World's largest sargassum bloom, why it's probably here to stay

Florida researchers have discovered the world’s largest sargassum forest in the Atlantic Ocean, a sweeping bloom of macroalgae gorging on natural and manmade nutrients that has waged a suffocating assault on Sunshine State beaches the past two years.

The thriving pelagic flora, which stretches from the west coast of Africa to the Gulf of Mexico, was measured using 19 years of NASA satellite images and detailed in a study published this month in Science magazine.

University of South Florida and Florida Atlantic University scientists found the seaweed’s growth spurts occur in years when runoff from the Amazon River includes large amounts of fertilizer and when an upwelling in the eastern Atlantic brings cooler water and nutrients from the bottom of the ocean to the surface.

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For beachgoers the bloom can be a smelly nuisance, but at least one Palm Beach County fisherman noted another characteristic to the floating wrack line that has piqued the interest of an FAU researcher — there are fewer critters in the weed.

“Just one little clump used to have tons of little creatures, crabs and shrimp, and we’d be sure to throw it back in the water right away if it got on the boat,” said Bill Taylor, who runs Black Dog Fishing Carters at the Jupiter Inlet. “So we were excited to see it when it came back, but it’s upsetting because there is no life in it.”

Brian LaPointe, a co-author on the recent seaweed study and researcher at FAU’s Harbor Branch Oceanographic Institute, said the lack of the typical marine animals in the sargassum has become a common observation.

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He said there are several factors that could contribute to fewer animals in the seaweed, including that its growth has simply outpaced that of the organisms that call it home, spreading the critters out across larger areas.

“But there are more serious concerns,” he said. “In this unique community, everything is in balance, but when you get too much biomass, things change.”

LaPointe said higher nitrogen to phosphorus ratios and reductions in oxygen levels at night when no photosynthesis occurs could also have negative effects on life in the floating rainforest. LaPointe said similar discharges from the Mississippi River can feed sargassum blooms in the Gulf of Mexico.

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“It can alter food chains,” he said. “I’m kind of looking at sargassum as another canary in the coal mine for our planet.”

LaPointe’s co-authors are Chuanmin Hu, a USF oceanography professor who monitors sargassum with the school's Optical Oceanography Lap, and postdoctoral student Mengqiu Wang.

They’ve dubbed their discovery the “Great Atlantic Sargassum belt.”

Last year, the bloom contained more than 20 million tons of seaweed.

While the study points to two main culprits for the increase in sargassum — higher nutrient levels from Amazon runoff and upwelling — there are more pieces to the puzzle. Too much runoff from the Amazon can dilute salinity levels in the ocean, causing fewer sargassum blooms. Warmer ocean temperatures have also shown to “suppress sargassum growth.”

“This is all ultimately related to climate change because it affects precipitation and ocean circulation and even human activities, but what we’ve shown is that these blooms do not occur because of increased water temperature,” Hu said in a USF article. “They are probably here to stay.”

Taylor said the seaweed doesn’t hurt his fishing business, but trollers who draw lines through the water are getting tangled in it.

“I want to protect this little piece of the world here,” Taylor said. “The most important resource we have is our water.”

Kmiller@pbpost.com

@KmillerWeather