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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 down 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 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 and wave-action have broken it up considerably, and the products of its disintegration litter the lake floor. They are large flat angular slate-like stones. Moon (1934), who has studied this region of the lake, refers to it as the ‘Bannisdale’ shore and contrasts it with the ‘drift’ shore which lies to the south. The drift shore consists of stones and boulders but these are round, not flat and angular, and there are finer particles between them. This shore has been formed by the erosion of a mound of boulder clay or glacial drift, somewhat after the manner shown in Figure 3. The hinterland of the Bannisdale shore is covered by woodland, but that of the drift shore has been cleared of woodland at some time and is now pasture. This is not coincidence; where the underlying slate is not covered with glacial drift, the topsoil is often so thin, and rocky outcrops are so frequent, that cultivation of the land is not feasible; but where the rock is covered by boulder clay, it has been worthwhile to remove the forest and bring the land into agricultural use.

Fig. 4 Windermere, north end showing reed-beds. Reed-beds are stippled

Figure 4 shows that at the south end of the drift shore there is a bay – High Wray Bay – which is somewhat protected. Only the comparatively rare easterly gales will blow right into it, and the range of direction of wind from which it gains no protection at all is but 30°. High Wray Bay is floored with sand.

Sandy Wyke Bay farther north is more sheltered. The range of direction of wind which will blow straight into it is only 20°. But a glance will show that the amount of exposure is not to be measured entirely by the angles drawn in Figure 4. If a wind blowing in the direction of the more southerly of the two pecked lines bounding the High Wray Bay angle veer slightly, it will still drive waves into part of the bay, and it must shift through nearly another 30° before complete protection is obtained. But if a south-easterly wind that just blows full into Sandy Wyke veer but a few degrees, the projecting coastline will shelter the bay almost completely. Sandy Wyke Bay is also sandy, but there is a big reed-bed growing in it.

Only a north wind will blow right into Pull Wyke South Bay, but it will traverse so short a stretch of water that the waves raised will not be of significant size. This bay is floored with fine mud. The vegetation shows the zonation typical of quiet conditions. In the shallowest water there are various emergent plants such as reeds, rushes, sedges, and horsetail; in deeper water there are plants with leaves floating at the surface, such as water lilies; and beyond them are plants, such as pondweeds, stoneworts, and quillwort, which live totally submerged throughout life.

The phenomena described above are of such general occurence that, in spite of the diversity of lakes, a ‘typical’ lake is a useful concept. There are two main types of lake that may be styled ‘atypical’. Lakes that have a large surface area and little depth do not stratify. Lough Neagh, possibly even Bassen-thwaite and Derwent Water in the Lake District, are examples. Lakes of this type, however, have not been studied thoroughly. and all that can be said at present is that they have been found to be unstratified in summer at a time when epilimnion and hypolimnion are clearly demarcated in other lakes. Of course, any body of water in temperate climates will show some stratification after a hot day; the important point is how long stratification lasts. It is possible that these large shallow lakes may stratify throughout an occasional summer when sunshine is unusually abundant and wind unusually scarce. Information should be available from Lough Neagh soon, as the New University of Ulster has established a station there. It is difficult to make observations sufficiently often unless a laboratory is available, and the ideal, described in the next chapter, is an arrangement of thermometers in the lake connected to a recorder in the laboratory.

The other kind of atypical lake is known technically as meromictic, and its peculiar feature is permanent stratification. The density difference that prevents mixing is due to substances in solution, not to temperature, and is often but not invariably due to peculiar geological conditions. The condition could arise in any lake where production is high and circulation low. Poor circulation occurs in areas where strong wind is rare, and the effect of lack of wind will be enhanced in a lake with a small surface area relative to its depth, and with not much water flowing in. An abrupt transition from winter to summer and from summer to winter is another factor that plays a part. Given these conditions one may postulate that the meromictic condition arose in the following way. If at the end of a summer the hypolimnion is greatly enriched by decomposition of organisms produced in the upper layers, it will be denser than the water from which they have come when both layers are at the same temperature. It is not difficult to suppose a year in which the cycle of events has resulted in both being at 4° C. The epilimnion will float on the hypolimnion. If there is little wind and ice forms soon, this state will endure until the spring. If there is little wind then to upset this delicate state of balance, and plenty of sun to increase the density difference by warming the upper layers, stratification will have lasted a year. By the following autumn the accumulation of two years’ production will have increased the density difference due to solutes between hypolimnion and epilimnion and the chances of their remaining unmixed during the following season are greater. The longer the two remain separate the more the energy required to mix them, and the less likely mixing becomes. It is believed by Professor I. Findenegg, who discovered the condition, that certain lakes in the Carinthian province of Austria became meromictic in some such way.

So far no definition of the word ‘lake’ itself has been attempted. Our colleague, Mr F. J. H. Mackereth, has been heard to say that a lake is no more than a bulge in a river. This idea is more useful to a chemist than to a biologist, but it is salutary that a biologist should remember how much of what takes place in a lake is governed by what is washed in from the drainage area. A lake is a piece of water of a certain size but at what size the word pond becomes applicable is a matter of opinion. It is one of those continuous series, frequently encountered in biology, where the difference between two ends is enormous but any lines drawn in between them to separate categories are arbitrary. One definition maintains that any piece of water which is so shallow that attached plants can grow all over it is a pond. The pedant has no difficulty in picking holes in this definition and pointing out that the depth to which attached plants extend varies very much with the transparency of the water; a cattle pond only a foot deep may be without vegetation in the middle because the light is cut off by innumerable small organisms which live in the open water and batten on the nutrients supplied by the dung. Or the nature of the substratum may be unsuitable for attached vegetation. Another school holds that, if a body of water becomes divided into epilimnion and hypolimnion and remains so divided throughout the summer, it is a lake and not a pond. Stratification, however, depends, not on size, but on the relation of depth to surface area and also to exposure to wind. The latter also determines to some extent whether the edges are eroded by wave action or not, and therefore blurs the definition according to which a lake is large enough for its shores to be eroded and a pond is not. In a restricted area, or for a given purpose, a worker may find a useful distinction between a lake and a pond, but in general no scientific distinction can be made.