An algal bloom is a rapid increase or accumulation in the population ofalgae (typically microscopic) in a water system. Cyanobacteria blooms are often called blue-green algae. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number ofphytoplankton species are involved, and some blooms may be recognized by discoloration of the water resulting from the high density of pigmented cells.

(From: Wikipedia, July 2015)