Good News, Florida. The Giant Seaweed Blob Has Shrunk.

For months, Florida’s usually picturesque coast was plagued by a rotting tangle of seaweed, known as sargassum. Then, as quickly as the stinking mass arrived, it began to disappear.

Beachgoers rejoiced, posting pictures of white sands and sparkling waters to social media. Scientists said they had expected the sargassum in the Gulf of Mexico to wane eventually — but not so fast, or by so much.

“That is a surprise,” said Chuanmin Hu, a professor of oceanography at the University of South Florida, noting that there was still “a lot of sargassum” in the Tropical Atlantic. “The good news is the sargassum season for Florida is very likely over for this year. But for the Eastern Caribbean, it’s not over yet,” he said.

Last month, the amount of sargassum in the Gulf of Mexico dropped by a staggering 75 percent, Dr. Hu and colleagues at the University of South Florida Optical Oceanography Lab noted in a bulletin published last week.

Sargassum — a type of macroalgae that is naturally abundant in the Sargasso Sea — has long been seen floating in mats across the North Atlantic. But in 2011, scientists began to observe extraordinary accumulations of the seaweed extending in a belt from West Africa to the Caribbean Sea and Gulf of Mexico, according to a 2019 study.

The immense bloom has continued to grow almost every year.

In March, scientists said they expected the blob to come ashore in Florida and elsewhere along the Gulf of Mexico. At sea, the blob provides habitat for fish, crabs and sea turtles, but on land it began to rot, emitting toxic fumes and fouling the region’s beaches over the busiest summer months.

“Avoid touching or swimming near seaweed,” the city of Deerfield Beach, Fla., warned residents on Facebook, noting research that suggested the bloom may contain bacteria. In Miami-Dade County, tractors with rear-mounted blades traversed 17 miles of coastline daily to “mix and blend” the seaweed, the county said.

Like other plants, sargassum has a natural life cycle, Dr. Hu said, and scientists expected it to decrease in the Gulf of Mexico by around September.

But scientists don’t know why the decline was so rapid. One theory is that strong winds caused by recent tropical storms could have dissipated the sargassum into smaller clumps, or sunk it to the ocean floor, Dr. Hu said, making it hard to see from a satellite. “There could be other reasons, we just don’t know,” he added.

While floating sargassum can benefit marine animals by providing shade and shelter, it begins to die once it comes ashore, degrading water quality and polluting beaches, scientists say. The decaying algae also releases hydrogen sulfide, a colorless gas that smells like rotten eggs, and can cause respiratory problems in humans. Some types of bacteria may colonize rafts of sargassum and marine plastics, which, once ashore, can pose a risk to public health, according to a recent study published by researchers from Florida Atlantic University.

Sarah Collier, who works for Seabird Key, a private island vacation rental in Key West, said that her community had been on edge since hearing of the sargassum blob earlier this year. “It’s such a relief,” she said of news that it was dissipating in the Gulf of Mexico, noting that she had not personally seen more seaweed than usual.

But while tour operators, holidaymakers and locals in Florida are enjoying sargassum-free beaches, the situation is not as rosy further south.

The beaches of the Dominican Republic, Haiti, Puerto Rico, the U.S. Virgin Islands, British Virgin Islands, the Lesser Antilles Islands, Barbados and Trinidad, will likely remain threatened by seaweed for the next one to two months, Dr. Hu said.

He and his colleagues predict that over the next two to three months, the amount of sargassum in the Gulf of Mexico will remain minimal, and that it will either decrease or remain stable in the Caribbean Sea. While that’s good news for the residents of Florida’s east coast, as well as the Florida Keys, sargassum is still likely to wash ashore in parts of the Caribbean, they said in the bulletin, noting that it was “difficult to predict exact timing and location for individual beaching events.”