A record amount of sargassum accumulated throughout the Caribbean and nearby areas in May, and is expected more this month, according to a new report.
Brown and thorny algae are suffocating the coast from Puerto Rico to Guyana and beyondinterrupting tourism, killing wildlife and even releasing toxic gases that forced a school on the French Caribbean island of Martinica to temporarily close.
The quantity, 38 million metric tons, is the largest amount of algae observed in the Caribbean Sea, the western and eastern Atlantic and the Gulf of Mexico since the scientists began to study the great Sargazo belt of the Atlantic in 2011Said Brian Barnes, an assistant research professor at the University of Southern Florida who worked in the report published Monday by the Optical Oceanography Laboratory of the University of Southern Florida.
The previous record was established in June 2022, with about 22 million metric tons.
1 /16 | The coasts of Puerto Rico are dyed brown with the arrival of Sargazo. Large concentrations of Sargazo began arriving at the Laguna del Condado, in San Juan, during the month of May 2025. – Carlos Giusti/Staff
“The peaks seem to continue growing year after year,” he said.
But scientists still don’t know why.
“That is the million dollar question”said. “I don’t have a supremely satisfactory response.”
There are three different types of sargassum in the Caribbean and nearby areas, which reproduce asexually while remaining afloat thanks to tiny bags of air. They thrive in different ways depending on sunlight, nutrients and water temperature, factors that scientists are currently studying, said Barnes.
Experts have also said that agricultural runoff, water heating and changes in wind, currents and rain could have an effect.
While large groups of algae in open sea are what Barnes called a ‘healthy and happy ecosystem’ for creatures that go from small shrimp to in danger of extinction, the sargassum near or on the coast can cause ravages.
It can block sunlight that coral reefs need to survive, and if the algae sink, they can suffocate the reefs and marine herbs. Once it reaches the coast, the creatures living in the algae die or are collected by the birds, Barnes said.
Huge stinky algae batteries are also a headache for the Caribbean, where tourism often generates a lot of money for the small islands.
“It is a challenge, but it is certainly not affecting every centimeter of the Caribbean,” Frank Comito said special advisor to the Caribbean Hotels and Tourism Association.
In the popular tourist place of Punta Cana in the Dominican Republicofficials have invested in barriers to prevent sargazo from reaching the coast, he said.
In the Dutch Caribbean territory of St. Maarten, crews with backhoes were sent at the end of May as part of an emergency cleaning after residents complained about strong ammonia smells and hydrogen sulfide, which can affect the respiratory system of a person.
“The smell is quite terrible,” Barnes said.
Meanwhile, in the French Caribbean, officials hope to use storage barges soon and an improved special ship that can collect several tons of algae per day.
The sargasso “disfigures our coasts, prevents swimming and makes life impossible for local residents”, The French Prime Minister, François Bayrours recently told reporters.
But Comito said such vessels are “massively expensive” and are not a popular option, and pointed out that another option, the use of heavy equipment, requires a lot of labor.
“You have to be careful because there could be affected turtle eggs,” he said. “It is not that you can enter there and rake and massively scrape the whole matter.”
Some Caribbean islands have financial difficulties, so most of the cleanliness is carried out by hotels, and some offer guests reimbursements or free transportation to the unqualified beaches.
Every year, the amount of sargassum expands to the end of spring, reaches its maximum point around the summer and begins to decrease at the end of autumn or principles of winter, said Barnes.
The new established record is hardly stationary: experts said they expect even more sargassum for June.