From AMS Glossary
The term applied to toxic algal blooms caused by several genera of dinoflagellates (Gymnodinium and Gonyaulax) that turn the sea red and are frequently associated with a deterioration in water quality.
The color occurs as a result of the reaction of a red pigment, peridinin, to light during photosynthesis. These toxic algal blooms pose a serious threat to marine life and are potentially harmful to humans. The term has no connection with astronomical tides. However, its association with the word "tide" is from popular observations of its movements with tidal currents in estuaries.