Sargassum, a smelly seaweed, may be coming soon to a Gulf beach near you. Here's what to know.

CORRECTION: This story is updated to reflect studies noted by the Sargassum Sea Commission that say the Sargassum on the eastern U.S., Africa and the Carribbean may originate in an area south of the Sargasso Sea.

If you’re heading to a beach in Florida, the Gulf of Mexico, or the Caribbean this summer, you may find yourself stepping over part of the massive Great Atlantic Sargassum Belt – enormous mats of brown seaweed washed up on shore after floating for months in the ocean.

Even if you don't walk on it, it may affect your beach stroll. Sargassum rots after it washes ashore and produces hydrogen sulfide gas, which smells like rotten eggs and can irritate your eyes, nose and throat.

• USA TODAY reports: 2023 could be big year for Sargassum.

The seaweed has existed for centuries, but climate change and inflows of nitrogen and man-made fertilizer from rivers have caused exponential growth over the past decade.

Large amounts of Sargassum are projected to hit the Gulf Coast shores, including Florida, this summer. Though the "seaweed season" is March through October, it started appearing ahead of schedule in the Key West area and in Cancun, Mexico.

Where does Sargassum come from?

Sources vary on where the seaweed comes from. A U.N. environmental agency says Sargassum originates in the 2-million-square-mile Sargasso Sea. Ocean currents carry it to Florida, the Gulf of Mexico and the Caribbean islands, where the sheer amount of it hurts tourism and the fishing industry.

However, the Sargasso Sea Commission says Sargassum on the eastern U.S., Western Africa and the Caribbean comes from a region south of the Sargasso Sea between the north equatorial current and the equator. Studies say that Sargassum found in the two regions are distinctly different, the commission says.

On the eastern Atlantic, the seaweed bloom near Florida and Mexico is considered one of the largest in history.

USA TODAY reported that more than 24 million tons of Sargassum blanketed the Atlantic in June 2022, up from 18.8 million tons in May, according to a report by the University of South Florida's Optical Oceanography Lab.

That report called the amount "a new historical record."

What is Sargassum?

Sargassum is a species of large brown seaweed, a type of macroalgae that floats in large masses. The Atlantic Sargassum belt reaches from the west coast of Africa to the Gulf of Mexico and can be viewed from orbit.

In 2018, NASA satellites photographed an estimated 20 million tons of Sargassum that stretched 5,500 miles across the Atlantic Ocean. The weight was more than 200 fully loaded aircraft carriers, NASA said.

Up close, Sargassum is a collection of leafy structures, branches, and oxygen-carrying berries called pneumatocysts, air bladders that help the mats float.

Is Sargassum dangerous to humans?

In the water, Sargassum is considered harmless to people.

On land, Sargassum begins to rot and produces smelly hydrogen sulfide and ammonia, which can irritate eyes, ears, and noses. People with asthma or other breathing illnesses may have trouble breathing if they inhale too much of it.

In open areas like beaches, however, moving air usually dilutes the gas to non-harmful levels.

The seaweed also contains tiny sea creatures, such as jellyfish larvae, that can irritate your skin on contact.

And don't use Sargassum in cooking, Florida officials advise. It may contain amounts of heavy metals such as arsenic and cadmium.

How long has Sargassum been around?

Sargassum has existed for hundreds of years. Christopher Columbus encountered it in 1492 and named the Sargasso Sea after the seaweed.

Historically, small amounts of Sargassum have washed ashore with other beach wrack material such as sea grass, sponges and soft corals. Beach wrack helps stabilize shorelines and feeds dune plants.

But Sargassum began attracting international notice in 2011 when it started washing up on Caribbean islands in much larger amounts. The trend has continued nearly every year since then in varying amounts, threatening tourism in some seaside communities.

What's driving the increase in Sargassum?

Climate change is considered a key factor. Sargassum thrives in warm water. A report by Florida International University also cites:

• Dust clouds from the Sahara: The clouds contain iron, nitrogen and phosphorus that fertilize seaweed and plankton and extend for thousands of miles over the Atlantic.
• More nitrogen: The Amazon and Mississippi rivers send large amounts of runoff fertilizer from farms and ranches into the ocean, which also feed the seaweed. 

Unlike most other seaweeds, which begin growing on the seafloor, Sargassum is holopelagic, which means it reproduces while floating on water.

Does Sargassum have any benefit?

Sargassum is classified as an Essential Fish Habitat by the National Oceanic and Atmospheric Administration. That gives the seaweed special protections, including how it is collected. It can't be removed directly from water – it has to wash up on shore first. 

Sargassum provides a source of food to a variety of marine wildlife, including birds, turtles, fish, and crabs, according to the Virgin Islands Department of Agriculture and Fisheries.

Green sea turtles use Sargassum mats as nurseries and loggerhead sea turtles use the seaweed to stay near the ocean surface, according to the Sea Turtle Preservation Society. Shrimp, crabs and fish also use it as a habitat.

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SOURCE USA TODAY Network reporting and research; sargassummonitoring.com; University of Florida Water Institute; Associated Press; Florida International University