Behind a massive kelp belt heading towards some Atlantic beaches

WASHINGTON — In the next few months, a 5,000-mile belt of seaweed lurking in the Atlantic Ocean is expected to wash away the beaches of the Caribbean, South Florida and Mexico’s Yucatan Peninsula.

The Great Atlantic Sargassum Belt—the name given to the biomass that stretches from West Africa to the Gulf of Mexico—contains scattered patches of seaweed in the open sea, rather than one solid clump of sargassum. This is not a new phenomenon, but satellite imagery taken in February showed an earlier-than-usual start to such a large open ocean congestion.

When Sargassum washes ashore, it becomes a nuisance, a thick brown algae that carpets beaches, emits a pungent odor as it decomposes, and traps humans and animals that step on it. For hotels and resorts, picking up trash from beaches can mean a round-the-clock operation.

Here’s a look at the Sargassum blooms this year:


Leafy brown algae adorned with what looks like berries. Marine timber floats in the open ocean and, unlike other seaweeds, reproduces on the surface of the water with air-filled structures that give it buoyancy.

Sargassum originates in a vast stretch of the Atlantic Ocean called the Sargasso Sea, which is far from the southeastern United States. Sargasso has no land borders; instead, its boundaries form four predominant ocean currents.

Tangled kelp stretches for miles across the ocean and provides breeding, food, and habitat for fish, sea turtles, and seabirds, according to the National Oceanic and Atmospheric Administration.

“It’s a dynamic, ever-changing set of parts of this large mass,” said Rick Lumpkin, director of physical oceanography at NOAA. “It’s not one big continuous blob heading straight for South Florida.”


Sargassum accumulates on beaches, where it quickly decomposes under the hot sun, emitting gases with the smell of rotten eggs.

In recent years, sargassum has covered beaches in some Caribbean islands and the Mexican Yucatan Peninsula in spring and summer. Beach towns and cities, as well as hotels, are struggling to cope with the massive amount of seaweed washed ashore.


According to University of South Florida oceanography professor Chuanmin Hu, some sargassum has already reached the beaches of Key West. But most of them will arrive in the summer, Hu said.

“What’s unusual about this year compared to previous years is that it started early,” Hu said. Algae usually blooms in spring and summer, but “we already have a lot of them this year in winter.”

South Florida, the Caribbean and the Yucatán Peninsula typically see sargassum accumulation during the summer months, Hu said, and the same can be expected this year.


It’s a lot, but it was worse.

Scientists estimate that the belt contains more than 10 million metric tons of sargassum this year. Lumpkin called it “one of the strongest years, but not the strongest” as scientists began closely observing biomass through satellite imagery in 2011.

He said there were more in 2018. Sargassum was also abundant in 2019 and 2021, he said.


Scientists aren’t entirely sure, partly because it wasn’t observed until 2011.

“We know that to get a lot of algae, you need nutrients and sunlight. Of course, there will be more sunlight as we get closer to the equator,” said Mike Parsons, professor of marine science at Florida Gulf Coast University.

Parsons and other experts say agricultural runoff seeping into the Amazon and Orinoco rivers, and eventually into the ocean, could explain the belt’s increased growth on the western side. Parsons said warmer waters are likely to help algae grow faster. Changes in winds, sea currents, rainfall and drought can also affect flowering.

“Perhaps in some years the entire belt is fed by dust containing iron and other nutrients coming from the Sahara desert more than others,” NOAA’s Lumpkin said.

It is not clear if climate change is playing any role. Hu said extreme weather, which happens more often due to climate change — strong winds, storms, more rainfall — could be one reason.


May be. As the sargassum decomposes, it releases ammonia and hydrogen sulfide, which explains the rotten egg stench. Short-term exposure is not enough to cause disease, but long-term exposure, especially for people with respiratory conditions, can be dangerous, scientists say.

Hu said it could be a problem for hotel workers and others who can spend hours cleaning decaying sargassum from beaches.

Left to rot on the beach, Sargasso can become a problem. It can harm coastal marine ecosystems and also promotes the growth of fecal bacteria.


The Associated Press receives support from the Walton Family Foundation to cover water and environmental policy. AP is solely responsible for all content. For all AP environmental content, visit

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