The dangers of a red tide

By Jennifer Gray and Rachel Clarke

Published August 8, 2018

Harmful algae blooms have been reported off all parts of the US coastline and were first observed in the 1700s. Florida experiences some of the worst, known as “red tides.”

Image: NOAA/Kai Schumann

A massive red tide has developed off Florida, killing thousands of marine animals.

People can also be sickened.

Hospital records along the west coast of Florida show a 50% increase in people going to the hospital for various types of respiratory distress and a 40% increase for gastrointestinal distress. People should not swim in the water, eat seafood from it, or breathe the air near it.

Larry Brand

Professor of marine biology and ecology at the University of Miami


Sunlight, temperature and nutrients can cause colonies of tiny algae that live in the sea to grow suddenly. Modern use of pesticides and chemicals that wash into the oceans provides more nutrients for the blooms to grow.

Image: NOAA

Florida red tides are caused by algae called Karenia brevis, which live primarily in the Gulf of Mexico.

Image: FWC Fish and Wildlife Research Institute

When the algae die, they release toxins.

Fish and other marine life ingest the toxins, which can kill the animals. Humans can become ill by eating shellfish containing the toxins.

It has killed fish, sea turtles, sea birds, a shark, dolphins and multiple manatees.

  • Onshore breeze can blow the toxins to land, causing coughs and breathing difficulties.

  • The tainted air can also trigger asthma attacks.

  • The massive amounts of algae can tinge the water a reddish brown, though it’s not always visible.

Image: NOAA/Kai Schumann

While red tides aren’t fully understood, scientists are working hard to determine what can trigger them. It is believed that increased nutrients in the water, pollution, water flow modifications and climate change all play a role.


Image: Chase Fountain, Texas Parks & Wildlife

There is no way to eliminate a red tide. While it creates a temporary dead zone, there is anecdotal evidence that a red tide can be beneficial to the marine ecosystem in the longer term.

Blooms can last from a few weeks to over a year, depending on currents, winds, temperature and other factors.

Image: Joe Raedle/Getty Images

The red tide off the west coast of Florida has already been going for longer than nine months, the most persistent in the Gulf of Mexico for more than a decade.