The seaweed invasion

It started washing ashore in the Caribbean eight years ago, the smelly, yellow-brown seaweed known as sargassum. Then, just as mysteriously, it disappeared.

Now the avalanche of algae is becoming an annual event — with increasingly dire consequences for the environment and economy of the region.

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This year, tons of the seaweed have fouled white-sand beaches from Miami to the Mayan Riviera. Local officials are fretting about economic fallout. Scientists are warning of harm to the largest reef system in the Americas.

In some places, the famously clear Caribbean water turns so murky it resembles gas-station coffee.

“It’s a little bit disgusting,” said Sara Fargeas, 29, a French tourist in Tulum with her husband, Alexandre. They’re at Playa Paraiso — but it’s anything but paradise. The sargassum stinks. And the water is full of it.

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In the last three months, more than 57,000 tons of sargassum have been raked and scooped up on Mexico’s Caribbean coast alone. Even the Mexican navy has joined the battle, sending ships to fish the seaweed out of the water.

The sargassum is highly unpredictable, moving with the winds and currents. In late July, the onslaught of seaweed suddenly eased along the Mexican coast, disappearing in some areas. But by mid-August, it was back.

Sargassum has been around at least since the days of Christopher Columbus. But until recently, it largely stayed in a part of the North Atlantic known as the Sargasso Sea.

Scientists announced this year that they’d managed to map the explosive new algae bloom. The sargassum had formed a 5,500-mile archipelago spanning the Atlantic, from West Africa to the Gulf of Mexico.


In the spring and summer, beaches emptied. Swimmers found the seaweed too slimy. And the decaying, blackened sargassum on shore emits an odor like rotten eggs. “Who’s going to eat here with the smell?” asked Manuel Vasquez, 36, a waiter in a beachside restaurant in Playa del Carmen.

The state of Quintana Roo is home to Cancun and the 70-mile-long Riviera Maya. It pulls in around $14 billion a year from visitors — more than half of Mexico’s tourism earnings. Officials have declared a state of emergency, describing sargassum as an “imminent national disaster.” They’re spending $30 million this year to remove it. 

The damage could go well beyond the tourist industry. Scientists say the coastline itself could be in jeopardy. That’s because sargassum is weakening the coral reefs that serve as a buffer to waves and the seagrass that anchors the sand. Without those protections, hurricanes could take increasingly big bites out of Caribbean beaches.

Scientists are only beginning to figure out what’s causing the massive sargassum bloom. They suspect that nutrients such as nitrogen and phosphorus, which are superfood for sargassum, are increasingly flowing into the ocean. This could be coming from fertilizer washed into the Amazon from farms in Brazil. But nature could also be contributing  — with winds churning up nutrient-rich material from the ocean floor.

In the open ocean, sargassum can be beneficial, serving as a habitat for fish, crabs and turtles. But once it approaches the shore, everything changes.

“It’s a major socio-ecological catastrophe,” says Jesús Ernesto Arias, who analyzes Mexico’s coral reefs at CINVESTAV, a government-backed research center. 

The mats of algae block the light that corals need to grow. And as the sargassum decomposes, it releases hydrogen sulfide and ammonium. That kills fish and crabs. Scientists suspect the seaweed is contributing to the spread of White Syndrome, a disease that’s wreaking havoc on coral reefs in Mexico. And it’s destroying seagrass — an underwater meadow that holds sand in place.

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More than 10,000 workers and volunteers have been raking, scooping and hauling the sargassum from Mexico’s beaches this year.

The navy has deployed two ships to capture the seaweed in giant nets. But out at sea, the algae often moves in long, thin strands. So it’s difficult to trap.

Mexican officials have boasted of the success of their cleanup operations. By early August, many beaches were clean.

President Andrés Manuel López Obrador said he hoped that soon “we’ll be able to say that the problem has been solved.”

But scientists say much of the algae was simply swept to other areas by climatic conditions.

As the great mass of algae in the Atlantic dies off each fall, it leaves remnants that form the basis for another big bloom the following season.

Chuanmin Hu, an oceanographer at the University of South Florida, led the team that for the first time mapped the seaweed. He says the problem is far from over.

“Most years in the next decade we will see a lot of sargassum,” he said.