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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 is stained with peat. The amount of light at different depths in Bassenthwaite, Windermere and Ennerdale is shown in Figure 11, expressed as a percentage of the intensity at the surface.
Table 2. The sequence of Lake District Lakes (Pearsall, 1921)
Fig. 11 Penetration of light into three Lake District lakes
Work on cores, started just before the war, had as its original aim the elucidation of the history of the lakes, but, like many another new line in research, an essentially opportunist activity, it proved most fruitful in a line other than the one aimed at and it revealed more about the land than about the water. However, as a lake is strongly influenced by events in the drainage area, the findings are relevant. Most animals disappear completely, but the shells of some waterfleas (Cladocera) and the heads of some chironomids do not decompose and persist in the cores. Similarly many algae leave no trace, but the siliceous skeletons of diatoms (e.g. Asterionella) that have lain in the mud for thousands of years are still identifiable. In contrast the pollen of nearly every plant that produces any does not decompose and, since that of almost all species is distinct, an examination of cores gives a picture of what the land flora was like when the particular layer of mud under examination was deposited. Research on pollen in cores from bogs and other places where soil has been accumulating since the Ice Age was in vogue all over Europe at the time, which was fortunate, because events in the lake cores could be related to events elsewhere, and some of these had been dated by one means or another. Chemical analysis of cores also yielded a large amount of information about the past.
The lower part of a core from Windermere consists of clay which, on examination, proves to be made up of alternating layers of very fine and coarser particles; it is accordingly referred to as laminated clay by Dr Winifred Pennington, who has described the cores. Above the laminated clay, which is pink, lies a grey layer and above that more pink clay, which may or may not be laminated. On top of this is a thick column of brown mud which extends nearly to the surface; it is capped by a fourth kind of soil, a black deposit which Pennington refers to as ooze.
The laminated clay contains very little organic matter and few remains of plants, and was almost certainly formed towards the end of a glacial period, for similar deposits are being laid down today in certain glacier-fed lakes. During the summer both coarse and very fine particles are washed into the lake. The coarse particles sink almost immediately but the fine particles remain in suspension for a long time. When winter comes the inflowing streams freeze, and so no particulate matter is brought into the lake, but fine particles left over from the summer are still settling. The result is a summer layer of coarse and fine particles and a winter layer of fine particles only.
The low organic content of this deposit and the scarcity of plant remains indicate that there was little life in the lake when the laminated clay was being laid down. In contrast the grey mud contains the remains of animals and plants, and the lake was evidently more densely populated during the period when it was being laid down. These organisms were associated with an improvement in the climate, which is known as the Allerød period, because it was first discovered near the place of that name. It was followed by a return of glacial conditions when the pink clay with few remains of organisms was laid down again. Professor H. Godwin has had dated by means of the C14 method, a technique which will no doubt be more widely used when facilities for it are more widely and more easily available, a sample from the Allerød layer and found it to be some 12,000 years old. The ooze at the top in which Asterion-ella suddenly becomes common obviously represents enrichment of some kind. Today the effluents from the sewage works are probably the main sources of enrichment, but this is recent. СКАЧАТЬ