Fresh Water Ecology
Uploaded by ihatesuchin on Jul 05, 2004
The nature of all aquatic environments depends upon the following factors which exert their effects in varying degree:
i) The depth of water
Has an important secondary effect by reducing the penetration of light and hence the degree of colonization by plants.
ii) Light
Water absorbs light of long wavelengths, particularly the blue part of the visible spectrum. Since this would not normally be sufficient for photosynthesis, many freshwater plants possess adaptations so that light does not become a limiting factor in their growth.
iii) Substrate
Streams that carry a large amount of sediment in suspension (e.g. due to a muddy stream bed) may have a decreased amount of light available to submerged aquatics. A muddy stream bed may also pose problems to the attachment or respiration of animals, and can lead to a build up of semi decomposed organic matter. This can lead to a high biological oxygen demand (BOD) and the water becoming anoxic, killing off most life.
iv) Temperature
Water possesses certain thermal properties which make it a unique environment. In particular, it has a high specific heat, high latent heat of fusion and the highest known latent heat of evaporation. The combination of these properties explains why temperature variations occurring in water are less than those on land. Moreover, when they occur, they take place far more slowly. Most aquatic organisms are well able to tolerate the normal range of temperature variations that occur throughout the year and are known as eurythermous. Species with a narrow temperature tolerance (stenothermous) tend to be restricted in their distribution to particular habitats such as the sources of mountain streams, where the temperature of the water changes little throughout the year.
Another important physical feature of water is that it achieves its maxiumum density at 4°C. Above and below this temperature it expands and therefore becomes lighter. This accounts for the fact that deeper waters never freeze solid in cold weather - an important consideration for the animals that live there.
v) Availability of Oxygen
Freshwater contains significantly less oxygen than air, causing aquatic animals to need to move much more of the surrounding medium than land organisms, to extract the equivalent oxygen. The availability of oxygen relates closely to the development of gills and other respiratory organs. Some burrowers in mud, such as the lavae of some Diptera make use of respiratory pigments like haemoglobin which aid oxygen uptake, indicating the degree of adaptation to anaerobic conditions.
Fast flowing water, because...