31 million tons of toxic seaweed is creeping toward beaches in Florida and around the Caribbean

CNN

15-05-2025

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The Atlantic Ocean has a toxic seaweed problem.
Floating in brown islands of algae, this year's sargassum bloom has already broken its own size record by millions of tons — and the growing season isn't done yet.
Now stretching across some 5,500 miles of ocean, the annual bloom is more than just an eyesore: Sargassum hurts ecosystems and economies wherever its overgrown arms reach. And they are spreading into Florida's waterways, coating marinas and beaches in the Miami area.
'Sargassum goes from being a very beneficial resource of the North Atlantic to becoming what we refer to as … a harmful algal bloom, when it comes ashore in excessive biomass,' said Brian LaPointe, a research professor at Florida Atlantic University's Harbor Branch Oceanographic Institute.
'What we have seen since 2011 are excessive inundation events all around the Caribbean region, the Gulf, as well as the South Florida region,' explained LaPointe, who has studied the seaweed for decades.
As it rots on shore, it emits harmful gases— an infamous stench.
It's a blight on beaches that repels tourists during the high-travel season, ultimately hurting towns that rely on tourism to fuel their economy.
Rising ocean temperatures due to human-caused climate change have spurred this sargassum surplus, supercharging the seaweed. In April, the University of South Florida estimated this year's bloom is already at 31 million tons — '40% more' than the previous record from June 2022, according to LaPointe.
The sargassum bloom itself is not a new phenomenon. It's long provided a home to species from sea turtles to fish as winds and tides push it from the coast of West Africa toward Brazil, up into the Caribbean and the Gulf of Mexico.
'Sargassum has been around for eons. Colombus ran into it right in the Sargasso Sea,' La Pointe told CNN. 'But what we are seeing now is above and beyond what we had historically.'
Sargassum's growth is also being driven by an excess of nitrogen in the water, LaPointe said — and that's a key factor behind this year's monster bloom.
Some nitrogen may be coming from the atmosphere, carried in the air from the burning of fossil fuels or dust from the Sahara Desert.
But there's one major source: agricultural fertilizers. Used in the American heartland as well as in the Amazon basin where there's been rapid deforestation for farming, the nitrogen-rich fertilizers are likely making their way into the Mississippi and Amazon Rivers as runoff, which then carries it into the Atlantic.
And the Amazon basin has notched its lowest water levels on record amid two straight years of extreme drought — the worst since records began being kept in 1950.
'What happens when you have a severe drought in the world's largest watershed? You get all this organic matter that dries up. Plants dry up and die. And then, when the rain hits, what happens? All those nutrients wash out,' LaPointe hypothesized, adding that 'first flush' events like this are full of concentrations of nitrogen and phosphorus at peak levels, which go on to feed the bloom's growth explosion.
Unsurprisingly, the constant inundation of stinky, brown seaweed along the coast is not good for economies driven by tourism.
This year's bloom has already been making an appearance along Florida's east coast, from the Keys to Saint Augustine, according to reports on a sargassum monitoring site, and southeastern Florida could see more in the coming weeks. The unwanted algae has also been spotted in popular destinations from Mexico to Barbados and farther south.
'It's not good for the environment, because what you're smelling is hydrogen sulfide gas which is toxic,' LaPointe said.
In some places, the beaches are cleared of seaweed from sunrise to sunset — an expensive endeavor combing up sargassum that inundates the coastline with every wave. Some of the machinery used to clean the beaches adds its own pollution to the scenic environment, too.
'Resorts have gone out to their beaches with heavy equipment like front-end loaders, bulldozers, dump trucks to try to remove the sargassum to make those beaches available,' LaPointe said, as tourists don't want mounds of sargassum to mar their tropical views. 'The tourists check out, and they don't come back.'
This is a major tangle for places like Mexico's Yucatán Peninsula, which exists in the heart of the sargassum belt between the Gulf of Mexico and the Caribbean Sea.
The problem has become so pronounced along the Riviera Maya, Mara Lezama, the governor of Quintana Roo, a state in the peninsula, has taken to social media to say her state is working with the Mexican Navy to collect the seaweed in the water while also installing a nearly 6-mile barrier in the water to protect Quintana Roo's Mahahual, Playa del Carmen and Puerto Morelos beaches.
The barriers, which are similar to booms that contain oil spills, are just over a yard deep and are designed to keep the seaweed from reaching the coast.
As it approaches, the decaying sargassum can also create health problems for animals and humans.
'When it arrives to the coastal area, it creates a shadow from the sun, so everything that is below — all the life is not getting sunlight. So, it starts to affect the ecosystem, coastal ecosystem, and many things die,' said Christian Appendini, professor at the Universidad Nacional Autónoma de México.
'Then when it gets over the beach, it starts to decompose. And when it decomposes, it releases all the contaminants it has.'
Ammonia is another problem emitted by the decaying seaweed, LaPointe noted. The chemical compound 'strips the oxygen out of the waters along our coastal ecosystems like mangroves, coral reefs, seagrass beds,' he said.
'When you see the mass inundation events along the beaches, say in the Mexican Riviera, for example, you don't see many fish or crabs,' LaPointe continued. 'If you do, they're probably dead because there's no oxygen in that water.'
With sargassum cementing itself as an ongoing problem, some are looking into putting the seaweed to good use, instead.
Appendini says research is ongoing to find ways to use the sargassum for biofuel, building bricks, or as membranes for cleaning water, since it is particularly absorbent.
'They absorb all the heavy metals and contaminants in the water,' he told CNN. 'That's also why sargassum can be very toxic, because when it's drifting in the ocean it's just assimilating all the toxic elements in the ocean like cadmium, arsenic and other minerals and elements.'
There's also the possibility of carbon sequestration by sinking the excessive biomass to the bottom of the ocean. And there's interest in possibly using sargassum to replace one of the globe's other problems: plastic.
'If we could harvest this sargassum and produce this biodegradable product that could replace single use plastics, that would begin to restore the oceans regarding the serious plastic pollution that we're seeing,' LaPointe said.
As the sargassum situation remains pervasive for more than a decade now, Appendini said the record-breaking bloom should make the world pay attention.
'I think the sargassum blooms are like a warning that we need to be more mindful of how we are developing in this world,' Appendini said. 'We need to change … how we do things.'
CNN's Norma Galeana contributed to this story.