Red tide is a term used for brownish or reddish areas of ocean, river, or lake water. The color comes from the presence of millions of microscopic organisms in the water. The discolored areas may range from less than a few square yards or meters to more than 1,000 square miles (2,600 square kilometers). They may last from a few hours to several months. Red tides appear in waters in most parts of the world. In the United States, they are often seen off the coasts of Florida, Texas, and southern California.
Many red tides are harmless. But some kill fish and other water animals, which then may float on the water or wash ashore in great numbers. The decaying bodies cause an unpleasant odor. Still other red tides do not kill sea life, but they make the shellfish that feed on them poisonous to eat. Harmful red tides are caused by several species of dinoflagellates (one-celled organisms). Some dinoflagellates produce a poison that paralyzes and kills fish. Dinoflagellates may also kill fish during red tides by using up nearly all the oxygen in the water.
Scientists do not fully understand why the dinoflagellate population suddenly increases, causing red tides. It is known that dinoflagellates accumulate when the nutrients, temperature, amount of sunlight, water currents, and other conditions in the water suit their needs. Red tides may decrease when other sea organisms eat the dinoflagellates.
