Life in Lakes and Rivers. T. Macan T.

of the channel between this bank and the second ridge, and water apparently stands in the north and south portions of the lagoon at all stages of the tide. Today (Fig. 7d) there are three dunes, and the Little Sea is an inland lake with water which is actually soft and rather poor in dissolved salts. Other, smaller, bodies of water have come into being and the slacks between the dunes are extensively marshy; man-made cuts traversing them testify to an attempt at some earlier date to drain them, presumably to obtain pasturage.

      Fig. 7c Formation of the Little Sea, c. 1849

      And so, thanks to the painstaking research of Captain C. Diver (1933), it is possible to reconstruct in detail the changes which brought Little Sea into existence. There are other sheets of fresh water of similar origin, but no one has pursued inquiries into their early history. Some have obviously been formed more simply, and Llyn Maelog and Llyn Coron in Anglesey, for example, lie in long transverse depressions which the sea has blocked at the ends with sand.

      Wherever man has had available an impervious soil he has tended to make ponds and lakes, to provide him or his animals with a water supply, for ornament or for sport. A favourite site is a narrow valley which can be flooded by the erection of a dam (Fig. 8), for building a dam is comparatively simple, while sufficient excavation to make a pond of reasonable size is a big and costly undertaking. Where there is hard impervious rock, fish-ponds are sometimes very numerous; for example, in the southern part of the Lake District the staff of the Freshwater

      Fig. 7d Formation of the Little Sea, c. 1924

      Biological Association have nearly fifty under observation within easy reach of their laboratory.

      On heavy clay soil the farmer frequently digs a hole in every field in order to form a pond from which his animals may drink. Many other pieces of water are the by-products of man’s activities. Quarrying for stone, or digging out clay for bricks, produces an impermeable basin which the rain will ultimately fill. Excavating sand and gravel for railway ballast and other purposes may extend down below the water-table so that a pond results. Underground mines and tunnels sometimes cave in and cause at the surface a depression which fills with water.

      The prosperity of the fifties and sixties and the boom in aquatic sports such as fishing and boating has meant that many gravel pits that might otherwise have been used for the disposal of rubbish have been saved as lakes. On the other hand, many small ponds are disappearing, because, with state aid to water supplies for farms, they are no longer necessary for watering stock. Indeed their use for this purpose is actively discouraged, since it has been shown that cattle contract Johne’s disease by drinking from fouled ponds.

      Fig. 8 Hodson’s Tarn, an artificial moorland fishpond

      These are some of the main ways in which bodies of fresh water have originated. There are others, less important in the British Isles, but a catalogue of them would serve no useful purpose here. Our main interest is with the plants and animals of water, and the next stage is to notice how lakes may be classified according to the biological processes going on within them.

      A lake receiving the drainage from rich cultivated land will be ‘productive’, because of the nutrient salts it receives, that is, a large quantity of plant and animal material will be produced in the upper layers. Many of these organisms will decay in the lower layer, which, if the lake is stratified, may become depleted of oxygen. A second condition is that the hypolimnion should be relatively small. A combination of good agricultural land and a shallow lake is typical of lowland country, and it is here that lakes with no oxygen in the hypolimnion generally occur. They are known as ‘eutrophic’. An ‘oligotrophic’ lake, that is, one in which the hypolimnion contains oxygen, is typical of mountain conditions where the drainage area is unproductive and lakes often occupy deep basins. For many years the difference was thought to be fundamental, and an elaborate classification arose on a foundation which had been simple originally. As knowledge accumulated, it became evident that the distinction was not as basic as had once been thought, and it is no flight of fancy to say that the edifice of classification was brought crashing about the ears of the assembled company by Professor H.-J. Elster, in a masterly review at the International Congress of Limnology in 1956. Since then the tendency has been to study the primary productivity of a lake, that is the amount of algal material produced in the open water during a year, and to arrange the lakes in a series according to the figure obtained.

      Shortly before the First World War, the late Professor W. H. Pearsall started a study of the Lake District lakes the basins of which were all formed in the Ice Age. Whether he was familiar with the continental ideas and ignored them, or whether he was not aware of them, we shall probably never know. Anyhow he arranged the lakes in a series with no attempt to delimit and define categories, although Esthwaite, at the productive end of the series is eutrophic, and Wastwater, Ennerdale, and Buttermere at the other are fine examples of an oligotrophic lake. This concept stimulated a great deal of work, and though Pearsall’s original ideas have been modified, the basic soundness of the idea has been revealed by research in several fields. Pearsall noted that the unproductive lakes lie in the hard Borrowdale volcanic rocks right in the main mountain masses. Consequently the valley sides are steep, the area of flat valley bottom is small (Fig. 9) and rain falling on the drainage area will flow over much bare rock and scree. Consequently it bears little in solution when it enters the lake. The unproductive land supports no more than a farm or two, and few other than farmers have been tempted to settle in the restricted area available. This, however, has also been influenced by the remoteness of the valleys which are distant from the main lines of communication.

      Fig. 9 Buttermere, an unproductive Lake District lake

      Fig. 10 Esthwaite, a productive Lake District lake

      Windermere and Esthwaite Water (Fig. 10) are the two most productive lakes. They lie in the south of the district in a zone of Bannisdale slates, which, though hard rocks, are softer than the Borrowdale Volcanics and have weathered more. Much of the drainage area is floored with the products of weathering and is relatively flat. Obviously rain-water seeping through soil will dissolve out more than water trickling over solid rock, and so the streams and rivers entering Esthwaite and Windermere bring with them a higher concentration of nutrient salts than those flowing into Ennerdale. But the flat land also attracts the farmer and the cultivator who seeks to improve the soil by adding manures to it. Some of these find their way into the lake, and so the difference between the two is enhanced. Within the last century Windermere, particularly, has become a residential resort. The result is that much human sewage enriches its waters and makes still greater its difference from Ennerdale.

      Esthwaite Water is a relatively shallow lake and, as already stated, eutrophic.

      Position in the series developed by Pearsall was based on three factors, first the percentage of the drainage area which is cultivated, second the percentage of the shallow water region which is rocky, and third the transparency of the water. The first two factors are fundamental; the third is partly fundamental and partly a result, because the transparency of the water depends both on the amount of mineral matter in suspension and on the quantity of life present, provided there are no extraneous factors like staining from peat or pollution by mine washings. In the Lake District none of the larger lakes except Bassenthwaite contain peat-stained water, and pollution from mine washings, though it does occur, is fortunately rare. Table 2 shows the Lake District lakes arranged according to these three factors. The figures in the last column show the depth at which a white disc, 7 cm. in diameter, could just be seen.

      On the whole there is a serial increase or decrease in each of the three columns. The most notable anomaly is the low transparency of Bassenthwaite, occasioned by its being the only lake of which the water