Life in Lakes and Rivers. T. Macan T.

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 erode the shoreline rapidly. Many lakes are surrounded, partly or entirely, by moraine deposits known by various names such as glacial drift, boulder clay, till, or sammel. Waves eroding a shore of this type leave in situ only the larger stones and boulders and carry away the finer particles, which eventually come to rest in deeper water away from the shore, or in some sheltered bay. The coarsest particles will be moved the least, the finest the greatest distance, and there will therefore be a graded series passing into deeper water farther away from the shore. The processes of erosion and deposition result in what is known as a wave-cut platform and are illustrated diagrammatically in Figure 3.

      Fig. 3 Diagram of the erosion of a boulder clay shore to give a wave-cut platform

      Sometimes the material removed is not carried out at right angles to the shore but at an acute angle so that, when it settles, it forms a spit. Such formations are of importance to animals and plants because they create areas of quiet water which are the resort of certain species unable to tolerate the conditions on a wave-beaten shore.

      Deltas are even more important features of the lake shore. They may be no more than bulges in the shoreline, or, at the other extreme, they may cut a lake in two. Good examples of deltas at all stages are to be seen in the Lake District lakes. The delta of the Measand Beck stretched two-thirds of the way across Haweswater, before this lake was dammed in 1941 to provide more water for Manchester. A stage farther can be seen in the valley of Buttermere and Crummock Water, which were left by receding glaciers as one large lake. Since then Sail Beck, flowing in from the east, has cut the original lake into two and its delta now provides the half-mile of flat land in the valley floor between the two lakes. Another pair of lakes, Derwent Water and Bassenthwaite, show a still more advanced stage. Here again the two were formerly one, but the River Greta has poured so much silt and gravel into the original lake that there is now a full two and a half miles of plain separating the north shore of Derwent Water from the south of Bassenthwaite.

      Some of these deltas are much too large to have been brought down by the little streams existing today, and much of the material was probably swept down during the last stages of the Ice Age by the bursting of ice dams and other minor cataclysms.

      We may pass from generalities to describe a portion of the shoreline of Windermere, for it illustrates several of the points already made, and is referred to later when the fauna is discussed. The shoreline in question is bounded to the north by a ridge of rock jutting into the lake. The sides of this promontory, which is known as Watbarrow point (Fig. 4) are smooth, and run down at a steep angle to a depth of nearly 100 feet. To the south the same kind of rock, Bannisdale slate, is exposed at the edge of the lake, but weathering