Did you know that elevated concentrations of nutrient compounds have a number of effects on natural waters? Once in a stream, bacteria will vigorously consume any dissolved oxygen they encounter in order to metabolize these compounds; where the concentrations are high enough, they can swiftly rob a stream of its entire oxygen content, thus effectively smothering fish and invertebrates. It is for this reason that the tendency of sewage waters to consume oxygen is a key measure of how polluted they are.
Waters which have been stripped of their dissolved oxygen are often termed ‘anoxic’. The absence of grazing aquatic animals in such waters turns them into ideal habitats for certain types of algae (the so-called blue-green algae) which tend to form unsightly scums on the water surface, and which can emit carbon compounds that are highly toxic to mammals.
Pathogens and nutrients from humans and farm animals are not the only important sources of pollution: many industries have the potential to release toxic metals (e.g. the mining and metal-finishing industries) and/ or synthetic organic compounds such as pesticides and fuel residues, many of which are known or suspected to cause cancer in mammals.
Ecology and hydroecology
Ecology is the science of the interactions between different organisms, and between organisms and the environment. Hydroecology specifically focuses on the relationships between organisms and the water environment. The first question to ask is: what controls does water exert on animals, beyond the obvious ones of supplying drinking water and washing away waste. Perhaps surprisingly, the key control is on oxygen supply.
Coming up for air: controls on aquatic life
Humans have a justifiable fear of drowning: in common with other mammals, we cannot breathe under water, and will quickly drown if we try to. It is easy to forget that most other marine animals are just as dependent on oxygen; however, they receive all they need by extracting dissolved oxygen from the water. As such, they are indirectly dependent on the atmosphere, too, as this is the source of all of the oxygen which dissolves in sea and fresh water. The amount of oxygen available in water is actually quite limited: usually less than 15 milligrams per litre. The gills of fish and aquatic invertebrates are amazingly efficient at extracting this oxygen. So, indeed, are the root tissues of aquatic plants.
As long as a water body is in good contact with the atmosphere, the principal controls on oxygen availability are water temperature – oxygen is more soluble in cool water than in warm – and the rate at which it is consumed by biochemical processes. Thus excessively dense populations of fish can lead to anoxic conditions. This can be a problem, for instance, in the vicinity of fish farms.
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