What is sargassum? Here's why seaweed is piling up along Florida beaches Since 2011, a fleet of seaweed patches double the size of the contiguous U.S. has cycled from West Africa to Florida, threatening beaches from Martinique to Miami. This year, it could grow bigger.

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

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It weighs more than 10 million tons. It stretches more than 5,000 miles. And if the wind hits just right, it can wash up on shores along Florida and Mexico, where it is bad for local ecosystems and economies. We're talking about the great sargassum belt - a blob of floating seaweed that scientists say could get bigger than ever this summer. NPR's Emily Olson explains why.

EMILY OLSON, BYLINE: When you look at sargassum up close, it doesn't look too threatening.

BRIAN BARNES: Like, if you pulled up a handful of blades of grass, and then you, like, threw some peas in there, and those peas are what keeps it afloat.

OLSON: This is Brian Barnes of the University of South Florida. He says that in the open ocean, sargassum is harmless, even serving as a habitat for fish, birds and crustaceans. But when the currents or wind push sargassum toward the coast, trouble can follow.

BARNES: This is a thick mat of floating algae. It can either cover or smother things like coral or sea grasses.

OLSON: And then once it gets to shore, it decays, which can block beach access and decrease tourism. The stuff isn't just messy. It smells like rotten eggs, and it releases gases like hydrogen sulfate, which can cause respiratory issues. But it's not as simple as throwing it into a dump, either. Here's Brian Lapointe, a researcher with Florida Atlantic University.

BRIAN LAPOINTE: It has fairly high concentrations of arsenic. There's a concern that, through leaching, it could affect the groundwater, for example.

OLSON: He says the sargassum blooms have always been there, but since 2011, they've been growing bigger and bigger. He has a theory about why.

LAPOINTE: One thing we know is that humans have greatly altered the global nitrogen cycle. We've more than doubled the amount of reactive nitrogen on our planet.

OLSON: In other words, people on land are using more fertilizer, cutting down forests and creating wastewater. He says all of that sends nitrates downriver, which eventually shoot out into the ocean. It's like food for sargassum. Combine it with the right light and temperatures, and the seaweed flourishes.

LAPOINTE: It looks like this year could be a new record.

OLSON: It's already starting to show up on beaches around the Florida Keys and Mexico's Yucatan Peninsula, but the peak is still likely a few months away.

Emily Olson, NPR News.

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