Before spring break season is over, beaches across the Gulf Coast will begin to stink.
By mid-April, as businesses in South Florida and across the Gulf Coast juggle an influx of vacationers, the region’s beaches will likely face another unwelcome visitor: enormous mats of rotting sargassum seaweed.
The leading edge of several thousand mile long train of floating sargassum is already beginning to pile up on the beaches of resorts in Mexico’s Yucatan peninsula, along Caribbean islands and in Key West, Fla.
But with an estimated 13 million tons of seaweed out there, those early arrivals are just “the tip of the iceberg,” said marine biologist Brian LaPointe.
LaPointe runs one of the nation’s leading seaweed labs, at Florida American University. He said the bloom wasn’t isn’t a cause for “panic” — but added that for a bloom to be “that big, that early, just doesn’t bode well.”
But much about the blob, from its origins to its ultimate destination, remains under active scientific debate.
Here is a list of things we know about the threats posed by the seaweed bloom — and what we still don’t.
What is washing ashore?
Clumps of seaweed that have broken loose from a far larger belt of floating sargassum that has established itself in the mid-Atlantic over the past decade.
Unlike kelp or eelgrass, which are types of seaweed fixed by their roots firmly to the seafloor, sargassum is a form of free-floating seaweed that is most highly concentrated in the Sargasso Sea.
The Sargasso Sea is the world’s only sea not bounded by any land. Instead, it is a region of the Atlantic surrounded by swirling ocean currents that have long trapped ships, ocean detritus and huge populations of sargassum.
Those swirling currents had long kept sargassum in — until about a decade ago, when the current sargassum crisis began, and cities like Miami and Fort Lauderdale began having to spend tens of millions per year on dealing with it.
What caused the seaweed crisis?
The acute cause seems to be two years of “unprecedented” wind events — in 2009 and 2010 — which blew record quantities of sargassum out of its home waters.
While clumps of sargassum had always escaped to wash up on Atlantic and Caribbean beaches, there had never been enough before to establish a stable population, said Rick Lumpkin, who runs the physical oceanography division at the National Oceanic and Atmospheric Administration (NOAA) observation lab in Miami.
Other factors helped the new sargassum colony to establish itself in its new home after 2011, Lumpkin said. Like any plant, sargassum benefits from heat and fertilizer. The heat came from rising ocean temperatures caused by climate change. And the fertilizer came from, well, fertilizer.
Many scientists link the sargassum blooms to the overuse of fertilizers in the watersheds of the great rivers that pour into the Atlantic: the Mississippi, Orinoco, Congo and Amazon.
Those fertilizers — which those rivers’ powerful outflow push far out to sea — are used in industrial agriculture to make up for soil depleted by tropical deforestation or season after season of growing the same crops.
But particularly in the case of agriculture in the Amazon and Orinoco, that nutrient runoff also ended up boosting the population of floating algae in the mid-Atlantic and the Gulf of Mexico.
Other factors likely contributed, Lumpkin said. Churning winds off the west coast of Africa pulling nutrient rich waters to the surface and the colossal Sahara Desert dust storms also seem to have helped played a role.
These led to huge sargassum blooms across the Atlantic in 2018 and 2019 — blooms that may even have been bigger than this year’s.
But what’s unusual this year, however, is how “how much sargassum has entered the western Caribbean,” Lumpkin said.
Where did sargassum seaweed come from?
We don’t really know. Scientists only detected the incoming sargassum belt once it had already congealed into a mass big enough to be visible from satellites.
But in the broadest sense, the sargassum beginning to wash up on Caribbean beaches came from what scientists call the Great Sargasso Belt — that new colony that established itself in 2011 outside the Sargasso Sea.
The western extremity of that belt, sheared off by currents, is now on its long collision course with the Caribbean.
Is the seaweed dangerous?
Not directly — at least not to people.
It can even make the ocean marginally safer, said Stephen Leatherman, an oceanographer at Florida International University specializing in beach health.
The thick drifting mats of sargassum are so thick that they block the action of riptides that could otherwise pull people out to sea, Leatherman said.
But few would want to swim among a sargassum bloom anyhow. As it decays on beaches and among mangroves, it releases noxious-smelling chemicals like hydrogen sulfide — a malodorous chemical reminiscent of rotten eggs that will deter beachgoers.
Leatherman described it as “a floating forest,” with its own unique ecosystem of hundreds of other plants and animals.
One particularly dangerous resident of that forest is the highly venomous Portuguese man-of-war.
But as the seaweed is pulled ashore, all that stuff goes along with it, creating a host of new problems. Fish, crabs and jellyfish that move among the sargassum get washed ashore and rot on the beach.
NOAA’s Lumpkin recounted pulling a little crab out of the mat of sargasso that had washed ashore near Miami. He recalled thinking, “That guy had gone on a really long ride.”
Could a sargassum colony — or the species that ride along with it — eventually establish the kind of permanent presence in the Caribbean that it has in the wider Atlantic?
“That’s a fascinating question,” Lumpkin said. “We don’t know the answer.”
Is it killing off coral?
The overwhelming danger posed by the massive seaweed blooms is ecological. As the floating plants pile atop each other, they form layered mats several feet deep and sometimes stretching for miles — so large, Leatherman said, that sea turtles sometimes get stuck underneath them and drown.
The drifting mats also can block all light from reaching underwater plants beneath, posing a deadly threat to coral reefs.
The rise of sargassum blooms has happened alongside a wider loss of stony corals in the Florida Keys, LaPointe said, although he stressed that the cause still hadn’t been proven.
And as the floating seaweed breaks down among the mangroves that ring the Gulf of Mexico, hydrogen sulfide and other byproducts suck oxygen out of the water column, creating dead zones that suffocate fish and “everything underwater that can’t swim away,” LaPointe said.
To make things even worse, those rotting bodies add to the stink.
Where is the seaweed headed and when will it get there?
Likely the beaches of the Gulf Coast and islands of the Caribbean Sea, but it is hard to say much beyond that, experts told The HIll.
That belt is “not one single mass — it’s lots and lots of clumps and passages that get sheared off,” said Lumpkin of NOAA.
Some of those clumps have gotten sheared off from the larger floating forest, and are washing ashore in Cuba, the Yucatan and South Florida — and in particular the three east-facing counties of Miami-Dade, Broward and Palm Beach.
“But the mass of it is still heading into the Gulf, and we’re going to see a steady increase in the amount passing along,” Lumpkin said.
In its current trajectory, a long train of seaweed is following the Caribbean current west, where it is catching the loop current around the Yucatan Peninsula and threading the 90-mile strait between Cuba and Florida, after which the Gulf Stream will carry it back into the Atlantic.
But while “that’s the pathway, wind can blow it off course,” Lumpkin said — a north wind could blow it against Cuba, a south wind against the Florida Keys.
LaPointe of Florida American returned to the metaphor of a hurricane far from land: the trajectory is too uncertain to warn any specific resort or beach community in time to avoid a wave of disappointed guests.
That could change starting this July. LaPointe’s lab has gotten a grant from the Environmental Protection Agency to begin work with the Florida National Marine Sanctuary to begin trying to track specific sargassum clumps with enough detail “to tell hotels what’s coming for you.”
What will places do about it?
In the past, Miami plowed algae under the beaches, but it has become such a problem now that the beach sand can no longer contain it, Leatherman said.
On some of the rockier Caribbean islands, where pocket beaches lie inside a protective bottleneck of peninsulas, the solution to sargassum is simple: stretch a boom (of the sort that might be used to stop an oil spill) across a narrow bottleneck.
And in Mexico’s Yucatan, an entrepreneur calling himself Mr. Sargasso has begun selling bricks made from clay compacted with the seaweed pulled from beaches — which he calls “sargablocks.”
But Florida’s long, flat beaches can’t be protected by booms, and the state has no brick industry.
Fort Lauderdale has disposed of sargassum for years by dragging it out to giant parking lots, laying it in sheets, letting the rain wash off the salt, and then composting it into mulch, which it gives out for free. It’s a solution that, in essence, repurposes uses the nutrients running off the denuded Amazon to nourish Florida lawns and crops.
But with the seaweed bloom arriving in such quantities so early in the season — months before Florida’s summer rains begin — composting may not be an option, Leatherman said.
That will likely give the region’s cities little choice but to drag it to the landfills at a cost of $35 million to $45 million per county.