Beach seaweed: Sargassum is the new mess of algae taking over shorelines and ruining vacations.

This past April, scientists started to understand that this year’s “blob” was a lot larger than it should be. It was even moving rapidly through the Atlantic Ocean.

The blob they were watching was the sargassum bloom—an annual event that sends mats of light-brown algae floating across the Caribbean, through the Gulf of Mexico, and eventually, in some cases, up the East Coast of the United States. They were studying satellite images of the equatorial Atlantic Ocean and began to notice that a 5,000-mile-long belt of sargassum, holding over 20 million tons of the seaweed, was drifting through the water—the largest bloom ever recorded so early in the year.

What that meant wasn’t exactly clear. To some, the algae represented a looming nuisance that threatened tourism. To others, it was a full-blown public health crisis. Some reporting suggested that the sargassum hosted high levels of a flesh-eating bacteria, prompting the Florida Department of Health to issue an advisory warning people not to swim in excessive amounts of seaweed. A doctor at Baylor College of Medicine warned that the seaweed can emit hydrogen sulfide as it decomposes—a chemical asphyxiant that can lead to nausea, eye irritation, and, in the most extreme cases, death (although she also noted that on the open-air beaches, where the seaweed would mostly wash up, those risks were low).

For communities in the Caribbean and the Gulf of Mexico, the near-term impacts were easy to predict. A lot of sargassum would pile up on beaches, where it would decompose, attracting flies and causing a big stink. Keeping beaches sargassum-free has become an annual chore at resorts in destinations like Tulum and Punta Cana—work that requires massive nets, bulldozers, backhoes, flatbed trucks, shovels, pitchforks, and a lot of time and labor. It’s become such a burden that the Mexican navy has even been building barriers in the ocean to keep the algae offshore, and deploying special sargassum patrols to intercept it before it reaches shore. The sargaceros who clean the algae up in Mexico work long shifts in punishing sunlight, and make the equivalent of $260 a month shoveling sargassum off the beaches and hauling it into trucks that bring it to landfills. All signs indicated that this would be a particularly brutal season.

The biggest groans, however, didn’t come from the sargaceros tasked with actually moving those slimy masses. Vacationers were quick to denounce the Sargassum Summer, turning to resources such as sargassummonitoring.com and the Florida Department of Environmental Protection’s Algal Bloom Dashboard to help them attempt to plan seaweed-free vacations. T-shirts and tank tops designed for the Cancun crowd popped up, with phrases like “Spring Break 2023: Sargassum Sucks” and (I’m sorry) “Not the ’Gasm I Was Expecting” emblazoned over cartoon drawings of sargassum fronds. In recent years, as blooms have expanded, travelers in the affected areas have taken to TripAdvisor to vent about their experiences, penning reviews of beach resorts with titles like “Sargassum seaweed is taking over the place,” “Holy Sargassum,” and “Too Much Seaweed.”

“Although the resort tried its best to keep up with it,” wrote one user, “the smell of rotting seaweed destroyed the beach experience.” All of this, I should add, before reports of flesh-eating bacteria began to emerge.

For Amy Siuda, a scientist at Eckerd College in Florida who studies the seaweed, the recent surge in sargassum has brought sudden, unexpected attention to her research. “Nobody really cared about what we were doing before it started washing ashore on the beaches,” she said.

Siuda has been working with sargassum her entire career. When she finished her undergraduate studies in around 2000, she began working at the Sea Education Association—an organization based out of Woods Hole, Massachusetts, dedicated to research on the high seas. S.E.A. researchers teach aboard their vessels while collecting data on plastics, marine microorganisms, and, of course, sargassum.

“I was sailing in the Sargasso Sea constantly and seeing this incredible ecosystem that we would pass by and collect in our nets and examine,” she said. “I’ve been really interested in the ecology and distribution of sargassum for a while now.”

The Sargasso Sea, where Siuda began her career, is named after the seaweed that for centuries (at least) has been found there in large quantities. Instead of the coastlines that define the boundaries of most bodies of water, the 600-mile-wide Sargasso is surrounded by several ocean currents—the Gulf Stream, the North Atlantic Current, the Canary Current, and the North Equatorial Current—that meet there. Because of that convergence, it’s the natural landing place for a lot of the stuff that’s floating in the Atlantic, such as plastic, but the currents also serve to slingshot pelagic sargassum, the majority of which has historically originated there, out into the rest of the ocean. The new belt of Sargassum, however, represents a change in that dynamic.

Around 2011, people living in the Caribbean, outside the usual inundation zone, began to see it washing up on their beaches in unprecedented quantities. Shorelines known for vast, pristine stretches of open white sand were suddenly buried in shoulder-height piles of decomposing sargassum that, in the most dramatic cases, stretched out to the horizon, obscuring the scenic turquoise water. Nobody knew where it was coming from.

“At first everybody was like, ‘Oh, it must just be coming from the Sargasso Sea,’ ” she said, but to her and others who had been following the algae blooms for years, that didn’t make sense. Scientists at the University of Southern Mississippi did some backtracking work, following currents backwards. The new bloom, they discovered, was coming from the equatorial Atlantic, between South America and Africa. Siuda and her colleagues looked at their old data and realized that the new seaweed had physical differences from most of the sargassum they’d been collecting for decades. The larger, newer mass was a previously esoteric variant of one of the two known pelagic sargassum species, dubbed Sargassum natans VIII, meaning this was different seaweed from a different part of the ocean. The Star Trek–sounding name, though just a typical scientific classification, was fitting: The suddenly abundant seaweed was a deep-sea mystery originating in a far-flung part of the ocean–exactly the kind of phenomenon that the crew of the Enterprise investigated over hundreds of episodes.

And as far as algae goes, sargassum is pretty famous. The drifting mats of seaweed have perplexed naturalists and sailors for centuries. Christopher Columbus made a note of it in his journal when he first crossed the Atlantic, widely credited with being the first written account of the seaweed. Fifteenth-century sailors shared stories of ships that had become ensnared in the algae, marooning their crews at sea. Naturalist Alexander von Humboldt studied it when he traveled to America in 1799, astutely observing that the seaweeds “seem to indicate the presence of currents.” In Rachel Carson’s The Sea Around Us, she described the “millions of tons of sargassum weed” and its “involuntary passengers, many fishes, crabs, shrimp, and innumerable larvae of assorted marine creatures” that ride it through the Atlantic. Those accounts, written centuries apart, all emphasized the seaweed’s enigmatic allure, and while scientists have learned a lot in those intervals, mysteries persist.

As an avid beachcomber, and someone who spends a lot of time in the ocean, I’ve encountered sargassum a few times. I’ve seen mats of it drifting between lobster buoys near the coast of Maine, and bobbing in the surf off of Puerto Rico. In the Northeast, where I live, it turns up on beaches in late August and September, noticeable to people who know it, but never in quantities significant enough to cause any problems.

I like to pick it up and examine it. Like everything else that washes up on the beach, it hints at ecological trends and natural events at sea that are otherwise invisible to the many people who only encounter the ocean in the dozen-or-so feet of water right along the shore. Each little strand is a reminder of the incredible network of ocean currents that carry sea life around the Atlantic Ocean. The algae drifts through the Caribbean, around the Gulf of Mexico, and up along the northern coast of the United States. A clump of sargassum can make these journeys repeatedly over the course of its lifetime, circulating on ocean currents in a loop like an enthralled child who refuses to get off a roller coaster.

The floating mats (even of the newly dominant variant) are teeming with life. Scientists have discovered 110 “sargassum obligate” species—animals that live exclusively in the seaweed—as well as a broader community of sea life that makes use of it. Many animals, like the sargassum pipefish, the sargassum slug, and the sargassum frogfish, have bodies that resemble in pattern, color, and shape the brown tendrils of seaweed. The eggs of a flying fish that uses sargassum as a laying substrate have even evolved to mimic the little gas-filled “bladders” that keep the algae afloat. The mats are drifting caravans of sea creatures—many of the animals permanent residents of the floating seaweed, hiding from larger marine life and feasting while they travel through the water.

“As far as we know, they’re not eating the sargassum itself,” Siuda explained. “They’re kind of feeding on other things on the sargassum, starting with microscopic algae that can be on the sargassum, to other things that are attached, like hydroids, which are little feathery colonial organisms. Then there’s crabs and nudibranchs and snails that are moving around and feeding on the other stuff.” In turn, those animals serve as food to larger economically and ecologically important fish that spend their lives in the open ocean. Mahi-mahi, juvenile swordfish, and tuna are all found foraging in conglomerations of the seaweed in the open ocean.

While the seaweed provides shelter and foraging for countless animals out in the ocean, in shallow seas it can cause big problems, inundating coral reefs and seagrass beds, which depend on sunlight to survive. As the seaweed decomposes, it also sucks up oxygen in the water.

“They’re basically smothering them,” said Siuda.

While some shoreline communities are in panic mode, not everyone in the path of the sargassum “blob” is getting their shovels and wheelbarrows out just yet. The Friends of Palm Beach, an environmental organization that leads beach cleanups near the Florida municipality, takes a different approach.

“We don’t clean up the sargassum,” explained founder and president Diane Buhler. “We clean within it and remove all the trash and plastics that reside there.” While in large quantities, she noted, the seaweed can prevent turtle hatchlings from emerging from the sand, and while the smell can be off-putting, overall, the seaweed is all right with her.

“Sargassum,” she claimed, “is beneficial to the beach.”

Indeed, over time, masses of seaweed that wash up on a beach can help firm up shorelines against erosion by waves and wind. While turtles might have trouble hatching on beaches that are matted with algae, large amounts of sargassum also mean more places to rest and hide for the ones that make it out to sea.

“It’s amazing habitat offshore,” said Siuda, “but there’s a question about the balances.”

When it comes to ecology, it’s always about balances—a simple truth that applies as much to the interactions between plankton species floating on a frond of algae as it does to the paths of miles-wide bands of seaweed drifting around the Atlantic Ocean. A good deal of the research that Siuda and her colleagues do is focused on the movements of the seaweed, and how the ecology is affected by different conglomerations, including these sudden inundations from the equatorial Atlantic.

Each individual sargassum “organism,” or clump, she explained, is about the size of a basketball. With so many clumps drifting around in the currents, the possibilities for different habitat arrangements are virtually infinite. The size and shape of each aggregation of clumps determines what other creatures it can support—at least, Siuda and her colleagues suspect it does. It’s still an open field of study, turned upside down in the last decade by the surge in sargassum natans VIII. “It’s like island bio-geography,” she explained. “If there’s this really big mat, it might be able to support a lot of individual organisms, but if it’s just an individual clump, there might be fewer things around it.”

When Rachel Carson wrote The Sea Around Us, it had been centuries since Columbus observed sargassum on his transatlantic voyage, but there were still big unanswered questions. Scientists at the time were debating where it came from, with some claiming that it began its life attached to the seafloor, and others arguing that it reproduced while floating at sea. Today, over 70 years later, the seaweed is still hiding big secrets.

“Nobody knows how long sargassum lives!” Siuda exclaimed. “How long does a clump last? Can we follow a clump along its distribution path? We’re still doing experiments in the lab on growth rates of different types of sargassum.”

The biggest question of all, one of dire importance to communities in the inundation zone, is why there’s suddenly so much of it.

“Something caused it to bloom like crazy,” said Siuda, “and we don’t know what it is.”

Some of the responses to this year’s extra-large bloom carried familiar tones of the climate despair that we’re used to experiencing when something in the environment is off, whether it’s a severe tropical storm or a chemical spill or wildfire smoke. The sargassum was a canary in the coal mines of our dying oceans, maybe, or a punishment for our sins against the planet. The exact cause, however, is still unclear. Almost all research on sargassum conducted over the past three centuries has been focused on the Sargasso Sea.

“Part of the problem is nobody’s been paying much attention to the equatorial Atlantic,” she said, “so we don’t have the historic record to compare to.”

Even if it’s not just the direct result of climate change, that’s not to say that humans have nothing to do with it. One plausible explanation for the sudden surge would be a change in the nutrient contents of rivers in Africa or South America that feed the ocean, probably from agricultural pollution. It’s also possible that a sudden upwelling of water is pushing nutrients from the deep ocean toward the surface, where sargassum reproduces.

It’s also not clear exactly what will happen next. Will this big bloom shrink down to a more manageable size, as some data indicate? Will annual blooms continue to increase in size, as they have since 2016? Is there some still-undiscovered threat hiding like a sargassum frogfish in those coils of algae? Nobody knows for sure. The massive sargassum “blob” feels a little like the enigmatic, menacing “Airborne Toxic Event” in Don DeLillo’s White Noise, retooled for the fictional universe of Finding Nemo (or, if you’re experiencing it on the beach at Cancun or Key West, a Jimmy Buffett song.)

Still, it’s not as if the seaweed is going to swallow anybody whole. In fact, while it can look dense when it’s piled up on shorelines, in the ocean, for the most part, it’s pretty dispersed.

“There’s this idea that there’s this big “blob” of sargassum out in the middle of the Atlantic Ocean, and it’s not quite like that,” Siuda said. “Yes, there’s a lot of sargassum out there right now, but it’s distributed in patches. It’s not this massive blob all moving at once.Some of it gets aggregated because of the way the currents are moving, and then it gets dispersed as the wind picks up or the currents change.”

Some of the more dire public health concerns, too, are overblown. Siuda was on the boat with the scientists who collected the flesh-eating bacteria that recently made headlines. The samples were taken over a decade ago.

“That’s from data from, like, years ago, from 2012, that was just published,” she said. But the bacteria is just vibrio. “You find it in the ocean! It’s not surprising that it was found on sargassum, and it’s not like these blooms now have more, or anything. I touch sargassum with my bare hands all the time.”

For as long as they’ve been contemplating its mysteries, people have been debunking sinister, overblown myths about sargassum. Rachel Carson, for her part, rejected the long-held notion that the algae had been responsible for stranding ships in the 1400s, pointing out that even in the Sargasso Sea, it wasn’t concentrated enough for that. “The dense fields of weeds waiting to entrap a vessel never existed,” she wrote, “except in the imagination of sailors, and the gloomy hulks of vessels doomed to endless drifting in the clinging weed are only the ghosts of things that never were.”

She may have spoken too soon. In recent years, as the Caribbean has been inundated with sargassum, reports of vessels stranding themselves in the weeds have trickled onto boating forums and blogs, with sailors describing sargassum jamming their propellers and steering equipment, and clogging the intakes of jet engines.

One user, going by UricaneJack, wrote:

“I remember watching some old horror movie when I was a kid. About ships getting trapped in sargasso weed. Crews having been stuck there from the days of pirates on the Spanish Main.

“It might have been prophetic.”