Life in Lakes and Rivers. T. Macan T.

      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.