A giant blob of seaweed twice the width of the continental United States is headed for the shores of Florida and other coastlines throughout the Gulf of Mexico.
The blob is threatening to dump smelly and possibly harmful piles across beaches, dampening tourism season.
Sargassum — the specific variety of seaweed — has long formed large blooms in the Atlantic Ocean, and scientists have been tracking massive accumulations since 2011.
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But this year's bloom could be the largest ever, collectively spanning more than 8,047 kilometres from the shores of Africa to the Gulf of Mexico.
This year's sargassum bloom began forming early and doubled in size between December and January, said Dr Brian Lapointe, a researcher at Florida Atlantic University's Harbor Branch Oceanographic Institute.
The mass "was larger in January than it has ever been since this new region of sargassum growth began in 2011," he told CNN International's Rosemary Church.
Traveling west, the blob will push through the Caribbean and up into the Gulf of Mexico during the summer.
The seaweed is expected to show up on beaches in Florida around July, Lapointe said.
"This is an entirely new oceanographic phenomenon that is creating such a problem — really a catastrophic problem — for tourism in the Caribbean region, where it piles up on beaches up to 121 cm deep," Lapointe said.
Here's what you should know about why these masses happen and how they affect both humans and ocean life.
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What is sargassum?
Sargassum is a catchall term that can be used to refer to more than 300 species of brown algae, although Sargassum natans and Sargassum fluitans are the two species most commonly found in the Atlantic.
When adrift at sea, the algae can have upsides for ocean life.
"This floating habitat provides food and protection for fishes, mammals, marine birds, crabs, and more," according to the Sargassum Information Hub, a joint project among various research institutions.
"It serves as a critical habitat for threatened loggerhead sea turtles and as a nursery area for a variety of commercially important fishes such as mahi mahi, jacks, and amberjacks."
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Is sargassum safe?
The problems with sargassum arise when it hits the beaches, piling up in mounds that can be difficult to navigate and emitting a gas that can smell like rotten eggs.
Sargassum can also quickly turn from an asset to a threat to ocean life.
It comes in such "large quantities that it basically sucks the oxygen out of the water and creates what we refer to as dead zones," Lapointe said.
"These are normally nursery habitats for fisheries… and once they're devoid of oxygen, we have lost that habitat."
Sargassum can be dangerous to humans, too, Lapointe added.
The gas emitted from the rotting algae — hydrogen sulfide — is toxic and can cause respiratory problems. The seaweed also contains arsenic in its flesh, making it dangerous if ingested or used for fertiliser.
"You have to be very careful when you clean the beaches," Lapointe said.
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Why is there a sargassum problem?
Just like plants and crops on the ground, the proliferation of seaweed can shift year to year depending on ecological factors, affected by changes in nutrients, rainfall and wind conditions, said Dr Gustavo Jorge Goni, an oceanographer at the National Oceanic and Atmospheric Administration's Atlantic Oceanographic and Meteorological Laboratory.
Sea currents are also influential on sargassum's growth and how much it accumulates, Goni added. Phosphorus and nitrogen in the sea can serve as food for the algae.
Those elements can be dumped into the ocean from rivers, which gain concentrations of phosphorus and nitrogen from human activities such as agriculture and fossil fuel production, according to the US Environmental Protection Agency.
For now, researchers are looking into ways to thwart the seaweed's impact on beaches, possibly by sinking it to the bottom of the ocean or harvesting it for use in commercial products such as soap, Goni said.
Goni cautioned that research into these sargassum accumulations is new, and it's likely scientists' understanding of how the algae grows will shift over time.
"Whatever we believe we know today," he said, "it may change tomorrow."
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How does the buildup affect travel?
Before traveling to coastal areas this spring or summer, research whether sargassum is at your destination or might show up there, Lapointe said. Plan ahead so your vacation won't disappoint.
There are sargassum Facebook groups, with members posting about what they recently saw on beaches, Lapointe said.
"It's already affected the travel industry," he said.
Unfortunately, sargassum can build up overnight so you might not be able to predict its effects on a trip, Lapointe said.
"This is why we're trying to work on these early warning systems — high resolution in coastal areas, which takes higher-resolution satellite imagery to do a better job showing what's actually coming into a beach within the next 24 or 48 hours," he added.
Satellite images from within the last week show sargassum isn't an amorphous mass moving across the ocean, but rather teardrop-shaped blobs trailed by long, thin strands of seaweed.
Within the last week, sargassum blobs have been spotted about 346 kilometres from Guadeloupe, in between the islands of St. Vincent and Bequia, 914 metres off Martinique and off the coast of Key Largo, Florida.
How is sargassum cleaned up?
Mounds of algae accumulated on beaches cost millions of dollars to clean up, and removal efforts can also harm marine life, according to the Sargassum Information Hub.
In Barbados, locals were using "1,600 dump trucks a day to clean the beaches of this seaweed to make it suitable for tourists and recreation on the beaches," Lapointe said.
In shallow waters, sargassum can be removed using fishing nets towed by light boats or by hand, according to the Dutch Caribbean Nature Alliance.
In the US, cleanup is often done with Barber beach rakes pulled by a tractor, Lapointe said. But once there's an accumulation of more than a foot of sargassum, the rakes don't work as well, he added. That's when front-end loader dump trucks can be helpful, but they can be harmful to beach health.
Use of dump trucks to remove sargassum can become problematic.
"Oftentimes you have sea turtle nests on beaches that are being run over by tires of this heavy equipment crushing the eggs," Lapointe said.
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What happens if sargassum isn't removed?
If sargassum isn't cleaned from beaches or is used as fertiliser, the arsenic in its flesh could leach out into groundwater, which could be a health hazard for humans, Lapointe said.
An excessive amount of rotting sargassum can also support the growth of fecal bacteria.
And in 2018, a massive bloom that ended up on the beaches of South Florida coincided with the biggest red tide ever seen on that coast, Lapointe said.
Red tides occur when toxin-producing algae blooms grow so out of control they discolor coastal waters. Red tide organisms can live on sargassum and be transported by it.
The toxins in red tides can harm marine life, and sargassum buildup on beaches can prevent sea turtle hatchlings and adults from getting to sea, Lapointe said.
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Will this happen every year?
Experts don't know whether a sargassum bloom of this size will happen every year, Lapointe said.
"It's hard to project because we don't know everything we need to know about the drivers (behind this)," he said.
"We know it's variable from year to year and that the trajectory is generally going upwards. So based on what we've seen in the past, we're thinking we could continue to see this worsen for years to come. What will it be like in 10 years? Will it be double the size it is now?"
More funding to do the research that could answer these questions is needed, he added.
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