|a discoloration of sea water caused by an explosive growth in phytoplankton density: sometimes toxic to fish life and, through accumulation in shellfish, to humans|
A population explosion of certain species of dinoflagellates, a kind of protozoan found in plankton. The dinoflagellates color the water red or reddish-brown and secrete a toxin that kills fish. Red tide usually occurs in warm coastal waters.
red tide definition