.) in weight with a few specimens of 3–4 lb. (1.4–1.8 kg.). It contained minnows almost throughout, and grayling in the lower half; thus, although there are no marked physical changes, the river enters the third of Carpenter’s zones in this part of its course.

At Croft, 100 feet (30 m.) above sea level, there are several important changes in the river. It enters a great clay plain, laid down during the Ice Age, and flows across this in a meandering course, though with a fair flow. The stretch is too fast for a typical lowland course judged on purely physical grounds, and it is probably in the third of Butcher’s five zones, for Ranunculus fluitans is one of the commonest plants; but it is in the last of Carpenter’s four classes since coarse fish abound. Other changes are due to the confluence of the River Skerne, a large tributary, which is more calcareous and much more heavily polluted than the Tees. These three factors, different kind of bed, more calcium, and more sewage products, all influence the biology of the river below Croft. but it is not possible to measure exactly how great a part each one plays.

Most of the bed of the river is of medium-sized stones and gravel but there are occasional patches of sand. In water shallower than five feet typical plants are the water crowfoot, Ranunculus fluitans, and various species of pondweed, Pota-mogeton. These plants can colonize the gravel and sand, and when they have formed a large patch they cause a stagnant area on the downstream side. Silt settles here and accumulates rapidly if there is heavy pollution upstream. It is colonized by such plants as Nitella, the stonewort, Elodea, the Canadian pondweed, and Potamogeton crispus, the curly pondweed.

There was usually sufficient water in the Skerne to dilute the sewage it received to below the danger point, but in one summer there was a long hot dry spell as a result of which all the oxygen was used up, and the toxic products of decomposition without oxygen were liberated into the water. The Skerne itself had a thick coat of sewage fungus on its bed, and this organism extended for some distance down the Tees below the confluence of the two rivers. Its range varied widely according to the season of the year. In winter when, owing to the low temperature, the rate of decomposition of sewage is slow, it extended a long way downstream from the mouth of the Skerne, but in summer, when decomposition is more rapid, its range was less.

A green filamentous alga, Cladophora glomerata, the Blanket Weed, abounds where nutrients are plentiful, as they are below a sewage outfall where the organic matter has undergone the initial stages of decomposition. It appeared in the Tees towards the end of May and grew rapidly to form a thick carpet in the shallow water a long way down the river from Croft. Then the first flood in July would usually sweep it all away, and it would be seen no more until the following year. If it lasted long enough, it trapped a deposit of silt and enabled rooted plants to grow in places where otherwise the flow was too fast. Before the estuary was reached the algal community typical of the upper, unpolluted reaches had become re-established on the stones.

Nitrogenous compounds and other products of decomposition were brought into the Tees by the Skerne, and the calcium concentration was increased to some 30 parts per million. There was less oxygen in this stretch during the summer than there was farther upstream, and the lowest value was reached during the time when the development of Cladophora was at its height. The dense growth of this plant, respiring in the hours of darkness, used up much of the oxygen and reduced the concentration to between 50 and 60% of the saturation value. This is well above the point at which deleterious effects on fish are likely, and trout flourished in this, the last freshwater reach of the Tees, not uncommonly attaining a size of 1–1¹/₂ lb. (.45–.75 kg.). Coarse fish, chiefly dace and chub, were abundant, and fishing was a popular pastime.

Fig. 12 Longitudinal section of the Tees estuary showing the salinity at high and low tide

In the estuary, surveyed by Alexander, Southgate and Bas-sindale (1935), the most important natural phenomenon is the salinity. The fresh water tends to float on the sea-water and the result is a marked stratification. Figure 12 shows the average conditions at high tide and at low tide, but it gives rather a distorted picture because it is necessary to use such different scales. Horizontally an inch represents about three miles, but vertically it represents only about fifty feet. The surface current of fresh water draws up some water of higher salinity from below it, and to replace this there is an upstream creep of water of high salinity along the bottom. The whole mass moves up and down with the tide as shown in the figure. It is estimated that the mean time for all layers of a body of water to pass through the estuary is about six days in dry weather, decreasing to about two and a half under average winter conditions.

The estuary has been much changed by the hand of man, and it must be admitted with regret that the Tees is typical rather than otherwise of larger British estuaries. From about midway nearly to the sea there is an extensive industrial conurbation. This requires a navigable channel so that its products may be removed by sea, and accordingly the natural tendency of the river to drop silt where it is checked by its meeting with the sea is counteracted by the continual activities of dredgers. The river is a convenient main drain and, at the time of the survey, the sewage from rather more than a quarter of a million people was discharged into it untreated. So were a variety of industrial waste products, of which the most important were tar acids and cyanides. Both these decompose gradually in the water.

Much water is taken in to cool condensers and machinery, and this results in a slight rise in the temperature of the estuary. Oxygen, it need hardly be said, is not plentiful in solution in the water. The amount used up depends on the temperature and also on the salinity, being greatest at salinities of between 15 and 25 parts per thousand. The lowest concentration of dissolved oxygen recorded during the survey was 9% of saturation.

The curly pondweed, Potamogeton crispus, the starwort, Callitriche stagnalis, and the two mosses, Fontinalis antipyretica and Eurhynchium rusciforme, which are abundant throughout almost the whole length of the freshwater part of the river, penetrate a little way into the brackish water. A few seaweeds penetrate a short distance from the sea but only four extend beyond the fringe of the brackish water region. Fucus vesiculosus, one of the brown bladder wracks, extends to beyond the middle point of the estuary, growing on wharves and piles between tidemarks; and three species of filamentous green algae occur throughout the brackish region.

It is difficult to determine exactly which fish dwell permanently in the estuary, as so many of the species recorded are migrants passing through, or casual invaders, but the threespined stickleback appears to be a regular inhabitant, extending down to at least the upper reaches of the polluted part. The effect of the pollution on the fish, particularly the regular migrants, and on the lower animals is described in Chapter 14.

Reviewing the River Tees in the light of the classifications put forward at the beginning of the chapter, we find that it includes all of Carpenter’s classes, for the lowest reach, immediately above the estuary, is dominated by coarse fish. On the other hand the last two classes, numbers 4 and 5, of Butcher’s botanical classification are not represented, for the current is nowhere so sluggish that the water crowfoot ceases to be the dominant plant.

A contrast to the Tees is provided by the south country rivers rising in the chalk downs. Butcher has surveyed the plants of the Itchen, and there was a fisheries research station on the nearby Avon for several years before the war. Much of the gathering ground is chalk down. Rain falling on this sinks in and percolates relatively slowly so that it may not reach a hill-foot spring for months. The effect of heavy rain is, therefore, dissipated and it will not produce a marked flood wave as in the Tees. The other effect of the chalk is, of course, to render the water highly calcareous, and Butcher quotes a figure of 92 parts per million of calcium in the River Itchen.

Then the slope is not so steep. Moon and Green (1940) give a profile of the Avon and show that between Christchurch, which is at the mouth, and Salisbury the fall is about 150 feet in 39 miles, which is a little less than 4 feet per mile (0.075%). The river rises some 20 miles from Salisbury at an altitude of about 350 feet, so this upper reach, for which we have not been able to find accurate data, is somewhat steeper, and the figure for the whole river will be greater, but still far below that of 30 feet per mile (0.57%) for the Tees.

The springs giving rise to the Avon headwaters are usually at the foot of the chalk and often flow in wide valleys floored with gravel. Sometimes the streams have been broadened so that they flow over wide areas in which water-cress is cultivated. In dry weather the water-table often sinks below the surface of the gravel covering the impermeable stratum which is the true valley floor and the stream disappears. Sometimes, owing to the time which rain takes to seep through the chalk, there may be a long interval before the effect of a dry spell or a wet spell is manifest in the river.

Below Salisbury the Avon has been put to a variety of uses by man. One of the characteristic features is water meadows, although the method of farming under which they were engineered is now obsolete. The principle is to take water from the river in a main canal, which can be filled by the manipulation of sluices across its mouth and a barrage across the river. From this main canal the water is led into many subsidiary channels, from which it eventually runs over the land. It is gathered up in a complementary series of collecting channels and led back to the river at a lower level. The advantage of this system was that the grass could be watered at certain critical times of the year, and the farmer was independent of the capricious rainfall of this country. The significance of water meadows in the economy of the river today is that a great deal of flood-water finds its way on to them and runs back to the river slowly. This is a second reason why the effect of flooding is much less fierce in the Avon than in the Tees.

Dams and weirs are thrown across the Avon not only to deflect water for irrigation purposes but also to pen up a head of water to provide power for mills. Weirs and side channels to take excess water when the level of the river is high are usually to be found in connection with mills, and the result is that the river does not flow in a simple single channel but in a maze of anastomosing channels.

The water is rich in nutrient salts and, since there is no great scouring by floods, the rivers flowing from the chalk are heavily overgrown with a variety of aquatic plants. Butcher records that the commonest plants of the River Itchen are: Ranunculus pseudofluitans, water crowfoot, Sium erectum, the lesser water parsnip, and Apium nodiflorum, where the current is fastest; Hippuris vulgaris, the marestail and Sparganium simplex, the simple bur-reed, where it is somewhat less rapid; and Elodea canadensis, the Canadian pondweed, and Calli-triche stagnalis, starwort, in the slowest reaches. The vegetation forms such thick beds that it has to be cut and removed to let the water pass, and also to make fishing possible.

Besides the game fish, for which these rivers are famous, there is a plentiful and varied population of coarse fish.

The Avon has no torrential head-stream region nor a typical meandering lowland reach. The whole river occupies a place somewhere in between these two, but it cannot be made to fit exactly into any of the various schemes of classification. There is no steady loss of gradient from source to mouth, as there is in the theoretical river, but a mosaic of faster and slower reaches due to the various artificial obstructions which man has thrown across the river.

A third river worthy of notice is the Lark, another of those surveyed by the Ministry of Agriculture and Fisheries team in connection with pollution (Butcher, Pentelow and Woodley, 1931). It is a small river rising in the East Anglian heights and flowing in a west by north direction to join the Ouse. The water is highly calcareous like that of the River Avon.

Only a comparatively small portion was surveyed, but this stretch is fraught with interest because it illustrates yet another effect of human interference. It may be remarked here that no south country river of any size is in a ‘natural’ state, and any account of it must dwell at some length on the modifications imposed by man.

The River Lark was once navigable as far up as Bury St Edmunds, though the last few miles were kept open with difficulty because the gradient was rather steep and the amount of water available was small. Eventually river traffic ceased to pay, and the locks fell into disuse. They are now derelict and the river flows in a bed which, having been widened to take barges, is too large for the volume of water which flows down it. This disproportion is particularly marked in one stretch which is now heavily overgrown with two emergent reeds, Glyceria aquatica, reed poa, and Sparganium erectum, the branched bur-reed. These plants probably established themselves first on beds of silt in shallow water. Their gradual spread would impede the current still more and result in further deposition of silt, and the process has continued and was still active at the time when the survey was made. The dense growth of reeds tended to deflect the current to the side, where it encountered and eroded a soft sandy bank, and so made yet bigger the area in which conditions were suitable for reeds. A stage had been reached where, when the reeds began to grow up early in the summer, above them the river flooded even though there had been no unusual rainfall, and below them a miller was hard put to it to obtain sufficient head of water to drive his mill.

Beyond this stretch overgrown with reeds there is a stretch overgrown with submerged pondweeds. In parts of it the current is sufficiently strong to keep a gravel bottom clear of silt and the water crowfoot is the dominant plant. Elsewhere the current is sluggish, the bottom is muddy, and the chief plant is usually Potamogeton lucens, the shining pondweed. In some places it is replaced by a community in which Spar-ganium simplex and Sagittaria sagittifolia, the arrowhead, are the dominant species. There was no evident difference in the river to account for these two distinct communities and at first they provided something of a puzzle. But a study of the activities of the human beings interested in the river at length provided the clue, and it was noticed that the bur-reed-arrowhead community was found in those parts where weed-cutting was most frequent.

Finally the river runs through a stretch of fenland before joining the Ouse, but unfortunately the survey stopped at the head of this stretch. The fenland river offers the extreme example of the lowland course. Left to itself it would follow a tortuous channel beset with marshes and stretches of open water. Changes of course might occur and the stream might split up and lose its identity in a number of small channels as does the Euphrates today. Figure 6 shows the lower part of the Euphrates and the Tigris, and gives a good picture of a lowland course which has hardly been interfered with. In Britain no fenland river is left to itself. The fen soil is valuable for cultivating and the rivers are important as the means whereby the water pumped up out of the fens is got rid of. Vegetation, which would impede the flow of water, is removed and the channels are constantly dredged. Flood-banks are raised on either side, often at some distance from the river’s brim, so that an expansion in width is possible when the river rises above its natural banks. Water left behind by a flood stands for a long period in this land between the river and the flood-bank, and the resulting ‘washes’ are characteristic features of the fenland landscape.

Most waterways were not created by man, though he has modified some of them considerably, but there is one group that owes its existence to human effort – the canals. The Exeter ship canal was built in the sixteenth century and a few artificial waterways persisting to this day date from even earlier times. But the title of ‘father of inland navigation’ is usually bestowed upon the third Duke of Bridgwater, at whose instigation a canal from Worsley to Manchester was built and opened in 1761. The commercial possibilities of this new means of transport were quickly exploited, and in the next forty years nearly 4,000 miles of canal were put into operation. After about 1800 the activity began to wane as the challenge from rail and road became ever greater. Today some of the canals have disappeared and others, though still containing water, are no longer used.

Even a used canal is surprisingly rich in animal life and an unused one is highly productive. Canals are almost confined to the lowlands and so their water is usually hard and rich in nutrient salts. There is sufficient flow to keep these replenished; but there is no danger of excessive flow after heavy rainfall of the kind which may wash away so much plant and animal life from the canal-like stretch of a river.

Furthermore canals link up all the main river systems draining central England. Boycott (1936) writes: ‘And about the middle of last century a snail could start in the Thames at London and travel in uninterrupted water to Norfolk or Leeds or Kendal or Newtown in Montgomery or Hereford or Trowbridge, or by slipping into the upper waters of the Avon in the Vale of Pusey even to Christchurch or Southampton.’

CHAPTER 6

ANIMALS AND PLANTS

The environmental background having been sketched, it is necessary to devote this chapter to an account of the different kinds of animals and plants which occur in fresh water.

We start with four paragraphs written for those naturalists who have not had a biological training, and who have studied only vertebrates. The number of different kinds – or species to use the proper term – is usually much greater in an invertebrate than in a vertebrate group and, since the animals are so much smaller, the differences are less obvious, and sometimes undetectable by the naked eye. Many people are surprised to learn that the word mosquito, for example, covers in Britain no less than forty different species. We have endeavoured to give some idea of the scope of the freshwater fauna by mentioning the number of known species in each group.

Latin names are unavoidable, since only a few species have an English name. Invertebrates are usually referred to by two names, and the first of these, the genus or generic name, corresponds to the surnames in the human community; though the Smith difficulty has been avoided by making a rule that the name of every genus must be different. The second name is the species name, and, like our Christian names, many are used over and over again; for example, lacustris