ometimes growing our food makes the seas turn red - or pink, violet, orange, yellow, green or brown. These episodes are called 'red tides', as this is the commonest colour. They happen when marine populations of algae explode: each may replicate itself a million times in just two to three weeks, until they cover the surface of the sea. They often devastate tourism. Some of the algae are toxic and can poison people when they eat shellfish.

ed tides can happen naturally, but increasingly they are stimulated by pollution by fertilizers used on farmlands and washed down to the sea. This is one aspect of eutrophication, when nutrients from fertilizers and sewage cause excessive growth of water plants. It can turn parts of the sea into waste areas. A dead zone appears off the coast of Louisiana each year thanks to pollution from the Mississippi, and fisheries in the Black Sea have been badly hit. The fertilizer can make seaweed grow so much that it smothers coral reefs. And the life of rivers and streams can also be damaged.

utting the pollution pays. A study in the Balkans showed that the cost of reducing pollution by half was more than recovered through improving beaches alone.

Photo: Khan Kuyucu/UNEP/Topham

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