New England red tide dinoflagellate, Alexandrium tamarense . Most red tides are harmless, but a few species of phytoplankton cause red tides that are poisonous to marine animals and to humans. Of the more than 60 different species of phytoplankton that cause red tides, only four or five have been identified as toxic. Red tides are not new.
Red tides occur on both sides of the Atlantic, off Florida, and along the Pacific coast into Alaska.
The organism that causes toxic red tide in New England is a microscopic one-celled alga called Alexandrium tamarense . Alexandrium propels itself through the water using two tiny whip-like extensions called flagella. Its life cycle includes a dormant cyst stage that can survive cold winters in bottom sediments. The cysts, which also contain toxin, are the seeds for future blooms. These cysts facilitate the spread of toxic red tides into new areas since they are easily transported by tidal currents, dredge material disposal, and transplanted shellfish.