Red tides are caused by a population explosion of microorganisms called dinoflagellates. The seawater becomes toxic to most life, and kills fish in the area. AP/Wide World. Reproduced by permission.

Red tide is a condition in which a huge area of seawater turns to a reddish-brown hue. This rusty-red discoloration is caused by an exploding population of tiny single-celled microorganisms called dinoflagellates, which are usually found in ocean water, but occasionally in lakes and rivers as well. Red tides have occurred naturally since oceans were formed, but today they are becoming more common because of human influence. During summer months, the warm Sun and an abundance of food in the water create optimal conditions for the breeding of dinoflagellates, which are a type of phytoplankton. This multiplication, or bloom, happens rapidly, and the seawater becomes extremely dense with dinoflagellates; sometimes their numbers can reach many...