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Scientists Don’t Think Sargassum Will Impact Regional Shores

March 22, 2023

By Isabella McDonnell, SCCF Marine Laboratory Research Assistant

As Southwest Florida continues to endure the aftermath of Hurricane Ian and the effects of red tide, another potential issue has entered the news cycle. Reports of a large patch of Sargassum algae forming in the Atlantic create new worries. It is similar to red tide in that it can cause negative health effects and dissuade tourism. 

The formation of large patches is a relatively new issue, observed and studied since 2011. Small islands of floating algae have been observed offshore for centuries, but have recently increased over a hundred-fold. The unusually large blooms can now be monitored via satellite. Currently, this year’s bloom is twice as wide as the United States. Last year, a record 22 million metric tons of Sargassum coated the Atlantic.

As of now, SCCF Marine Lab scientists believe it is unlikely that the seaweed will affect the Southwest Florida coast. The east coast of Florida is very likely to be impacted due to the Gulf Stream acting as a conveyor belt, running right next to the coast. The algae has already begun washing ashore in Key West, and more is expected in the Keys before the end of the month.

As seen in this image, the Loop Current is far from Sanibel and Captiva. It would take many days of a strong west wind for clumps to reach the islands. Satellite images of the bloom can be found on the University of South Florida’s Optical Oceanography Laboratory’s webpage

Gas bladders, called pneumatocysts, cause the algae to float on the surface. Offshore, the algae provide important habitat for many animals including juvenile sea turtles. Many creatures find temporary and permanent homes within the leaves. It serves as a nursery for young species, a resting place for seabirds, and a lifelong habitat for a variety of fish species.  

Nearshore, the algae can have a negative effect on coral reefs and seagrass beds by shading the seafloor from sunlight. When the algae washes ashore, there are few solutions.  Beaches can be covered in 5-6 feet of rotting algae. Using machinery to remove the algae can be fatal to sea turtle and shorebird nests and can worsen shoreline erosion. 

When the algae decomposes without oxygen, or anaerobically, this can lead to the release of hydrogen sulfide gas, which can smell like rotten eggs. Should you encounter the algae, walking through or touching the algae is not recommended.

High levels of nitrate, phosphate, and ammonium are tied to the growth of Sargassum, red tide, and other harmful algae blooms. Researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute found that nitrogen levels had gone up by 45% in the area where sargassum forms since the 1980s.

Runoff from river basins is believed to be a major contributor to the blooms. Even those far from the coast are urged to limit fertilizer use in order to stop these issues from continuing to worsen. 


Sargassum beaching event on Martinique in 2015. 

Photo courtesy of Jean-Philippe Maréchal via USF’s Optical Oceanography Laboratory website.


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