This section describes some of the chemical and physical properties of still and flowing waters. These help determine the shape and nature of the lake, pond or waterway, and affect the type of plant and animal life which it supports.
Oxygen and carbon dioxide
One of the problems faced by aquatic life is that of exchange of gases. Not only is diffusion of gases in solution slower than in air, but the amount of oxygen which can be dissolved in a given volume of water is much smaller than in the same volume of air. The balance of oxygen and carbon dioxide can therefore rapidly become critical. Oxygen is added to the water by surface aeration and by plant photosynthesis during the day. It is depleted by animal and plant respiration and by the oxidation of decaying organic remains.
In moving water, circulation continually brings water from the depths to the surface allowing exchange of gases. By contrast, exchange at the surface of a still pond is very limited. During the night, a thickly vegetated pond may become almost depleted of oxygen, which only slowly builds up during the day. The effect is worsened in warm weather, as warm water contains less dissolved oxygen than the same volume of cool water. Deoxygenation of the lower layers of a deep lake may kill off all but the anaerobic bacteria. If this bottom layer is disturbed, toxic byproducts of anaerobic decomposition may actually pollute the upper layers.
pH
pH is a measure of the acidity or alkalinity of water and relates to the concentration of hydrogen ions. pH 7.0 is neutral, below 7.0 being acid and above being alkaline. Aquatic organisms, especially animals, are generally adapted to a very restricted range of pH values. Changes due to pollution or to the release of acids by decomposition of organic remains cause shifts in the species balance. The presence or absence of certain invertebrates can be used to estimate the pH and pollution level of water bodies.
Light
Plants depend on sunlight for photosynthesis and food production, and are therefore found near the surface.
Animals, not depending directly on sunlight, can live at lower depths or in the bottom ooze. Light penetration is usually limited by staining (due to dissolved matter), turbidity (due to suspended matter), and shading by floating vegetation.
Thermal stratification
Still waters become thermally stratified because the sun’s heat, absorbed near the surface, cannot penetrate the depths. Shallow ponds may stratify whenever the day is warm, calm and sunny but become uniform again at night as the exposed surface layers cool and sink to mix with the deeper layers.
In deep lakes, overturn is seasonal rather than daily. Starting in spring, a sharp distinction develops between the upper, warm ‘epilimnion’ and the lower cold ‘hypolimnion’. Between them is a narrow transition zone, the ‘thermocline’, in which temperature drops rapidly with depth. During the summer the layers do not mix, and the difference between their respective temperatures and content becomes more marked. The hypolimnion receives no oxygen, but only the organic debris showering down from the epilimnion. The epilimnion in turn receives none of the products of decay, and by the end of the summer its nutrients are exhausted and vegetation declines. Stratification continues until autumn, when the surface gradually cools to bring the epilimnion and hypolimnion to about the same temperature and allow storm induced eddies to mix them again.
Water current
Streams and rivers exhibit two types of flow. ‘Laminar ’ flow is smooth and occurs in a thin band along the beds and banks where the water and channel are in constant, steady friction. This causes the laminar flow to slow relative to the central core of moving water. ‘Turbulent’ flow occurs elsewhere, due to irregularities in the bed or banks which cause the water to eddy. Except in very small or artificial channels, this is the dominant force. The velocity along different lengths varies with the channel’s slope, relative width and height, and also over time, according to the volume carried.
A natural watercourse is adapted not to its average or mean flow but to its volume when bank-full, which may occur only a few days a year. Similarly, the bed material may reflect extreme rather than average conditions. For example, a stream strewn with large boulders is clearly moulded by torrential spates rather than by average flow or volume. The nature of a stream bed depends largely on the velocity of the water flowing over it, and not on the parent material in which it is formed.
Wave action
Much of the specific shape and character of the margins of open water bodies, particularly of large lakes, is determined by the varying effects of wind on different shoreline materials and on the orientation of storm winds to the shore.
If the shore is rock, waves break up the weaker types of rock and leave harder material projecting as ridges, but the total effect is small. If the shore is soft sand or peat, waves quickly erode the edge, perhaps into distinct cliffs. If the edge is glacial drift or till, as in the moraine-enclosed lakes of the North, waves carry away finer particles but leave larger stones and boulders in place. The disturbed materials grade from coarse near the shore to fine in deeper water, and are often redeposited in a platform, the shoreward edge of which is steepened as it is cut by the waves. Where the material is carried along the shore by wave generated currents, rather than directly out from it, a spit forms as the material settles. This may create a sheltered inlet important to plants and animals which cannot survive the more exposed positions. In such places the process of eutrophication and gradual filling-in of the lake can most easily be observed. The relative stability of many intermediate shorelines is quickly destroyed by an increase in wave action. This is why motorboats, especially when towing water skiers, may cause serious erosion problems on canals and much-used lakes and Broads.
Silting
Deltas form where inflow streams drop sediments into still water. Provided they are not too disturbed by currents, the sediments fan out and become graded from coarse material near the main inflow channel to finer particles in deeper water and to either side. Once raised above the water, deltas develop a wetland or terrestrial vegetation appropriate to alluvial soils.
Spate streams may quickly fill in small ponds with deposits brought down in flood. These may form deltas which progressively extend across the open water. To preserve such ponds it is necessary to deflect the current against reinforced embankments and to encourage vegetation as a natural silt trap. Periodic digging-back is also essential.
