Terry Wendell, age 12, of Sarasota, Florida, for his question:

What causes red tides?

Every few years, the offshore waters of Florida and California turn reddish brown and soon the beaches are strewn with the bodies of dead fish. In the summer of 1971, these red tides infested both Florida and California. Scientists worked intensively, though they did not learn all they wanted to know. But they suspect that the causes of the fish slaughter may be somewhat different in the two areas.

The red tides in Florida were caused by a miniature organism with the scientific name of Gymnodinium breve. The California red tides were caused by a different organism named Gonyaulax polyhedra. Under certain conditions, these microscopic marine algae reach a fantastic population explosion. Their teeming cells tinge the water with muddy brown, brownish red and sometimes almost blood red.

In nature, population explosions always end in disaster for the exploding species, and usually for all other species in the environment. The teeming algae soon die in teeming numbers. These plant cells decay and the process of decay uses oxygen. For a long time, scientists supposed that fishes in the red tide waters died because their oxygen supply was depleted by masses of the decaying algae. This may grove to be part of the story. But researchers now have evidence that the red tide algae also produce poisons that are fatal to the fish populations.

The Florida algae release their deadly toxin while they are living. This toxic chemical has been isolated and studied in some detail. It is a light weight molecular substance that attacks the nerves and blocks their electrochemical impulses. 'Apparently this toxin also attacks the nervous systems of other backboned animals. Researchers hope to learn a lot more about it.

This summer, California researchers learned that their red tide algae do not release their toxin while they are living, but after they die. This toxin has not been isolated as precisely, but the evidence suggests that it is similar to that from the Florida algae. Apparently, it also attacks the nervous systems of the fishes.

The next step is to learn what factors start these population explosions in the red tide algae. At present, nobody knows the answers, though marine biologists have several suggestions. Whatever these causes are, they must he present only once in every two or three years    when red tides tend to occur.

It is suggested that water temperature may be a factor. Some researchers are investigating the possible role of various chemicals dumped into the offshore waters by streams and rivers. Some of these might be man made pollutants though, at present, most researchers think this is unlikely. Pollution is dumped into the ocean every day and red tides occur every two or three years, as they did in the past.