Читать книгу Life in Lakes and Rivers - T. Macan T. - Страница 8

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.