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.
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
