Red tide is a term used for a brownish or reddish discoloration occurring in ocean, river, or lake water. It is caused by the rapid reproduction of a variety of toxic organisms, especially the toxic red dinoflagellates, which are members of the genera Gymnodidium and Gonyaulax. Some red tides are harmless, but millions of fish may be killed during a “bloom,” as the buildup is called. Other red tides can poison shellfish and the birds or humans who eat the contaminated food. Scientists do not fully understand why the “bloom” occurs.