What is an algal bloom that occurs in salt water called?

An algal bloom that occurs in salt water is commonly called a red tide. The name refers to dense concentrations of microscopic marine organisms, especially dinoflagellates, that can discolor seawater reddish-brown, although blooms may also appear green, yellow, or even remain visually unnoticed.

The term is best known from recurring blooms along Florida’s Gulf Coast, where Karenia brevis produces brevetoxins that can harm fish, marine mammals, birds, and people who consume contaminated shellfish or inhale sea spray. Other regions have different organisms and toxins, including Alexandrium in the Gulf of Maine and Pseudo-nitzschia along parts of the California coast.

“Red tide” is a familiar but imprecise expression. The phenomenon is not caused by ocean tides, and not every bloom is red or toxic. Scientists increasingly use “harmful algal bloom,” or HAB, when a bloom produces toxins or causes ecological damage. Some marine blooms are harmless, while others kill wildlife by removing oxygen from the water as the organisms decompose.