Massive swaths of sargassum are blanketing the beaches and coastlines of the Caribbean and Gulf of Mexico, suffocating shorelines and snuffing out marine life while spooking visitors in areas that depend heavily on tourism. According to a report released Friday by the University of South Florida’s Optical Oceanography Lab, some 27 million metric tons of self-sustaining sargassum were recorded in the Atlantic, Caribbean, and Gulf of Mexico in July.
The prickly seaweed covers miles of coastline, releasing a foul smell as it rots onshore.
This year marks the first time that scientists have observed sargassum growing in large quantities directly within the Caribbean and Gulf of Mexico, rather than being swept into those areas by ocean currents. Brian Barnes, a marine scientist and research assistant professor at the University of South Florida, noted that the algae usually reproduces asexually as it travels from Western Africa.
"None of this is normal," Barnes said. "Every year is a little bit of a new story." The Great Atlantic Sargassum Belt has been growing since 2011, and experts suggest that agricultural runoff, warming waters, and changes in wind, current, and rain could be contributing factors.
The current sargassum, which began reproducing in the region earlier than usual, stems from a record 40 million metric tons reported last year. "The 2025 bloom was so big it never really left," Barnes explained. "It was enough to start a new bloom locally." Sargassum levels in the Caribbean reached record levels in June, while the amount present in the Gulf hit 5 million metric tons in June, nearly double last year’s record.
Scientists expect 2026 to be the second-largest amount of sargassum recorded in the Atlantic, Caribbean, and Gulf of Mexico, noting that June alone saw 34 million metric tons, which was only 10% lower than the year-earlier period.
Mexico announced Thursday it plans to invest $115 million to tackle the blooming algae, which poses an environmental and economic threat. The government estimates that up to 9,000 tons reach its Caribbean coastline on the worst days, yet with current resources, Mexico says it can only gather 1,200 tons of sargassum a day.
In the state of Quintana Roo, one of Mexico’s tourism gems, sales fell 30% in the first half of the year, leading to layoffs. One local worker, Vázquez, expressed his sadness over the state of the once-lively place and fears for his job security.
The impact extends across the region. In the U.S. Virgin Islands, people are walking up to 25 feet (seven meters) on mats of seaweed before even reaching the water, according to environmental consultant Amy Dempsey. "This is by far the worst I’ve seen it in the last five years," she said.
"We are getting slammed in the Virgin Islands." Much of the record sargassum reported last year ended up along Puerto Rico’s coastline, prompting the governor to declare a state of emergency.
Waldemar Quiles, the U.S. territory’s natural resources secretary, noted he does not recall ever having that much concentration in so many places at once. Emmanuel Maldonado González, an environmental sociologist at East Carolina University, added, "It’s not a one-off event anymore. It’s becoming a growing hazard."
The smell of rotten eggs permeates communities where large piles of decaying sargassum emit ammonia and hydrogen sulfide. The U.S. Environmental Protection Agency warns that the effects of these gases on people can be severe, potentially causing nausea, headaches, and lung irritation. Officials on the French Caribbean island of Martinique recently opened a shelter to house those affected by the toxic gases after residents protested.
Crews in Martinique have collected more than 11,500 tons of sargassum so far this year, including 638 tons in four days alone this month, using sea barriers and manual collection for composting and bioenergy production.
While patches of sargassum in open waters provide a healthy breeding ground for wildlife, including turtles, fish, and seabirds, Barnes noted that once it is closer to shore, it reduces oxygen in the water, smothers mangrove roots, and blocks sunlight from reaching coral and sea grass.
Furthermore, once it washes ashore, it can prevent newly hatched sea turtles from reaching the ocean, while the heavy machinery used to remove it risks destroying fragile turtle nests.
"The coastlines are not built for this," Barnes said. Researchers at the University of South Florida and the U.S. National Oceanic and Atmospheric Administration used artificial intelligence to find that from 2003 to 2022, macroalgae blooms increased by more than 13% per year in the tropical Atlantic and Western Pacific, reaching 16.9 million square miles (43.8 million square kilometers).
