What Is a Red Tide?

A red tide is a colloquial term for a harmful algal bloom (HAB) in which certain species of microscopic algae — primarily dinoflagellates like Karenia brevis — multiply rapidly, sometimes coloring the water red, brown, or green. These blooms produce potent neurotoxins called brevetoxins that can kill fish, marine mammals, sea turtles, and birds in massive numbers. Humans can be affected through eating contaminated shellfish (causing paralytic or neurotoxic shellfish poisoning) or by breathing in aerosolized toxins near the coast, which irritates the respiratory system. Red tides occur worldwide but are particularly common along the Gulf Coast of the United States, where blooms can stretch for hundreds of kilometers and last for months.