Acidity. Continuing increase in soil acidity reduces vital mycorrhizal activity, causes leaching of nutrients such as magnesium and calcium, and leaves phosphate in an insoluble form. In soils formed over limestone and chalk, the effects of acid rain are much less damaging.
Excess nutrient levels in water and soils (especially from fertilizers and farm silage) encourage increase in algae and a corresponding loss of dissolved oxygen. This process (called eutrophication) has a serious effect on plant and animal survival. It is seen most strikingly when fish in rivers and lakes are killed in this way.
Heavy metals may be released into the air or into rivers as by-products of chemical industries and the burning of fossil fuels. Cadmium, lead and mercury are three commonly discharged elements. While plants are more tolerant of these substances than animals, there is a slow increase within the plant cells, and more importantly the levels of chemicals increase dramatically as the plants are eaten and the chemicals move up the food chains (see also DDT p58).
Pesticides. Recent legislation has led to a greater awareness of pesticide effects on the environment. However, herbicide and insecticide leaching through sandy soils into watercourses continues to be a threat if application of the chemicals occurs near watercourses. A herbicide such as MCPA can kill algae, aquatic plants and fish.
A high water table can have a marked effect on a habitat if the effect is prolonged. The anaerobic conditions produced often lead to root death in all but the aquatic species present in the plant community.
There is constant monitoring by Government agencies for the factors mentioned above. This is especially so in National Parks, National Nature Reserves (NNRs), and Sites of Special Scientific Interest (SSSIs). Environmental scientists and laboratories have a range of techniques for assessing levels of these factors. Chemical tests for common nutrient substances and for pH can be performed in the field. More sophisticated analysis is required for heavy metals and pesticides. A common five day test for water quality called 'Biological Oxygen Demand' (BOD) enables a confident assessment of a water sample's pollution level. An unpolluted sample would register at about 3 mg of oxygen per unit volume per day whereas a sample polluted by fertilizers could be about 50 mg of oxygen per unit volume per day.
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