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Everything You Need To Know

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Everything You Need To Know

If you’re planning on visiting any East Coast beaches in the U.S. this summer—or vacationing in destinations in Central or South America, bordering the Atlantic Ocean—keep an eye out for leafy, brown piles of seaweed. There’s a good chance you’ll immediately notice its presence: there may be a rotten egg smell lingering in the air.

Each year, this brown algae, called Sargassum, blooms and then drifts along sea currents between the Americas and Africa. The currents eventually push giant mats of the seaweed westward until they wash up on shore.

Since February, new satellite images have shown this year’s Great Atlantic Sargassum Belt creeping west, but reaching farther south than usual. Scientists are keeping an especially close eye on Florida and the Caribbean, where some of the seaweed has already landed in the Key West area.


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And that’s a problem—the longer you’re exposed to Sargassum, the more harm it can cause.

Once it hits shore, Sargassum piles up and begins rotting in the air, releasing effluvia that is harmful to those with asthma or breathing difficulties. Beside emanating hydrogen sulfide, the same compound that gives rotten eggs their characteristic stench, the biomass can contain heavy metals like lead, arsenic, and mercury, which are toxic to people and animals, according to the Miami-based Atlantic Oceanographic and Meteorological Laboratory, part of the National Oceanic and Atmospheric Administration (NOAA).

Here’s everything you need to know about the Great Atlantic Sargassum Belt in 2023.

Where Does Sargassum Come From?

Sargassum is scattered in separate blobs floating on the open sea, extending 5,000 miles from West Africa to the Gulf of Mexico. It’s an unusual form of algae, because it reproduces on the surface of the sea instead of originating from the ocean floor. Speckling the algae are berry-like protrusions filled with gas (mainly oxygen) that help the whole seaweed belt float.

large algae bloom in atlantic ocean makes way to florida beaches

Seaweed washes ashore on March 16, 2023 in Fort Lauderdale, Florida. Reports indicate that this summer, a huge mass of Sargassum seaweed that has formed in the Atlantic Ocean is possibly headed for the Florida coastlines and shores throughout the Gulf of Mexico.

Joe Raedle//Getty Images

The algae gives its name to the Sargasso Sea, and it’s part of an oceanic ecosystem, serving as a habitat for various species of fish, sea turtles, crabs, birds, and other animals, according to NOAA. For the eponymous sargassum fish, the seaweed is a permanent home, complete with food, hiding spots, and breeding grounds. Even as the floating habitat loses its gases and sinks to the seafloor, undersea life gains valuable carbon, which potentially plays an important part in the food chain.

Why Is There So Much Sargassum This Year?

handful of sargassum seaweed

Ali Bentley//Getty Images

While Sargassum isn’t new, it’s blooming months earlier than usual this year, so the accumulated patches are moving toward the shore faster than usual; the brown algae typically appears along the coast in later spring and summer, Chuanmin Hu, a professor of oceanography at the University of South Florida (USF), tells ABC News. Most of the seaweed will still appear only in the summer, according to scientists’ estimates.

Hu has been tracking the belt since 2006 at USF’s College of Marine Science, analyzing data from satellites, including NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS). He expects Southern Florida, the Caribbean, and the Yucatán Peninsula to experience most of the algae on its beaches in the summer months, where it’s likely to impact tourism.

In 2019, Hu observed an unusually large Sargassum mass. His research at the time determined that it could be enlarging in size due to increased deforestation and fertilizer use along the Amazon River. Every spring and summer, the river’s contents flow into the ocean; nutrients from fertilizer may be following the typical ocean currents to end up in Sargassum zones, Hu explained in 2019 on the USF College of Marine Science website.

“The evidence for nutrient enrichment is preliminary and based on limited field data and other environmental data, and we need more research to confirm this hypothesis,” Hu said at the time. “On the other hand, based on the last 20 years of data, I can say that the belt is very likely to be a new normal.”

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The Sargassum blooms are a newer population occurring much closer to the equator than in previous decades, oceanographer Ajit Subramaniam of the Columbia University Climate School in New York says in a 2019 Nature film. Based on satellite data, scientists estimated that it was a whopping 24.2 million metric tons, probably even bigger than this season’s large algal bloom.

How Is Sargassum Managed at the Beach?

Beach workers clearing the mess, as well as fishers and divers, risk irritation to their eyes, noses, mouths, skin, and airways. Rosa Rodríguez Martínez, a biologist who studies reefs and coastal ecosystems for the National Autonomous University of Mexico, says beach workers in Puerto Morelos, Mexico, were exposed to dangerously high levels of hydrogen sulfide gases during the Sargassum landfall in 2022. “At one spot (in a decomposed pile of seaweed) it reached 56 parts per million. That’s very high. Above two, that can be dangerous for people with respiratory problems,” Rodríguez Martínez tells the Associated Press.

Scientist Shelley Ann-Cox, who specializes in fisheries management in Barbados, told Nature that sea urchin divers there struggle on two fronts. For one, seaweed clogging the shoreline prevents boats from getting to sea. And, if they spend time around the Sargassum while diving, they experience skin irritation and even get the “berries” that keep the leafy algae afloat in their ears. “Fisher folks have reported Sargassum mats as far as their eyes can see,” she said of the 2019 Barbados beaches.

With a somewhat smaller—but comparably massive collection of seaweed fast approaching—we’ll know by summer whether Sargassus will be as problematic in 2023.

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Before joining Popular Mechanics, Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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