Review the effect of acid rain on the local distributions of fish and aquatic invertebrates. What are the biological reasons for the disappearance of these animals as pH falls?.
Humans produce about 160% of the natural emissions of sulphur. This is compared with about 5-10% in the case of carbon dioxide and nitrogen (Krebs, 2001). Combustion of fossil fuels, according to Krebs (2001) has altered the sulphur cycle more than any of the other nutrient cycles'.
Sulphur is quickly oxidised in the atmosphere to sulphate (SO4) and redeposited rapidly on land or in the oceans. One clear manifestation of this alteration of the sulphur cycle is the widespread problem of acid rain (Krebs, 2001).
In areas uncontaminated by either industrial emissions or calcareous dust, precipitation usually has a pH value close to 5.0 (Schindler, 1998). Distilled water, which contains no carbon dioxide, has a neutral pH of 7. Liquids with a pH less than 7.0 are acid, and those with a pH greater than 70 are alkaline (USGS). Uncontaminated rainfall usually has a slightly acidic pH because it contains small amounts of both weak and strong acids of natural origin. In most areas within several hundred kilometres of industrial emissions precipitation has a pH value of much less than 5.0 (Schindler, 1998). Around Washington D.C. for example, average rain pH is between 4.2 and 4.4 (USGS).
Schindler (1998) says; it is now clear that acid rain has already caused widespread acidification of many aquatic ecosystems in the north-eastern United States, Canada, Norway, Sweden, and the United Kingdom'. Many fish invertebrates are sensitive to acidification, with some disappearing at pH values as high as 6.0 (Schindler, 1998).
Aquatic environments vary widely in their sensitivity to acidification. When water moves through limestone or dolomite rocks and soils, it picks up calcium and magnesium carbonates. These minerals neutralise incoming acids.