Why Is a Huge Seaweed Blob Headed for Florida?

The Great Atlantic Sargassum Belt may be a nuisance for beachgoers this summer, but millions of marine creatures call this seaweed home.

By Stephen C. George
Apr 1, 2023 6:00 PMJun 5, 2023 7:24 PM
Sargassum en masse
(Credit: StephanKogelman/Shutterstock)


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You’ve likely been hearing it in the media for weeks now: A giant mass of seaweed, some 5,000 miles wide, is headed for Florida, where it will engulf beaches with tons of icky and decaying seaweed, emitting a noxious gas all the while. And this year, it's been reported, the seaweed is bringing a potentially deadly surprise with it when it makes landfall.

Inevitably, stories about this massive swath of seaweed tend to call it a blob, stirring up images of some 1950s B-movie monster coming to absorb the Sunshine State, beaches, tourists, retirees and all. In reality, the so-called blob, while big, is well-known to scientists and has been carefully studied for more than a decade. And it won’t just affect Florida. Here’s what we know about it — and the nasty bacteria that public-health experts are now warning beachgoers about this summer. 

It’s Not a Blob, It’s a Bloom

A 2018 image of the belt, which is large enough to be seen from space. (Credit: NASA/Earth Observatory)

First, let’s call the blob what it is: an algal bloom. In this case, it's made from a type of brown algae, a seaweed known as sargassum.

This particular conglomeration of sargassum even has a formal name: the Great Atlantic Sargassum Belt, stretching from Africa to the Gulf of Mexico. Although considered the largest seaweed bloom on the planet, it’s not one giant, uninterrupted bloom, but several, spread out across the Atlantic.

Read More: How Did 5,500 Miles of Seaweed Spread Across the Atlantic?

What is Sargassum?

Sargassum itself has been floating in the ocean for a long time, around 30 million years. Unlike other types of seaweed, which can attach itself to the sea floor, sargassum floats wherever wind and tide take it, and is kept on the surface of the water by a number of gas-filled pods.

Sargassum is also unique in that it is holopelagic, meaning that it goes through its entire lifecycle on the open sea, rather than reproducing or originating from the ocean floor. Despite its ability to float, sargassum can be quite heavy — the current belt has been estimated to weigh anywhere from 6 to 10 million tons. 

The Origins of Sargassum Seaweed

(Credit: Public domain/Wikimedia Commons)

Sailors have known about sargassum for hundreds of years — Christopher Columbus is said to have discovered it in the 15th century during his voyages (or at least he was the first to document it in his journals). The word derives from a Portuguese term for a similar-looking plant. Although types of sargassum can be found in just about every ocean on the planet (except the Arctic Ocean — too cold), the seaweed generally originates in the Sargasso Sea, a large area of the Atlantic whose borders are defined by ocean currents rather than any land boundaries.

In the days of wooden sailing ships, this sea was particularly feared. Columbus, who is also credited with discovering the Sargasso Sea, noted concerns that the massive clumps of floating seaweed might trap a ship or hide dangerous shallows and reefs. While it’s possible that seaweed strands could foul a ship’s rudder, in general, the innocuous mats of sargassum posed little threat to a large ship.

The real danger for mariners was part of the Sargasso Sea that included the infamous doldrums, an area dreaded for having little to no wind. A becalmed sailing ship could be stuck on the sea for weeks or months, leaving a stranded crew with dwindling food and water supplies. But you can’t blame seaweed for that. 

Sargassum Isn’t Always Bad

Sargassum serves as a habitat for as many as 70 different species, including crabs. (Credit: Jaimie Tuchman/Shutterstock)

Sargassum provides a unique ecosystem for a variety of marine creatures, according to the National Oceanic and Atmospheric Administration. Baby sea turtles love the stuff and use the large mats as a kind of floating nursery. As many as 70 other species, including types of shrimp, crabs, fish and even birds, have adapted specifically to the floating islands of sargassum as a habitat.

Like all seaweed, sargassum is also excellent at sequestering carbon dioxide and producing oxygen via photosynthesis. That’s all well and good for the environment. Unfortunately, other aspects of the environment are contributing to the seaweed’s massive growth, creating new concerns.

Why Is Sargassum a Problem Now?

(Credit: StephanKogelman/Shutterstock)

Thanks to climate change, even fractionally increased global temperatures translate into warmer water, accelerating algal growth. Meanwhile, changing ocean currents impact how far and wide the sargassum belt will spread. Since 2011, scientists have been monitoring the growth of the belt, which is big enough to be seen from space.

This isn’t even the first time that “giant blob of seaweed” stories have spun through the news cycle. A few years ago, several outlets, including Discover, began featuring news of the expanding belt, which has been as wide as 5,500 miles, clogging beaches throughout the Caribbean, and putting a serious dent in the tourism revenue of many resorts.

Read More: A Smelly Seaweed is Crossing the Atlantic Ocean in Massive Clumps

The same thing will happen again this summer. Already sargassum is being reported on beaches and will continue to wash up on shores throughout Florida and the Gulf of Mexico.

What does that mean for humans? Well, if you live in or plan to visit an area within range of the drifting belt, you may well see lots of sargassum landing on beaches, to a depth of several feet.

It would be foolhardy for anyone to wade through it, and you really shouldn’t try. Never mind that you might get stung by any number of tiny sea creatures still inhabiting the seaweed: This year, public-health officials have issued a warning that vibrio bacteria have been detected in significant quantities in the sargassum. While the bacteria is common in coastal waters, concentrated exposure to it — say, by pushing your way through some sargassum and getting infected through a cut or wound — can lead to serious illness. In a worst-case scenario, vibrio exposure can cause necrotizing fasciitis, the dreaded "flesh-eating bacteria" type of infection that could lead to disfigurement, amputation of affected limbs and even death.

Such massive amounts of sargassum can cause other problems, too, as the giant mats of seaweed have the potential to absorb all of the oxygen from large areas of water, creating dead zones where other marine life can’t live. And once the massed piles of sargassum start to accumulate on the beach, it all begins to decay.

You can’t spell sargassum without gas and this seaweed will surely deliver it when it makes landfall. Decomposing sargassum emits ammonia and hydrogen sulfide which, aside from having a supremely unpleasant odor, could also pose health risks in concentrated amounts. At the very least, the rotting seaweed can cause plenty of irritation to your eyes, nose and throat — yet another reason to avoid going near it.

Sargassum Solutions

(Credit: Kamira/Shutterstock)

Already, resorts from Florida to the Caribbean have spent millions of dollars each of the past few years, deploying personnel and equipment to remove stinking sargassum piles from once pristine beaches.

But to truly solve the problems posed by such a massive amount of seaweed, what really needs to happen is sustained efforts not only to combat climate change but also to reduce the amount of nutrients that flow into the ocean from places like the Amazon River.

Run-off rich in nitrogen and phosphorus from agricultural and other industries is believed to be a major contributor to sargassum’s gargantuan growth. Until that can be remediated or reduced, you can expect future stories about the Great Atlantic Sargassum Belt, when it may well be even bigger than it is today.

Read More: Algal Blooms May Have Threatened These Ancient Cities

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