Red tide is the common name for phenomenon created when toxic algal blooms turn seawater red, killing marine life and making water unsuitable for human or animal use. Red tides are caused by several species of dinoflagellates and diatoms, microscopic unicellular phytoplankton that live in cold and warm seas. A red pigment, called peridinin, which collects light during photosynthesis, colors the water red when large numbers of the plankton populate an area.

Red tides are a danger because the toxins released by large numbers of these plankton can paralyze fish and bioaccumulate in the tissues of shellfish and filter-feeding mollusks. Predators of the shellfish, including humans, consume the toxins causing paralysis and death.

Species of dinoflagellates that cause red tides generally belong to the "red tide genera," Gymnodinium and Gonyaulax. Though several are known to cause red tides, a single red tide is nearly always tied...