What Is a Brown Tide? Scientists Fear Florida Red Tide Could Merge with new Outbreak to Create Super Bloom

The annual outbreak of algae called a brown tide has begun off the coast of western Florida and scientists are hoping it doesn’t meet up with the red tide that they’re already facing.

The brown tide near Florida is made of Trichodesmium, a cyanobacterium, or blue-green algae. The bloom occurs every year in the Gulf of Mexico and has been known to extend so far that’s it’s visible from space. 

Records of the brown tide date back to the 1700s, according to the Florida Fish and Wildlife Conservation Commission. Captain James Cook of the British Royal Navy first wrote about large, brown blooms in the water that looked like sandbars.

Since the algae can form colonies that are big enough to be seen by the naked eye, sailors sometimes refer to it as “sea sawdust”. The tide is brown when it’s healthy, but when it begins to decay it turns green, then pink or red, then white. The toxins also smell sweet when they are decaying, like freshly cut hay. 


While some strains of Trichodesmium produce toxins, scientists haven’t found that those toxins can harm animals in the water or people. And Trichodesmium is not usually a source of food for other organisms living in the water.

But the brown tide feeds off iron in the air, blown to the Gulf of Mexico from the Sahara Desert. The algae has the unique ability to fix nitrogen. And the red tide, Karenia brevis, which already surrounds 130 miles of Florida coast, can feed from it, in turn, to fuel further spread. 

Red Tide Dead Fish Fish are seen washed ashore the Sanibel causeway after dying in a red tide on August 1, 2018 in Sanibel, Florida. The red tide could potentially use the brown tide as a new food source. JOE RAEDLE/GETTY IMAGES

As of Friday, August 17, Manatee County, Florida, had collected 150 tons of dead fish for disposal over the previous two weeks, Manatee County Information Outreach Manager Nick Azzara told Newsweek. Right now, the Florida Fish and Wildlife Conservation Commission is reporting that the outbreak is in offshore waters, so scientists are hoping the two tides don’t end up combining.

“As the Trichodesmium bloom dies and degrades, Karenia can potentially use the nutrients that are released,” Vincent Lovko, a marine scientist told the Bradenton Herald, while noting it’s not necessarily a merge. The red tide would use the brown tide as a food source, potentially extending its life.

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