Life in Lakes and Rivers. T. Macan T.
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Название: Life in Lakes and Rivers
Автор: T. Macan T.
Издательство: HarperCollins
Жанр: Природа и животные
isbn: 9780007406135
isbn:
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 СКАЧАТЬ