Red tide is a term used to describe the brownish or reddish coloration of waters that sometimes occurs in oceans, rivers, or lakes. It is caused by algal blooms—the rapid reproduction of a variety of toxic, single-celled organisms. These organisms include dinoflagellates and diatoms, mostly of the genera (plural form of genus, a category or classification made up of species sharing similar characteristics) Gymnodidium and Gonyaulax, which contain a red pigment called peridinin.
In some cases red tides are harmless. However, during intense algal blooms, dangerous levels of toxins are released into the water. These toxins kill fish (millions of fish may die in a red tide) and accumulate in the fatty tissues of shellfish. Humans and other animals that eat contaminated shellfish may become paralyzed or even die.
Scientists do not fully understand why red tides occur. However, there seems to be a connection between the warm waters of the El Nino current and algal blooms.
Sources: Ashworth, William. The Encyclopedia of Environmental Studies, pp. 324-25; Cunningham, William P., et al. Environmental Encyclopedia, pp. 689-90; Science Digest, vol. 75 (February 1974), pp. 64-69.