Life in Lakes and Rivers. T. Macan T.

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.