3 The natural aquatic environment
3.1 Water, the medium of life
The list of necessities for the provision of life includes various nutrients and water: water is one of the basic resources needed for the process of photosynthesis. Since it is an excellent solvent, water, even in its 'natural' state, is never pure H2O but contains a variety of soluble inorganic and organic compounds. Water can also carry large amounts of insoluble material in suspension. The amounts and types of impurities vary with location and time of year, and determine some of the characteristics of a particular watercourse.
One of the most important determining factors is the presence of organic material in solution or in suspension. Organic material can be used as 'food' by the organisms living in natural water, provided the material is biodegradable. Biodegradable materials are those organic substances which can be decomposed by micro-organisms (usually bacteria and fungi) into inorganic substances. We have met the idea of food chains and webs and the cycling of nutrients. The basis of a trophic system in a river is the content of inorganic and organic materials in it, the biodegradation of these by decomposer organisms, and the products of the photosynthetic activities of the primary producers.
In a water environment, as on land, the primary producers (green plants and algae) are eaten by herbivores (primary consumers) and these in turn are devoured by the secondary consumers (carnivores). The interdependence of these organisms gives a complex food web within which there are many food chains, the successive links in the chains being composed of different species in a predator-prey relationship.
For a river a typical food chain could be
Scavengers eat bottom debris, including dead organisms. If the latter are not eaten immediately, the decomposers (bacteria and fungi) break them down, releasing nutrients which can be taken up by plants.
Through this cyclic movement of nutrients, the water environment achieves an ecological equilibrium. In a given stretch of water a balance occurs between total production of living material and the occurrence of death and decomposition over a period of time. The river neither becomes choked with living organisms nor devoid of them – although, depending on location and geological conditions, the numbers and varieties of the biota vary enormously. The maintenance of equilibrium is dependent on the complexity of biota and the interlinking of food chains and webs.
A typical river has several sources in high ground which are characterised by steep gradients, swift current velocities, and erosion of the surrounding rocks and soil. As the gradient lessens, the current velocity decreases and the river deepens and widens. The river then tends to deposit stones, gravel and sand. This variation in the flow downhill has a direct influence on the types of organisms and substratum to be found at different points along the river. The whole length of the river can be subdivided into different zones, each characterised by its own typical fauna and flora.
In contrast to rivers, standing bodies of water such as lakes and reservoirs may be affected by thermal stratification. Figure 7 illustrates this effect for a typical lake. In the summer, there is very little mixing between the cooler, denser water at the bottom of the lake (hypolimnion) and the warmer, less dense water at the lake surface (epilimnion). Thus, stream and river water running into the lake will tend to stay in the upper layer. This water carries nutrients, so organisms flourish in the epilimnion and there is a high rate of primary production. In the hypolimnion, the dead remains of primary production settle out, forming a layer of bottom sediment. The lack of mixing between the layers (stratification) together with the absence of light penetration to the bottom of the lake determine the ecological characteristics of a deep lake or reservoir. In a deep lake, the absence of light prevents the growth of plant life in the bottom layers, although decomposer and scavenger micro-organisms can live on, and in, the bottom sediment.
In contrast to summer conditions, Figure 7 shows that in winter, thermal stratification is absent. This is because the density of fresh water is greatest at 3.98°C (about 4°C). Thus, when the temperature of the surface layer falls to this temperature, the layer will descend to the bottom of the lake, displacing any colder (but less dense) water which will now rise to the surface. The lake 'turns over' and mixing occurs at all levels, leading to uniform temperature and uniform conditions throughout. This mixing process can bring partially decomposed bottom sediments to the surface, where further biodegradation can occur. This can also cause a significant deterioration in water quality.
Thus water carrying the organic and inorganic nutrients supports and maintains the aquatic ecosystem. Where there is very little biomass, the conditions are said to be oligotrophic (nutrient impoverished). This may occur when the physical and chemical characteristics of the land through which the water passes are such that nutrients are sparse or are not dissolved out of the soil and rocks. The opposite situation is eutrophication; this is the gradual increase with time of nutrients and biota in a body of water, eventually leading to parts of lakes (especially) and rivers becoming choked with plants.