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Meet the sargassum belt, a 5,000-mile-long snake of seaweed circling Florida

People remove Sargassum in Playa del Carmen, Mexico, in April, 2022. Researchers expect this year will bring another massive bloom, choking local ecosystems and tourism economies.
Elizabeth Ruiz
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AFP via Getty Images
People remove Sargassum in Playa del Carmen, Mexico, in April, 2022. Researchers expect this year will bring another massive bloom, choking local ecosystems and tourism economies.

It stretches over 5,000 miles. It weighs over 10 million tons. And it's circling around the Gulf of Mexico and the mid-Atlantic, where the right combination of currents and wind could push it ashore.

If you haven't heard of the great Atlantic sargassum belt, or even if you have, chances are high that you'll see it pop into your news feed at least once this summer. After a decade of record-breaking blooms, 2023's sargassum mass is again shaping up to cause headaches (literally and figuratively) for beachside towns and tourists.

Here's what you need to know.

What exactly is the sargassum belt?

Sargassum is a type of leafy, rootless and buoyant algae that bunch up in islands and floats around the ocean.

In the open sea, healthy patches of sargassum can soak up carbon dioxide and serve as a critical habitat for fish, crabs, shrimp, turtles and birds.

But if sargassum moves closer to the coast, the seaweed can wreak havoc on local ecosystems, smothering coral reefs and altering the water's pH balance. Once ashore, clumps of sargassum can choke local economies by closing tourism sites, cutting off marinas and constricting fishing yields.

Sargassum begins to rot after about 48 hours on land, releasing irritants like hydrogen sulfide, a hazard to anyone with respiratory issues like asthma. Oh, and the resulting smell resembles manure or rotten eggs — not a great spring break aroma.

It used to be that sargassum rafts were disparate, sporadic bodies, causing little disruption to beach-going.

But scientists noticed a change in sargassum levels in 2011, when masses of the seaweed multiplied, gaining in density and size, becoming so big they were captured on satellite images.

Today, the patches comprise a 5,500-mile-long, 10 million-ton belt that circulates annually, starting near West Africa and snaking through the Gulf of Mexico back into the Atlantic.

More than double the width of the contiguous U.S., the mass fluctuates in size from month to month, with the high point generally landing in the summer.

"The low season of the cycle is now higher than the high point of the cycle five or six years ago," says Brian Barnes, a researcher with the University of South Florida's College of Marine Science.

"What we thought was just a massive bloom has only gotten bigger and bigger and bigger each year," Barnes adds.

What's behind these monster blooms?

Sargassum seaweed inundated the shores of Le Gosier on the French islands of Guadeloupe in November 2022.
Loic Venance / AFP via Getty Images
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AFP via Getty Images
Sargassum seaweed inundated the shores of Le Gosier on the French islands of Guadeloupe in November 2022.

The exact drivers of the growth are still a bit "shrouded in mystery," says Brian Lapointe, a research professor with Florida Atlantic University, who's been studying sargassum for over 40 years.

His hypothesis is that it has to do with how humans are altering the nitrogen cycle. We're using more fertilizer, burning biomass, cutting down forests and increasing wastewater from cities, all of which sends ammonium, nitrate and phosphate down major river systems.

Those elevated nutrients then shoot out over the surface of the ocean, acting as a fertilizer for sargassum patches.

"What we've found in studying these plants over the last four decades is that the ratio [of nitrogen to phosphorus] is going up, and that's exactly what's happening to all these major river systems," Lapointe said. "It's almost like sargassum is a barometer for how global nitrogen levels are changing."

Cleaning up major rivers from the Mississippi to the Orinoco would be the best step for mitigating excessive sargassum bloom, Lapointe says.

But in the meantime, the blooms continue to get bigger and bigger. Barnes and Lapointe both say that this year is already on track to break records.

What's happening with this year's bloom?

The University of South Florida's Optical Oceanography Lab, which tracks the mass using NASA satellite imagery, the latest bloom has already doubled every month from November to January.

And, thanks to ocean currents, the belt is continuing to migrate westward, threatening beaches along the Florida Keys, along with Mexico's Yucatán Peninsula and the eastern Caribbean.

An estimated 200 tons of sargassum already began washing up on beaches along the Yucatán Peninsula earlier this month, spurring local authorities to jump into cleanup operations.

Key West, Fla., is also seeing large, early amounts of sargassum piling up, restricting beach access just a month after beaches were closed in other parts of the state during a toxic microalgae bloom known as the "red tide."

Barnes says the two oceanographic events are totally unrelated, but "they both run on the same principle: You need a seed population, you need fertilizer, you need light and the right temperatures." It's a good year for those elements.

What happens if sargassum isn't removed from the beaches?

A special boat collects sargassum seaweed off the coasts of Le Robert on France's Martinique island in February 2022.
Thomas Coex / AFP via Getty Images
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AFP via Getty Images
A special boat collects sargassum seaweed off the coasts of Le Robert on France's Martinique island in February 2022.

Local officials in Florida and elsewhere sometimes use heavy machinery to clear beaches, but scientists say that can threaten local sea turtle nests and cause shoreline erosion.

Last year's record bloom offered a taste of what could happen without proper planning.

Following last year's record sargassum levels, the U.S. Virgin Islands declared a state of emergency and requested assistance from FEMA to handle the masses.

A desalination plant on St. Croix became so clogged with seaweed that local electrical-generating capacities were threatened.

In Barbados, local governments employed 1,600 dump trucks daily during the peak season to clean the beaches for tourists, LaPointe told NPR.

And desperate officials in the Cayman Islands tried a pilot program to pump the seaweed directly out of the water, but swiftly suspended the efforts after realizing how difficult it is to decompose the material, the Associated Press reported.

As evidence that necessity is breeding invention, private companies, too, have experimented with using sargassum as food, fertilizer, biofuel, construction material and medicinal products.

But Lapointe cautions that keeping the biomass at bay might be the safest long-term option.

The field of study is young, but "we're finding [sargassum] can contain heavy metals, including arsenic. It has fairly high concentrations of the toxin," he said. "There's a concern that, through leaching, that could impact groundwater."

In 2018, doctors on the islands of Guadeloupe and Martinique also reported more than 11,000 cases of "acute sargassum toxicity" during an eight-month period of intense beach buildup, Reuters reported.

Is this just the new normal?

The hassle and danger of living with sargassum yields is only complicated by how tricky it is to predict where the patches will wash ashore. Bloom sizes vary based on wind, river flooding, droughts and temperature.

But once the seaweed is grown, patches "aggregate where the currents push them," Barnes says. "We can see it out there in the open ocean and we can track movements offshore with accuracy. [...] But once you start getting into the scale of an individual beach, it becomes much more difficult."

Barnes is hoping his team can get better at predicting sargassum beaching with precision, in large part because he doesn't see this excessive bloom cycle "winding down anytime soon."

"It's only getting bigger and bigger and bigger each year," he said.

Lapointe agreed, framing it another way: "I remember seeing The Blob as a movie when I was a kid and it scared the you-know-what out of me. [...] This blob of seaweed is scarier. It's the real deal."

Copyright 2023 NPR. To see more, visit https://www.npr.org.

Corrected: March 16, 2023 at 10:00 PM MDT
An earlier version of the story incorrectly paraphrased researcher Brian Lapointe as saying the ratio of phosphate to nitrogen is increasing in sargassum tissue. In fact, he said the ratio of nitrogen to phosphorus is increasing. March 15, 2023 An earlier version of this story incorrectly stated that the estimated weight of the great Atlantic sargassum belt is 10 billion tons. In fact, it is 10 million tons.
Emily Olson
Emily Olson is on a three-month assignment as a news writer and live blog editor, helping shape NPR's digital breaking news strategy.