to a depth of nearly ten metres and this is the condition found after thorough mixing by strong wind. In calm weather the temperature tends to decrease, often in an irregular manner, from the surface to the thermocline. As the hypolimnion flows to and fro with each oscillation, irregularities on the bed of the lake set up eddies, and these produce the mixing of the water which led to the investigation originally.
The rivers and streams flowing into a lake are usually at a temperature well above that of the hypolimnion and accordingly will mix only with the epilimnion.
With the shortening of the days in autumn, particularly if there is a fine spell with cold clear nights, heat is lost by radiation during the hours of darkness. The epilimnion begins to cool down and eventually, sometimes not until December, a gale will obliterate it, mixing it completely with the hypolimnion.
There remains one other factor to mention before anything living comes into the picture. The sun’s rays have been considered so far only from the point of view of their heating properties; for the activities of plants light is more important. Light rays do not penetrate far into even the purest water, and in most waters there is something extraneous to reduce their penetration still more. Any sedimentary matter in suspension, any colouring such as that derived from peat, and living organisms themselves all absorb rays of light. (Fig. 11) shows that in three Lake District lakes light goes farthest into the pure and barren waters of Ennerdale Lake, less far into the richer waters of Windermere, and least far into the peat-stained and rich waters of Bassenthwaite. Since light does not penetrate far into water, plant growth is only possible in the upper layers, and is nearly always confined to the epilimnion.
May we recapitulate here, since so much of what follows depends on the physical conditions which have just been discussed. During the summer months the lake is divided into an upper warm epilimnion and a lower cold hypolimnion, which are to all intents and purposes completely separate (Fig. 1), and all plant growth takes place in the epilimnion.
Algae (minute floating plants) are present in the open water all through the winter but physical factors, notably the short days and the low light intensity, are unfavourable for rapid multiplication. When conditions are right for this there is a rapid and colossal increase in numbers which is checked when the substance in shortest supply is exhausted. Phosphate and nitrate are two important nutrients but in Windermere, the size of the population of Asterionella, the commonest diatom, is limited by the concentration of silica, which the alga requires for its skeleton. Once reproduction is halted, the population declines rapidly (Fig. 2). After the spring outburst various species of algae rise and fall in numbers, but the total attained is much less than that reached in the spring. The zooplankton (small floating animals) reach their maximum abundance a month or two later than the phytoplankton.
Fig. 2 Increase in phytoplankton and decrease in the concentration of certain salts in Windermere in 1936
The animals living in the mud at the bottom of the lake are in perpetual darkness and almost constant temperature. Little is known of their activities in any British lake, but P. M. Jónasson has shown that in the Danish Esrom lake the growth of a chironomid depends on the rain of dead plankton falling from above. This comes to an end in winter and the growth of the larvae stops. It starts again in the spring and proceeds rapidly, but is checked again when the oxygen is used up in the lower layers and the larvae can do little more than survive. They emerge early in the following year. Most larvae take two years to complete development but a few achieve it in one, but their eggs are all eaten by their brothers and sisters who have failed to develop as fast. The result is a big emergence every other year. Nearly all aquatic insects emerge as adults in spring or summer, presumably because of the physiological difficulties of flying in cold weather, and this must impose a seasonal rhythm upon their development.
A ring of green algal growth on the stones in the shallow water of a lake appears in spring, but most of the stoneflies and some of the Ephemeroptera of this region grow during the winter and pass the warm part of the year in the egg stage. This phenomenon will be discussed further when streams are described. One of the commonest animals in the reed-beds is Leptophlebia (Ephemeroptera) and this is a species that grows throughout the winter, but most of the fauna grows during the summer.
These various plants and animals are continually dying and decomposing, broken down by various agencies about which we do not know very much at the present time. Fungi and bacteria set upon their dead bodies and reduce them to fine particles and simple compounds, which serve as food for other organisms, so that there is a constant process of breaking down and building up in the epilimnion. But some of the decaying fragments, with the organisms breaking them down, fall through to the hypolimnion, and we must leave them for the moment to describe what has been happening there. More important perhaps is what has not been happening; there has been no plant growth, because it has been too dark, and therefore no utilization of the dissolved substances for want of which algae have been dying in the layers above. Evidently division into epilimnion and hypolimnion reduces the productivity of a lake.
The decaying matter which falls down to the hypolimnion continues to decay, though at a slower rate on account of the low temperature, and it uses up oxygen. There is no source from which the oxygen in the hypolimnion may be replenished, and consequently the concentration falls steadily all through the summer; it may reach nil if the lake is a productive one and the hypolimnion small – an important point, as will be seen in the next chapter.
The decaying matter may eventually reach the bottom, and here some of it is eaten by the animal inhabitants of the bottom mud, and some of it is broken down into simple substances by bacteria and other agents. Most of the organic matter found deep in the mud, where it must have lain for thousands of years, was washed in from the land. But these simple substances cannot reach the surface layers, where they could be used for building up more living matter, until hypolimnion and epilimnion mix in the autumn. By then biological activity is reduced, and by the time there is a big demand again for dissolved nutrients in the following spring, much of the supply will have been washed out of the lake. On the average, water takes nine months to pass through Windermere, and therefore during the winter there will be considerable depletion of the dissolved substances released from the hypolimnion by the autumn mixing. Again it becomes apparent that the formation of a hypolimnion prevents the development of the full potentialities of a lake.
Large fragments hardly decay at all in the cold mud at the bottom of deep lakes. Wasmund (1935) gives an account, illustrated by gruesome photographs of bodies, including three human ones, that have been brought up, generally in fishermen’s nets, after many years in the water.
Dr C. H. Mortimer (1941–42) has recently shown that, when there is oxygen at the surface of the mud, iron is present in the oxidized ferric state and forms a colloidal complex with various other substances. This colloidal complex tends to hold the simple products of decay, and therefore augments the locking-up process caused by the slow decomposition in the mud. But, if all the oxygen is used up, the ferric iron is reduced to the bivalent ferrous state, which goes into solution with consequent breakdown of the colloid complex. This liberates the other substances, and Mortimer was able to show, both in an artificial experiment in an aquarium and in a lake, that the disappearance of oxygen from the hypolimnion is followed by an increase in the concentration of silicate, phosphate, ammonia, and iron in the water.
The above are factors which affect the plants and animals living out in the open water of a lake and in the mud below it.
A different assemblage of living things inhabits the shallow regions near the shore, and this population too is affected by physical and chemical processes. The most important is wave-action. The effect of this factor depends on the nature of the land on which it acts. Waves beating upon rock will disintegrate the weaker patches and leave the harder ones projecting as ridges but the total effect is small; waves beating upon sand or peat, on the other hand, will
