【正确答案】RED TIDES Different events can change the balance of sea life./ Red tides are an example./ These can happen in oceans, rivers or lakes, anywhere in the world./ What happens is that algae suddenly increase in numbers./ Algae are single-celled organisms./ They are a normal part of sea life./ As they spread, or bloom, the water may turn red or brown. / Sometimes the water does not change color at all./ But deadly algae could still be present./ Some kinds of algae produce a strong poison. / This can build up in shellfish that eat the algae /and make them poisonous, but not kill them. / Other kinds of algae may kill sea life by reducing oxygen levels in the water. / In some red tides, thousands of dead fish appear on beaches. / Scientists do not know exactly why red tides happen. / But they say a combination of conditions all play a part./ These include water temperature, nutrients in the water and water flow./ Pollution could also play a part./