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A giant patch of seaweed is growing to record sizes in the Atlantic. Sorry, Florida.

It’s a stinky ecological wonder.

A white sand beach with umbrellas faces a line of turquoise water with dark bobs of seaweed, seen from above.
Dark patches of Sargassum seaweed can be seen in the water in Miami Beach, Florida, in March.
Jeffrey Greenberg/Universal Images Group via Getty Images
Benji Jones is a senior environmental reporter at Vox, covering biodiversity loss and climate change. Before joining Vox, he was a senior energy reporter at Insider. Benji previously worked as a wildlife researcher.

Florida is known for many things: amusement parks, warm weather, anti-LGBTQ politics, retirees, manatees.

Oh, and increasingly, seaweed.

This spring, large blobs of a Sargassum, a kind of algae, have been washing ashore in southeast Florida. It looks bad and smells worse: As the golden algae decompose, they release hydrogen sulfide, a gas that reeks of rotten eggs.

Florida and other regions of the Caribbean are used to influxes of Sargassum in the spring and summer, but the invasion appears to be getting worse on average, year over year.

The Great Atlantic Sargassum Belt in April 2023, as detected by satellite imagery. Darker red corresponds to denser quantities of seaweed; density decreases from yellow to green to blue.
Courtesy of Brian Barnes/Sargassum Watch System/University of South Florida

The source of this seaweed — which looks a bit like a brambly plant — is an enormous patch of Sargassum in the open ocean, known as the Great Atlantic Sargassum Belt. It stretches more than 5,000 miles from West Africa to the Caribbean. And the amount of algae it contains has exploded in recent years.

In March, satellite data detected 13 million tons of Sargassum in the belt, a monthly record, according to data from the University of South Florida, and April saw similarly large amounts of the algae. Some scientists speculate, based on recent trends, that seaweed blooms in spring and summer may continue to grow in the years to come.

It’s this growing belt that inundates beaches in Florida and the Caribbean with seaweed. Ocean currents push the patch westward, and mats of algae break off and wash ashore.

Mounds of Sargassum piled up on the beach in Akumal in Mexico’s Yucatan Peninsula on March 31, 2023.
Federica Grassi/Getty Images

All this seaweed can be a problem for tourism, fishing, and other coastal industries, especially on Caribbean islands like Puerto Rico and Trinidad and Tobago. In large amounts, hydrogen sulfide gas can also irritate the eyes and throat, although this is mostly a concern in enclosed spaces.

But there’s a flip side: Sargassum is an essential component of marine ecosystems in the open ocean, and it may even help fight climate change.

A golden rainforest in the open ocean

The stretch of ocean between Florida and West Africa may seem pretty desolate — there are few islands and a lot of open sea. Yet this region boasts a tremendous diversity of life, from young fish and crabs to turtles and seabirds, some of which are found nowhere else on Earth.

Sargassum is the linchpin of this open-ocean ecosystem. The seaweed floats in tangled, island-like mats that are concentrated in two main areas of the North Atlantic: the Great Atlantic Sargassum Belt and, just north of it, the Sargasso Sea.

A map showing the Sargasso Sea and the Great Atlantic Sargassum Belt.
José Luis López Miranda et al./Frontiers in Marine Science

The expanse of Sargassum here has been compared to a rainforest because it provides shelter and food for a variety of animals.

“It’s an essential habitat for so many species,” Brian Barnes, an oceanographer at the University of South Florida, said of Sargassum. “There are so many different organisms that have evolved to live in these random aggregations in the middle of the ocean.”

The Sargasso Sea is among the planet’s most interesting ecosystems. It’s the only sea with no land borders; it’s bounded instead by ocean currents that form a soft barrier between the sea and the rest of the ocean.

The sea is a nursery for jacks, tuna, mackerel, and other fish that people eat. It’s home to a number of species found only in Sargassum ecosystems, such as the Sargassum shrimp and Sargassum frogfish (which both look a whole lot like algae). The Sargasso Sea is also the only known place on Earth where American and European eels spawn.

A Sargassum frogfish.
Aleksei Permiakov/Getty Images

Sargassum is a nuisance when it washes ashore, and it can smother coral reefs and harm other coastal ecosystems. Yet beached seaweed also harbors critters that provide food for shorebirds, and it can fertilize plants growing along the beach, helping reduce coastal erosion, David Die, a marine scientist at the University of Miami, told CNN.

The giant Sargassum patch is growing. Why?

There have likely always been floating chunks of seaweed between the Caribbean and West Africa. But in 2011, something changed and the amount of Sargassum exploded. The scale was so massive that even satellite images could see it.

With few exceptions, the abundance of Sargassum in this region — which became known as the Great Atlantic Sargassum Belt — has been rising ever since. In the last six years, the amount of seaweed in the belt doubled compared to the previous six years, according to the University of South Florida, which tracks the blooms.

The amount of Sargassum in the Great Atlantic Sargassum belt has been steadily increasing since it first exploded in 2011.
Mengqiu Wang et al./Science

What’s causing this constant growth — and the 2011 tipping point — is still something of a mystery, though humans are likely partially to blame.

One explanation is tied to pollution. An enormous amount of fertilizer runoff and other chemicals enter rivers like the Amazon that flow into the Atlantic, where they can fuel algae blooms. Deforestation also increases the amount of nutrient-rich sediment that flows into rivers, and that sediment feeds algae, too.

Some researchers also suspect that rising ocean temperatures, as well as changes in wind patterns, may be behind the recent blooms.

Off the West Coast of Africa, cold, nutrient-rich water rises from the deep ocean, which feeds Sargassum in the eastern portion of the belt. Changes in wind patterns, possibly linked to climate change, can strengthen or weaken this coldwater upwelling. Under the right conditions, winds can also push the seaweed patch closer to the mouths of rivers, including the Amazon and Orinoco in South America, stoking the bloom even more.

A mat of Sargassum seaweed floating in waters near Cozumel, Mexico.
Heather Paul/Getty Images

A combination of these forces likely triggered the 2011 boom, and they continue to cause algae to balloon. Yet it’s still not clear how climate change will shape ocean currents and wind patterns — and what that means for the Sargassum belt.

Florida will be fine (at least in terms of seaweed)

Frightening media reports refer to a “giant blob” of seaweed barreling toward Florida. That’s not quite right. The Great Atlantic Sargassum Belt is not one big blob; it’s a vast stretch of ocean spotted with clumps of algae. And it’s not like the whole thing is about to ram into Florida.

Plus, Florida doesn’t get the brunt of the algae that does wash ashore, Barnes said. Caribbean islands, like Turks and Caicos, Trinidad and Tobago, and Hispaniola are the places that tend to get inundated, he said, and this can cause serious issues.

Last year, there was so much of the seaweed in the US Virgin Islands that the territory’s governor declared a state of emergency after Sargassum clogged a desalinization plant on St. Croix. Problems like these are real, and likely to get worse if the belt keeps growing.

Sargassum seaweed in the coastal waters in Cancun, Mexico, on July 17, 2019.
Alejandro Cegarra/Washington Post via Getty Images

Yet there are also some benefits to this abundance, beyond the value of Sargassum for animals and commercial fish in the open ocean. Like kelp and other seaweeds, Sargassum sucks up carbon dioxide, a greenhouse gas, as it grows. And at least some of that carbon is stored for long periods, meaning it doesn’t decay and send carbon back into the atmosphere.

“Carbon sequestration can be a positive outlook of this phenomena,” authors of a 2021 study said of the growing abundance of Sargassum in the Atlantic. This is especially important, the authors added, considering that many other marine environments that remove carbon, including mangrove forests and seagrass beds, have been destroyed. (Seaweed left on the shore, however, can decay and release carbon back into the atmosphere.)

Entrepreneurs are also trying to turn Sargassum into useful products, such as bricks, fertilizers, and food additives.

That’s not to say we should all be celebrating Sargassum. It can, however, be worth recognizing that this giant bloom is not some evil, uncontrollable force threatening to destroy our vacations. It is a phenomenon humans help fuel and on which many marine creatures rely.