By Meghan Bartels, Scientific America on March 17, 2023
A loose raft of brown seaweed spanning about twice the width of the U.S. is inching across the Caribbean. Currently, bucketloads of the buoyant algae are washing up on beaches on the eastern coast of Florida earlier in the year than usual, raising scientists’ concerns for what the coming months will bring.
The seaweed is made up of algal species in the genus Sargassum. These species grow as a mat of glops of algae that stay afloat via little air-filled sacs attached to leafy structures. The algae form a belt between the Caribbean and West Africa in the Sargasso Sea in the North Atlantic Ocean and then ride the currents west. Scientists say that reports of a massive blob of seaweed slamming into coastlines are overblown because the Sargassum algae are scattered across the ocean, and much of the seaweed will never reach the coast’s sandy shores. But in recent years researchers have generally seen larger so-called Sargassum blooms. And once the seaweed begins washing up on beaches and rotting, it can cause serious problems, local communities say.
Among annual Sargassum censuses in the Atlantic Ocean, “2018 was the record year, and we’ve had several big years since,” says Brian Lapointe, an oceanographer at Florida Atlantic University, who has studied seaweed for decades. “This is the new normal, and we’re going to have to adapt to it.”
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The seaweed “blob” has been dubbed the Great Atlantic Sargassum Belt, and though it’s sprawling, the algae in the belt cover only about 0.1 percent of the water’s surface, says Chuanmin Hu, an oceanographer at the University of South Florida, who has used satellites to study Sargassum for nearly 20 years.
Hu and his colleagues estimate the total mass of Sargassum in the Atlantic every month, tracking a yearly cycle that typically peaks in June. To do so, they use data collected by NASA satellites such as Terra and Aqua, as well as National Oceanic and Atmospheric Administration satellites. Last year the seaweed broke the record for the highest amount ever recorded in the Atlantic, with some 22 million metric tons of the stuff found across the ocean, according to the team’s calculations.
Hu says the team estimated that the Atlantic contained about six million metric tons of Sargassum in February and that he’s confident March’s mass will be higher. “This month there should be more. There’s no doubt,” Hu says. “Even in the first two weeks, I have seen increased amounts.”
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