Researchers Warn of Seaweed Surge

MIAMI — Piles of smelly seaweed have washed ashore on Miami Beach and Key Biscayne, a stinky reminder that this is sargassum season.

The brown seaweed is a common sight across the Caribbean and on South Florida beaches.

Researchers who track seaweed blooms expect 2025 to be another “major sargassum year” for the Atlantic Basin — but how much seaweed washes ashore on South Florida beaches depends on winds and currents.

Sargassum season runs from March through October.

“All it takes is the right currents, the right winds, to push a particular patch on shore and create an inundation,” said Brian Barnes, research assistant professor at the Optical Oceanography Lab, part of the University of South Florida’s College of Marine Science.

In the ocean, seaweed offers an important habitat for marine life. But when seaweed washes ashore, the rotten egg smell can put a damper on beach days.

Washed-up seaweed can also cause health problems for people with chronic respiratory conditions.

Here’s what else to know:

Why does seaweed smell so bad?

The brown seaweed that washes up on Florida beaches is known as sargassum. Once seaweed washes ashore, the piles start to rot and release hydrogen sulfide gas, which smells like rotten eggs.

What’s the Florida seaweed forecast?

While it’s common to see some seaweed at the beach, sargassum season can bring large amounts ashore.

Predicting which Florida and Caribbean beaches will be inundated with seaweed, and when, is complicated. That could soon change.

Barnes for several years has led a federally funded project to create a forecast model meant to help predict when and where large patches of seaweed will wash ashore.

He expects a test version of the model will be released later this year, with the initial forecast set to focus on the Florida Keys and the rest of South Florida. A similar model for Puerto Rico and the U.S. Virgins Island will likely be released in 2026.

The forecast model is part of a five-year $3.2 million grant awarded in 2023 from the federal National Oceanic and Atmospheric Administration. Barnes’ USF team has been working on the model with researchers from Florida Atlantic University in Boca Raton, Caribbean Coastal Ocean Observing System, NOAA’s Atlantic Oceanographic and Meterological Laboratory, and the U.S. Virgin Islands Department of Planning.

But what does current data tell us?

“There’s a lot of sargassum offshore, off the Keys, and even in the Gulf of Mexico,” said Barnes, noting that seaweed often collects in these areas before winds and currents shift the blooms toward the Florida Keys, Key Biscayne and Miami Beach. Whether the seaweed remains offshore or eventually floats onto beaches is what the future forecast model hopes to predict.

In Miami Beach, for example, “stronger than usual easterly and southeasterly winds have likely contributed to the increased amount of sargassum washing ashore,” city spokeswoman Melissa Berthier said in an email statement to the Miami Herald. “The prevailing winds push floating mats of seaweed closer to our coastline, resulting in the heavier deposits we have seen along our beaches recently.”

Are there seaweed hot spots in Miami?

While Florida might see blooms of seaweed this season, that doesn’t mean all beaches will be full of it through October. The sargassum will show up on beaches in clumps when the tide is high and the wind is blowing from east to west, pushing the seaweed toward shore.

The biggest accumulations of sargassum on Miami-Dade beaches happens during high tide twice a day, according to Miami-Dade County’s sargassum webpage. County crews generally get rid of smelly seaweed early in the morning, before most beachgoers arrive. “Unfortunately, when the second tide arrives in the afternoon, depending on the winds, it often brings another wave of sargassum to the shoreline,” the county said.

Key Biscayne had large swath’s of seaweed wash ashore on April 24. Some parts of Miami Beach’s shoreline have also seen more seaweed buildup recently compared to other areas.

On April 28, the Greater Miami and Miami Beach website’s webcam showed dark pockets of seaweed in the water and shoreline in the beach area of 17th and 18th streets in the early morning. The 10th Street beach area, on the other hand, didn’t have much seaweed in the water or shoreline, Berthier said.

While it’s hard to predict which beach will get a blanket of seaweed, Miami-Dade County has identified four seaweed hot spots: Haulover just north of Haulover Cut; Bal Harbour just south of Haulover Cut; Miami Beach between 26th and 31st streets; and beaches along the South Pointe jetty.

Sargassum season also coincides with Miami-Dade’s sea turtle nesting season. That means sea turtle nests need to be marked before any seaweed cleanups can begin in Miami Beach, considered a crucial nesting habitat for loggerhead, green and leatherback sea turtles.

Health problems from hydrogen sulfide

The hydrogen sulfide gas seaweed releases when it rots can irritate your eyes, nose and throat. Tiny sea creatures living in the seaweed, like jellyfish and sea lice, can also cause skin rashes and blisters.

“If you have asthma or other respiratory illnesses, you will be more sensitive to hydrogen sulfide. You may have trouble breathing after you inhale it,” according to the Florida Department of Health in Monroe County.

People who are exposed to hydrogen sulfide for long periods in an enclosed space, like workers in wastewater treatment plants, can get ill with dizziness, headache, an upset stomach and apnea, according to the federal Centers for Disease Control and Prevention.

“However, hydrogen sulfide levels in an area like the beach, where large amounts of air flow can dilute levels, is not expected to harm health,” according to Monroe’s health department. There’s also no indication the gas causes cancer in humans. Anyone experiencing severe trouble breathing should call 911.

Miami-Dade County also is keeping an eye on evolving information about the relationship between sargassum and a warm coastal bacteria that can cause rare and severe life-threatening infections, including an infection known as “flesh-eating bacteria.”

A recent study involving researchers at Florida Atlantic University suggest this warm coastal water bacteria, Vibrio vulnificus, can sometimes “stick” to microplastics, which are commonly found in our oceans and can get stuck in sargassum. Scientists found that the Vibrio bacteria in the open ocean has similar genetic characteristics to the Vibrio species that can cause disease in humans, according to NBC News. People usually get infected with the bacteria by eating raw shellfish, especially oysters, or through an open wound that has direct contact with saltwater.

“I don’t think everyone should be running away from sargassum as if it’s going to kill them,” Linda Amaral-Zettler, a marine microbiologist at Royal Netherlands Institute for Sea Research and one of the authors of the recent study, told NBC News. “That’s simply not the case. But I think we need to think responsibly about the potential risks.”

What should you do if your nearby beach has a lot of seaweed?

Health officials usually recommend people who live near the beach keep their windows and doors closed when a lot of sargassum washes ashore. Florida Keys health officials have recommended anyone with asthma or other respiratory problems stay away — or reduce their time — at the beach if there is seaweed. If you don’t want to risk getting stung by an animal living in the seaweed, don’t touch it and stay away.

And before you think about taking some seaweed home to cook, think again.

Florida’s health department said that’s not a good idea because sargassum might contain “large amounts of heavy metals like arsenic and cadmium.”

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