Life in Lakes and Rivers. T. Macan T.

plain, laid down during the Ice Age, and flows across this in a meandering course, though with a fair flow. The stretch is too fast for a typical lowland course judged on purely physical grounds, and it is probably in the third of Butcher’s five zones, for Ranunculus fluitans is one of the commonest plants; but it is in the last of Carpenter’s four classes since coarse fish abound. Other changes are due to the confluence of the River Skerne, a large tributary, which is more calcareous and much more heavily polluted than the Tees. These three factors, different kind of bed, more calcium, and more sewage products, all influence the biology of the river below Croft. but it is not possible to measure exactly how great a part each one plays.

      Most of the bed of the river is of medium-sized stones and gravel but there are occasional patches of sand. In water shallower than five feet typical plants are the water crowfoot, Ranunculus fluitans, and various species of pondweed, Pota-mogeton. These plants can colonize the gravel and sand, and when they have formed a large patch they cause a stagnant area on the downstream side. Silt settles here and accumulates rapidly if there is heavy pollution upstream. It is colonized by such plants as Nitella, the stonewort, Elodea, the Canadian pondweed, and Potamogeton crispus, the curly pondweed.

      There was usually sufficient water in the Skerne to dilute the sewage it received to below the danger point, but in one summer there was a long hot dry spell as a result of which all the oxygen was used up, and the toxic products of decomposition without oxygen were liberated into the water. The Skerne itself had a thick coat of sewage fungus on its bed, and this organism extended for some distance down the Tees below the confluence of the two rivers. Its range varied widely according to the season of the year. In winter when, owing to the low temperature, the rate of decomposition of sewage is slow, it extended a long way downstream from the mouth of the Skerne, but in summer, when decomposition is more rapid, its range was less.

      A green filamentous alga, Cladophora glomerata, the Blanket Weed, abounds where nutrients are plentiful, as they are below a sewage outfall where the organic matter has undergone the initial stages of decomposition. It appeared in the Tees towards the end of May and grew rapidly to form a thick carpet in the shallow water a long way down the river from Croft. Then the first flood in July would usually sweep it all away, and it would be seen no more until the following year. If it lasted long enough, it trapped a deposit of silt and enabled rooted plants to grow in places where otherwise the flow was too fast. Before the estuary was reached the algal community typical of the upper, unpolluted reaches had become re-established on the stones.

      Nitrogenous compounds and other products of decomposition were brought into the Tees by the Skerne, and the calcium concentration was increased to some 30 parts per million. There was less oxygen in this stretch during the summer than there was farther upstream, and the lowest value was reached during the time when the development of Cladophora was at its height. The dense growth of this plant, respiring in the hours of darkness, used up much of the oxygen and reduced the concentration to between 50 and 60% of the saturation value. This is well above the point at which deleterious effects on fish are likely, and trout flourished in this, the last freshwater reach of the Tees, not uncommonly attaining a size of 1–1¹/₂ lb. (.45–.75 kg.). Coarse fish, chiefly dace and chub, were abundant, and fishing was a popular pastime.

      Fig. 12 Longitudinal section of the Tees estuary showing the salinity at high and low tide

      In the estuary, surveyed by Alexander, Southgate and Bas-sindale (1935), the most important natural phenomenon is the salinity. The fresh water tends to float on the sea-water and the result is a marked stratification. Figure 12 shows the average conditions at high tide and at low tide, but it gives rather a distorted picture because it is necessary to use such different scales. Horizontally an inch represents about three miles, but vertically it represents only about fifty feet. The surface current of fresh water draws up some water of higher salinity from below it, and to replace this there is an upstream creep of water of high salinity along the bottom. The whole mass moves up and down with the tide as shown in the figure. It is estimated that the mean time for all layers of a body of water to pass through the estuary is about six days in dry weather, decreasing to about two and a half under average winter conditions.

      The estuary has been much changed by the hand of man, and it must be admitted with regret that the Tees is typical rather than otherwise of larger British estuaries. From about midway nearly to the sea there is an extensive industrial conurbation. This requires a navigable channel so that its products may be removed by sea, and accordingly the natural tendency of the river to drop silt where it is checked by its meeting with the sea is counteracted by the continual activities of dredgers. The river is a convenient main drain and, at the time of the survey, the sewage from rather more than a quarter of a million people was discharged into it untreated. So were a variety of industrial waste products, of which the most important were tar acids and cyanides. Both these decompose gradually in the water.

      Much water is taken in to cool condensers and machinery, and this results in a slight rise in the temperature of the estuary. Oxygen, it need hardly be said, is not plentiful in solution in the water. The amount used up depends on the temperature and also on the salinity, being greatest at salinities of between 15 and 25 parts per thousand. The lowest concentration of dissolved oxygen recorded during the survey was 9% of saturation.

      The curly pondweed, Potamogeton crispus, the starwort, Callitriche stagnalis, and the two mosses, Fontinalis antipyretica and Eurhynchium rusciforme, which are abundant throughout almost the whole length of the freshwater part of the river, penetrate a little way into the brackish water. A few seaweeds penetrate a short distance from the sea but only four extend beyond the fringe of the brackish water region. Fucus vesiculosus, one of the brown bladder wracks, extends to beyond the middle point of the estuary, growing on wharves and piles between tidemarks; and three species of filamentous green algae occur throughout the brackish region.

      It is difficult to determine exactly which fish dwell permanently in the estuary, as so many of the species recorded are migrants passing through, or casual invaders, but the threespined stickleback appears to be a regular inhabitant, extending down to at least the upper reaches of the polluted part. The effect of the pollution on the fish, particularly the regular migrants, and on the lower animals is described in Chapter 14.

      Reviewing the River Tees in the light of the classifications put forward at the beginning of the chapter, we find that it includes all of Carpenter’s classes, for the lowest reach, immediately above the estuary, is dominated by coarse fish. On the other hand the last two classes, numbers 4 and 5, of Butcher’s botanical classification are not represented, for the current is nowhere so sluggish that the water crowfoot ceases to be the dominant plant.

      A contrast to the Tees is provided by the south country rivers rising in the chalk downs. Butcher has surveyed the plants of the Itchen, and there was a fisheries research station on the nearby Avon for several years before the war. Much of the gathering ground is chalk down. Rain falling on this sinks in and percolates relatively slowly so that it may not reach a hill-foot spring for months. The effect of heavy rain is, therefore, dissipated and it will not produce a marked flood wave as in the Tees. The other effect of the chalk is, of course, to render the water highly calcareous, and Butcher quotes a figure of 92 parts per million of calcium in the River Itchen.

      Then the slope is not so steep. Moon and Green (1940) give a profile of the Avon and show that between Christchurch, which is at the mouth, and Salisbury the fall is about 150 feet in 39 miles, which is a little less than 4 feet per mile (0.075%). The river rises some 20 miles from Salisbury at an altitude of about 350 feet, so this upper reach, for which we have not been able to find accurate data, is somewhat steeper, and the figure for the whole river will be greater, but still far below that of 30 feet per mile (0.57%) for the Tees.

      The springs giving rise to the Avon headwaters are usually at the foot of the chalk and often flow in wide valleys floored with gravel. Sometimes the streams have been broadened so that they flow over wide areas in which water-cress is cultivated. In dry weather the water-table often sinks below the surface of the gravel covering the impermeable stratum which is the true