听力原文： RED TIDES Different events can change the balance of sea life./Red fides 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 sit. e-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 fides, thonsands of dead fish appear on beaches./ Scientists do not know exactly why red tides happen./But they say a combination of conditions all play apart./These include water temperature, nutrients in the water and water flow./Pollution could also play a part/ (1)