I’m Phoebe Zimmermann with the VOA Special English Agriculture Report.

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.

A number of different algae can cause red tides. A common form has the scientific name Karenia brevis. It is often linked with red tides in the Gulf of Mexico and the Caribbean area.

Algae poisons build up in the tissue of shellfish like clams, oysters and mussels. This makes them unsafe for several weeks after a red tide goes away.

People are also advised not to eat the organs of fish or shellfish like shrimp, crab or lobster. Scientist Richard Pierce says the poison produced by K. brevis does enter the meat of these creatures. But he adds that there have been no reports of people getting sick from eating healthy fish during such a red tide. Mister Pierce is director of the Center for Eco-toxicology at the Mote Marine Laboratory in Florida.

There are different kinds of shellfish poisoning, some more severe than others. Effects can include diarrhea, fever, stomach and muscle pain, and breathing problems. In severe cases people can die unless they receive treatment.

Experts say older people are especially likely to experience severe effects from algae poisons.

Some people who swim during a red tide report skin problems or shortness of breath. Scientists have more to learn about the possible risks of swimming in red tides or breathing algae poisons in the air.

This VOA Special English Agriculture Report was written by Mario Ritter. I'm Phoebe Zimmermann.