Dense aquatic accumulation of microscopic organisms produced by an abundance of nutrients in surface water coupled with adequate sunlight for photosynthesis .
The microorganisms or the toxic substances they release may discolour the water, exhaust its oxygen content, poison aquatic animals and waterfowl, and irritate the skin and respiratory tract of humans. Single species of algae , diatom s, or dinoflagellate s, reproducing every few hours, may dominate a bloom's population; the number of individuals per quart (litre) of water, normally about 1,000, can increase to 60 million. Blooms of the dinoflagellate genus Gymnodinium cause red tide s. The Red Sea is named for the occasional blooms of the alga Trichodesmium erythraeum . See also water pollution .