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.
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
