At a Glance

Sargassum is a vital habitat for fish and other sea creatures.But it can also affect tourism and local economies.Scientists say several factors are at play.

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Runoff from the Brazilian rainforest could be fueling this year's massive sargassum bloom, according to some scientists.

“It’s coming off the coast of Brazil … quite sure it’s because of all the agriculture and use of fertilizer in the Amazon River Basin,” Stephen Leatherman, the coastal sciences professor known as , told weather.com in a recent interview.

Leatherman, who teaches and conducts research at Florida International University and is well known for his annual ranking of "," points out that sargassum has been washing up on beaches for years.

(WATCH: Companies Striving To Find New Uses For Smelly Sargassum)

But scientists have been closely following it since 2011, when the bloom grew large enough to be seen on satellite from space. And this year, it may be bigger than ever.

By March, some tons of it were floating in scattered clumps along a 5,000-mile path from West Africa to the Gulf of Mexico, according to the University of South Florida’s Sargassum Watch System. That was a record for the time of year.

Beachgoers walk past seaweed that washed ashore on March 16, 2023, in Fort Lauderdale, Florida.

(Joe Raedle/Getty Images)

It’s called the Great Atlantic Sargassum Belt. Leatherman calls it "a sargassum seaweed monster."

And while it’s crucial as a habitat for marine life at sea, it can impact economies that rely on tourists, cost municipalities millions of dollars to clean up and lead to fish kills and other issues near shore.

Leatherman and other scientists say there are several factors at play in its proliferation.

What’s Happening In Brazil

From 2000 to 2021, Brazil lost more than square miles of mature tropical rainforest, according to the independent Global Forest Review.

Much of that was converted for agricultural uses, which means more fertilizer and other nutrients.

"The Amazon River, which is the largest river in the world, is now bringing all these nutrients, phosphates and nitrates to the coast, which feeds the sargassum which is right offshore. And it blooms. It just explodes in numbers," Leatherman said.

From there, ocean currents carry it to the Caribbean islands and into the toward Florida.

(WATCH: Is The Smelly Seaweed Invading Some Florida Beaches Dangerous?)

A 2021 study led by Florida Atlantic University sargassum expert Brian Lapointe showed that nitrogen levels in samples of sargassum pulled from the Atlantic Ocean rose between 1983 and 2019. The study put the blame on several factors, including sewage and fertilizer.

Lapointe also cites discharges from the Congo and Mississippi rivers.

Saharan Dust Could Also Carry Some Blame

The is a mass of very dry, dusty air that moves from the Sahara Desert over the tropical North Atlantic Ocean in the late spring into summer and early fall.

It’s a natural phenomenon and not generally dangerous, although it can cause concern for people with underlying respiratory conditions.

"That dust also contains some nitrates, phosphates and iron oxide, which is also a nutrient," Leatherman said. "So when we get those big dust, the Saharan dust years, we're also getting nutrients, which is adding to the sargassum problem.”

(WATCH: Don’t Let Giant Sargassum Seaweed Bloom Ruin Your Vacation)

In 2020, the Saharan Air Layer since record-keeping began about 20 years ago moved all the way across the U.S.

What Role Do Weather And Climate Change Play?

Ultimately, wind and ocean currents determine where the sargassum will go and how much it might muck up beaches in any given location.

When it comes to climate change, scientists aren’t yet sure. Leatherman and others say warmer ocean waters are contributing to the big blooms, but it’s not clear to what extent.

(P​aul Matadeen contributed to this report.)

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