Warm effluents discharged into bays and estuaries may raise the water temperature. This reduces the solubility of oxygen, and the oxygenation may be further reduced by increased oxygen consumption by animals and bacteria, and by reduced vertical mixing due to thermal stratification. The effect may be that the underlying layers become deoxygenated and foul. Migratory fish such as salmon may be discouraged from passing through the area. Warm water may favour pests such as shipworm and gribble, accelerating their growth and extending their breeding seasons, and it may also encourage the establishment of foreign pests. The fouling rate on ships' hulls is likely to be raised.
There are also some possible benefits. Growth rates of useful species such as edible bivalves may be improved and the area may be colonized by useful foreign species e.g. Mercenaria mercenaria in Southampton water. Use can also be made of the warm water from power stations to rear certain fish such as eels (see Section 9.6.3).
Of all the many types of pollution to which the oceans are subject, oil is the one to have caught the public's attention the most. This was the first type of large-scale marine pollution to become apparent to the public as beaches became oiled when ships sank during World War II.
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