Life in Lakes and Rivers. T. Macan T.

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СКАЧАТЬ water, he has built ponds which have received the name of dewponds. There are few subjects about which more nonsense has been written. One explanation offered, even by people who should know better, is that dew-ponds are made by a secret process which insulates them from the surrounding land. When heat is lost at night by radiation from the surface, warmth from the lower layers is conducted upwards, and therefore the temperature at the surface does not reach a very low point; but this upward conduction cannot affect the dewpond because it is insulated. Accordingly, it is alleged, the dewpond area gets very cold, the atmosphere above it is chilled and moisture is deposited. The difficulty about this theory is that, if the dewpond were so effectively insulated from the land below, it would get very hot when the sun shone on it by day and much water would be lost by evaporation. Furthermore, considerations of the respective latent heats of water and chalk (that is the amount which a given volume of each would lose in a given time) have been ignored. Several people have examined the problem both experimentally and theoretically and the whole fallacy has been exposed more than once. Mr A. J. Pugsley (1939) has returned to the attack in a small book published by Country Life, but it would be optimistic to expect that the myth has been exploded. There is certainly a secret process in the making of dewponds and it has been handed down from father to son in certain families, but its aim is the construction of a waterproof bottom which will last for many years without cracking.

      The dewpond, in effect, is a shallow pan of concrete or clay, and, though sometimes it is situated on top of a hill where it must rely entirely on rain, often it is located to take advantage of storm water, particularly where a road presents an impermeable surface. The belief that dewponds date back to the Neolithic Age is erroneous.

      We have now worked our way down to the south of England and come to the coast to study a freshwater lake which owes its origin to sand-banks thrown up by the sea. To the west of Bournemouth lies Poole Harbour, a big enclosed area connected with the open sea by a small entrance, which cuts through a narrow strip of land and so makes two peninsulas. The one which lies to the west is known as South Haven Peninsula, and a conspicuous feature of it is the Little Sea, a shallow lake over 70 acres (28 ha.) in extent. Particular interest lies in the fact that the origin and development of Little Sea can be traced in detail from the information given on old charts. The first of these, dating from the reign of

      Fig. 7a Formation of the Little Sea, c. 1600

      Henry VIII, is not very accurate, but from it and one or two later charts a fair deduction is that the peninsula then comprised only land of the Bagshot Sand formation, with a small more recent sandspit at the tip. This is shown in Figure 7a, but the sea and the area between tidemarks are omitted from this figure, as any attempt to include them would be based largely on conjecture. A chart of 1721 is remarkably accurate. It shows that a sandbank, thrown up parallel with the land existing in the previous century, has enclosed a lagoon which is apparently a sheet of water at high tide but at low tide an expanse of bare sand, except for water standing in drainage channels. There is a wide beach, shown stippled in Figure 7b, and a detached sandbank lying to the north of the channel draining the lagoon. Rather more than a century later, in 1849, a survey shows considerable development; a second ridge has

      Fig. 7b Formation of the Little Sea, c. 1721

      been thrown up parallel to the first in the northern half and marshy area indicates the depression between the two; a third is foreshadowed by a long sandbank which now bounds the outflow on the seaward side; it is shown white in Figure 7c, the convention used to denote land above high water, though strictly it should be stippled as, according to the chart, it was covered by the highest tides. The sea runs in and out 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 СКАЧАТЬ