Giant Blob of Seaweed Is Ready to Ruin Summer Vacations

For much of the year, an enormous brown blob floats, relatively harmlessly, across the Atlantic Ocean, providing shelter and breeding grounds for fish, crabs and sea turtles. Spanning thousands of kilometers, it can be seen from space.

But scientists say that in the coming months, the blob — a tangled, buoyant mass of a type of seaweed called sargassum — is expected to come ashore in Florida and elsewhere along the Gulf of Mexico. It will then begin to rot, emitting toxic fumes and fouling beaches over the busy summer months.

The seaweed, which can also cause pollution and threaten human health as it decays, has begun to creep onto the shores of Key West, Florida. In Mexico, “excessive” levels of the seaweed were recorded in February, choking beaches south of Cancún.

“You can’t get in the water,” Leonard Shea, a travel YouTuber, said in a recent video from the resort town of Playa del Carmen, Mexico. “It’s not an enjoyable experience.”

Sargassum 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 bloom has continued to grow almost every year.

While scientists are trying to understand exactly why and how the mass, known as the great Atlantic Sargassum belt, is expanding, it appears to be seasonal — coinciding with the discharge of major waterways, including the Congo, Amazon and Mississippi rivers.

The runoff from these sources feeds the bloom with nitrogen and phosphorus, said Brian Lapointe, a research professor at Florida Atlantic University who studies sargassum. Fossil fuel emissions and the burning of biomass — such as trees after deforestation — also produce nutrients that could be helping the sargassum to grow.

In January, scientists measured the largest bloom for that month on record. According to the National Oceanic and Atmospheric Administration, the sargassum blooms will disrupt Caribbean waters into midOctober.

When the sargassum comes ashore and begins to die, it degrades the water quality and pollutes beaches. It can also choke vital mangrove habitats and suck oxygen out of the water. The decaying algae also releases hydrogen sulfide, a colorless gas that smells like rotten eggs, and can cause respiratory problems in humans.

Last summer, the U.S. Virgin Islands declared a state of emergency, after “unusually high amounts” of sargassum piled up on its shores, affecting a desalination plant on the island of St. Croix. And in 2018, after a mass bloom that sprawled across about 8,850 kilometers in the Atlantic Ocean, doctors on the Caribbean islands of Guadeloupe and Martinique reported thousands of cases of “acute” exposure to hydrogen sulfide, according to a study published that year.

In Mexico, the navy has been recruited in recent years to scoop the seaweed from the ocean, and rake the country’s beaches.

Meanwhile, some entrepreneurs have proposed transforming the seaweed into animal feed, fuel or construction materials. But Dr. Lapointe warned that sargassum contains arsenic, which, if used in fertilizer, could potentially make its way up the food chain.

The most immediate threat, however, is to tourism. “It’s having catastrophic effects,” he said.