Читать книгу Life in Lakes and Rivers - T. Macan T. - Страница 11

DIFFERENT KINDS OF LAKES

Lakes, geologically speaking, are transitory features of the landscape. The biologist who studies a lake is likely sooner or later to find that the answer to some problem he is seeking to solve is to be sought in past history, and particularly in the way the lake was formed. Lakes and ponds have originated in many different ways and much ingenuity has been devoted to fitting them into schemes of classification. We shall not dwell on the groupings and subgroupings which have been suggested, but, in the first part of this chapter, prefer to take the lakes as they come, starting in the north of Scotland and travelling southwards.

The Great Glen is a tear in the earth’s surface, and Loch Ness, which lies in it, provides an example of a lake associated with faulting. Loch Ness is 21³/₄ miles (35 km.) long and a little under one mile (1.6 km.) in average breadth, so it is a long and narrow lake; its greatest depth is 754 feet (230 m.) and its mean depth 433 feet (130 m.), so it is deep and steep-sided. Its mean depth is greater than that of any other British lake by quite a big margin, though there is one, Loch Morar, which is deeper at the deepest part (1017 feet = 310 metres). All these features are characteristic of tectonic lakes, that is lakes formed originally by movements of the earth’s crust. Also in this class are some of the most striking lakes of the world, such as the Dead Sea and the lakes in the Great Rift Valley of Eastern Africa. These were caused partly by lateral tearing, as is the Great Glen, but there followed a lowering of a strip of the earth’s crust so that what is now the floor of the rift valley was once level with the high land on either side.

Most of the other lochs in the Scottish Highlands owe their present form to the work of ice when the country was covered with it during the Ice Age and so were the llyns of the Welsh mountains and the lakes of the English Lake District. Indeed, nearly all the larger stretches of water in Britain were formed by glaciers, at least in part. In some cases their basins were gouged out by glaciers flowing down mountainsides, usually in valleys cut by a stream in an earlier, more clement period. When the mass of snow and ice and rubble reached the bottom of the slope it dug into the ground and excavated a great trench. This trench became the basin of the lake when the glacier retreated with the onset of warmer conditions. The mass of material which the glacier plucked from the land it passed over was deposited in mounds or moraines at the snout of the glacier, and some glacial lakes are dammed up by a moraine which makes them deeper than if they were contained in the actual excavation alone. Glacial lakes, like rift valley lakes, are usually long and narrow and relatively deep; Windermere, for example, is 10¹/₂ miles long but only half a mile wide on the average, and 219 feet deep at the deepest point. Their sides tend to be parallel and any major irregularity in the shore line is often of more recent age. Lakes of this type were formed only in hard rocks where the relief was rugged, and steep valleys concentrated the glacier and directed its excavating effort to a circumscribed area.

Also of glacial origin are the smaller lochans, tarns, and the small lakes in cirques, corries, or cwms which are often to be found near the tops of mountains. They are frequently circular in outline and they mark the place where the snow or ice piled up and a glacier took its origin.

An ice sheet covering a plain did not excavate because its effort was dispersed and not concentrated, but it did give rise to lakes none the less. As might be expected these are of a different type; Loch Leven is an example and no fisherman requires a biologist or geologist to tell him that there is something fundamentally different between Loch Leven and the Highland lochs. As the ice sheet which covered Scotland began to recede, a large lobe of the glacier flowing down the Forth Valley became isolated in the centre of the Kinross plain. It was surrounded by clay, stones, boulders, and suchlike products of ice erosion washed along in the water from the melting glaciers, much of it coming through a pass in the Ochils from the Tay Valley. A considerable depth of this material was deposited on the plain, but in the middle there was this big block of ice melting slowly because of its large size, like an iceberg in the North Atlantic. When it finally disappeared it left a hollow where it had been sitting and this filled up with water to become Loch Leven. The shape of Loch Leven is quite different from that of either glacier-cut or rift valley lakes: it is not much longer than it is broad, one axis being 3²/₃ miles (5.7 km.) and the other 2²/₃ (4.1 km); its mean depth is only 15 feet (4.5 m.) and its greatest depth only 83 feet (25 m.).

The Cheshire Meres were formed in a similar way to Loch Leven, although subsidence of the land surface also played a part. Outside Britain there are many lakes of the same type: two groups, which are referred to in later chapters because they have been studied in much detail, are the numerous lakes in the Wisconsin area of North America and the Baltic lakes of Denmark, Germany, Poland and U.S.S.R.

Also characteristic of mountainous areas are the much smaller peat pools. These may occupy holes where stone for a wall or a house has been quarried or sometimes a rock basin of natural origin, but most of them are formed by the growth and then the erosion of peat. Some of the largest are to be seen on the Pennines, for the Pennines have flatter tops than the mountains in Scotland, Wales, or elsewhere in Britain, and it is on flat places that these pools develop. Vegetation, of which bog-moss (Sphagnum) is usually an important constituent, dies and accumulates over a long period of years, building up a bed of peat. At a certain stage, for reasons which are not at present understood, the peat becomes unstable, and hollows are eroded by the action of wind and rain. The surface becomes dotted with small pools and, as further erosion takes place, these coalesce. Finally a channel becomes eroded through the rim of the peat bed and all the water runs away. The building-up process then starts again.

Larger bodies of water on the Pennines are few, apart from man-made reservoirs which are now characteristic features of the landscape.

Lough Neagh in Northern Ireland is the largest sheet of fresh water in the British Isles, with a surface area of 153 square miles (393 km.²). It was formed in a way different from that of any of the other lakes so far encountered, and is volcanic in origin. There was no volcanic mountain like Etna or Fujiyama, but basaltic lava welled up from fissures in the ground. It flowed freely over the countryside and eventually solidified as a flat plate-like capping. Later it sagged in the middle and the depression so formed is Lough Neagh. The greatest depth is only 56 feet (17 m.), so it is even shallower than Loch Leven.

There remain to be explored the more recent geological formations of south-east England, and on them there are few large bodies of fresh water, though they are not on that account of any less interest to the freshwater naturalist. The best-known sheets of water are the Broads of East Anglia. The scientific mind, like Nature, abhors a vacuum and there was no dearth of armchair theories about how the Broads had been formed when Joyce Lambert, a botanist, and J. Jennings, a geographer, set out to collect some facts.

Dr Lambert pushed her way through the dense fens along straight lines from edge to edge and took borings at regular intervals. At many places the peat was of a different type at different levels, at others it was uniformly of a type that indicated accumulation at the bottom of standing water. In due course an elaborate and plausible explanation of the origin of the Broads was formulated and it might have remained the accepted one for a very long time, a great deal of hard work having gone into the collection of the evidence. However, Dr Lambert thought it prudent to continue her borings, and turned up evidence which demolished the theory. She found peat composed of different plant associations at different levels and peat that had accumulated uniformly in water so close together that the plane between them must be vertical; indeed there was evidence of columns of the former surrounded by the latter. There could be no explanation of this except excavation by human agency and the research became primarily historical.

No direct evidence has been found but the circumstantial evidence is convincing.

Documents of the thirteenth and earlier centuries refer to turbary rights in the region of the Broads, and there are records of much peat-cutting in parishes where there was no source of peat other than the fens where the Broads now are. There is no reference to water. After about 1350 there are few references in old documents to turbary, but frequent references to fisheries. There is, therefore, good reason to believe that the Broads are old peat-cuttings which became flooded between 1300 and 1350 probably as a result of some change in the relative levels of land and sea (Ellis, 1965).

A river tends to build up a deltaic plain at the end of its course and it inundates this plain every time it rises a little above its normal level. Parts of the plain will be under water only at the height of a flood, parts will be permanently marshy, and parts will be under water all the year round. This is the normal and accepted state of affairs in regions of the world where man has done little towards controlling and taming nature: the Rivers Tigris and Euphrates may be taken as an illustration (Fig. 6). In Britain, however, man has long since decreed that there is a place for everything and the place for water is within well defined banks; any breaking out and overflowing is an irregularity and often a catastrophe, and the victim of a flood is not consoled by the assurance that it is “natural”.

Fig. 6 Lower courses of the Rivers Tigris and Euphrates

The East Anglian fens originated when a flat clay-floored valley opening into the Wash was flooded by the sea after the Ice Age, owing to a slight lowering of the land level. Silt banks deposited by the sea gradually cut it off and it became a great inland lagoon. It was shallow, and rich in nutrient salts. Conditions were, therefore, good for plant growth and the resulting vegetation was luxurious. The dead remains accumulated and formed peat which filled up the lagoon rather rapidly, speaking in geological terms, till open water was left only in a few meres, which must have been very like the Broads today. Man coveted the rich soil and in the seventeenth century he successfully started drainage and reclamation. Now the meres have gone, the natural vegetation is to be found only in a few carefully tended preserves, and the fenland presents to the pond-hunter no more than an endless series of ditches, great and small.

Travelling a little farther south, we come to the chalk region; and a more waterless expanse than a chalk down cannot be found anywhere in the country. But even here there is something to interest the freshwater naturalist. Man has been wont to run stock over the downs for centuries and, in order to provide them with 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 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 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. The population, particularly the holiday-making one, has been increasing since the railway came to Windermere in 1847, but the transition from earth closets to running water sanitation has been slower, and it is doubtful if the lake was being seriously enriched from this source a century ago. If mud is being deposited on the bottom of Windermere at a rate that has been constant over the last few hundred years, whatever caused Asterionella to appear happened about two centuries ago. It therefore probably antedates the tourist completely, and is to be sought in improved agricultural practice of which there is some evidence early in the eighteenth century.

Mackereth’s conclusions from chemical analyses upset certain ideas of long standing. The lakes were richest chemically in their earliest days, when the land was covered with rock fragmented by the ice and exposing a great area of surface from which the rain could leach nutrient salts. Esthwaite was eutrophic at an early stage and presumably, therefore, the lakes were richest biologically when they were richest chemically. Some of the algal species identified in the cores support this view. The rocks are hard and when a fresh surface has been leached for a time water dissolves little from it. The lakes slowly became less productive. A climatic change and the arrival of man, who started to fell trees, resulted in more erosion and enriched the lakes, but as more stable conditions were established production fell again. A thousand years ago the Norsemen arrived, and since then man has been the most important agent affecting the lakes. The increased production in some lakes on account of their situation has already been described.