A mystery wave of seaweed is causing a stink in the Caribbean

A record amount of sargassum seaweed amassed across the Caribbean and surrounding areas in May, with more expected this month, according to a recent report.

The brown algae is blanketing shorelines from Puerto Rico to Guyana, disrupting tourism, killing wildlife, and emitting toxic gases. One school in Martinique was temporarily closed due to the fumes.

The volume of seaweed—38 million metric tons—is the largest quantity observed in the Caribbean Sea, the western and eastern Atlantic, and the Gulf of Mexico since scientists began monitoring the Great Atlantic Sargassum Belt in 2011.

Brian Barnes, an assistant research professor at the University of South Florida and contributor to the report published by the university’s Optical Oceanography Lab, confirmed the findings on Monday.

The previous record was set in June 2022, with approximately 22 million metric tons.

“The peaks just seem to keep getting bigger and bigger year after year,” he said.

But scientists don’t know why yet.

“It’s the million-dollar question,” he said. “I don’t have a supremely satisfying answer.”

Three different types of sargassum exist in the Caribbean and nearby areas, reproducing asexually as they remain afloat thanks to tiny air sacs. They thrive in different ways depending on sunlight, nutrients and water temperature, factors that scientists are currently studying, Barnes said.

Experts also have said that agricultural runoff, warming waters and changes in wind, current and rain could have an effect.

While large clumps of algae in the open ocean are what Barnes called a “healthy, happy ecosystem” for creatures ranging from tiny shrimp to endangered sea turtles, sargassum near or on shore can wreak havoc.

It can block sunlight that coral reefs need to survive, and if the algae sinks, it can smother reefs and sea grasses. Once it reaches shore, the creatures living in the algae die or are picked off by birds, Barnes said.

Huge piles of stinky seaweed also are a headache for the Caribbean, where tourism often generates big money for small islands.

“It is a challenge, but it’s certainly not affecting every single inch of the Caribbean,” said Frank Comito, special adviser to the Caribbean Hotel and Tourism Association.

In the popular tourist spot of Punta Cana in the Dominican Republic, officials have invested in barriers to prevent sargassum from reaching the shore, he said.

In the Dutch Caribbean territory of St. Maarten, crews with backhoes were dispatched in late May as part of an emergency clean-up after residents complained of strong smells of ammonia and hydrogen sulfide, which can affect a person’s respiratory system.

“The smell is quite terrible,” Barnes said.

Meanwhile, in the French Caribbean, officials expect to soon use storage barges and an upgraded special vessel that can collect several tons of seaweed a day.

The sargassum “disfigures our coasts, prevents swimming and makes life impossible for local residents,” French Prime Minister François Bayrou recently told reporters.

But Comito said such vessels are “massively expensive” and not a popular option, noting that another option — using heavy equipment —- is labor-intensive.

“You have to be careful because there could be sea turtle eggs affected,” he said. “It’s not like you can go in there and massively rake and scrape the whole thing.”

Some Caribbean islands struggle financially, so most of the cleanup is done by hotels, with some offering guests refunds or a free shuttle to unaffected beaches.

Every year, the amount of sargassum expands in late spring, peaks around summer and starts to decline in the late fall or early winter, Barnes said.

The new record set is hardly stationary — experts said they expect even more sargassum for June.