What Causes Red Tides?

Red tide in Victoria, British Columbia.
Red tide in Victoria, British Columbia. / James R.D. Scott/Moment/Getty Images

Every once in a while, the ocean turns the color of blood and scores of dead fish rise to the surface. The phenomenon might look like a biblical plague, but the source is far more mundane. It’s just algae.

Red tides, also called harmful algal blooms, occur when there’s a sudden population boom among specific kinds of algae, which in enormous quantities become visible to the naked eye. Not all harmful algal blooms are red; some are blue, green, brown, or even purple. They occur all over the world. In the Gulf of Mexico, the culprit behind red tides washing onto coastlines from Texas to Florida is usually a type of microscopic algae called Karenia brevis. It produces a toxic compound that affects the central nervous system of fish, sea turtles, manatees, and dolphins, often with fatal results. In addition to generating the neurotoxin, K. brevis can become airborne and cause symptoms in humans like coughing, skin irritation, and watery eyes [PDF]. Not all algae blooms produce toxins that harm humans or animals, however.

Algae blooms may occur naturally or be helped along by runoff from farms and coastal development that flows into streams and rivers, which make their way into oceans. In particular, runoff from agricultural areas—which may include fertilizers and livestock poop—can contain abundant nutrients, such as nitrogen, that turbocharge the algae’s growth. Blooms can smother marine life below the ocean’s surface by blocking sunlight and by sucking up oxygen in the water, suffocating fish and other animals.

One of the worst red tides on record occurred in 2018, when a mass of algae lapped at Florida’s gulf coast for more than a year. Hotels in the popular tourist destinations of Captiva and Sanibel islands lost $8 million due to canceled reservations, while the bloom cost the state of Florida $14.5 million to clean beaches of dead wildlife.

A version of this story ran in 2018; it has been updated for 2023.