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'Dead zones' and red tides

Some 50 intermittent dead zones have now been created in the world's oceans. One of the worst affected areas is the Gulf of Mexico, heavily polluted by nutrients and other chemicals brought down by the Mississippi River.The dead zone in the Gulf covered an estimated two million hectares in July 1999. Some 57 percent of the shellfish-growing area in the Gulf has been closed as a result of associated health risks.

'Red tides' of highly coloured algae, often poisonous as well as oxygen-depleting, have become a major problem in such countries as Australia, China and the Philippines. Whether the nitrogen that creates them comes from human-based activities still has to be proved, but the

Most of the sewage in developing countries is discharged directly into local rivers or the sea without treatment. Such pollution leads to algae blooms and poses a threat to human health, leading to gastro-enteritis and other diseases. The pollution of beaches by sewage is an aesthetic rather than health problem. But it can also deprive countries of substantial tourist revenue.