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
