It might have been one of Alfred Hitchcock’s fanciful tales of the supernatural: a 5,000-mile wide blob of murky seaweed creeping menacingly across the Atlantic before dumping itself along the US shoreline.
But now giant clumps of the 13m-ton morass labeled the Great Atlantic sargassum belt are washing up on Florida’s beaches, scientists are warning of a real-life threat from the piles of decomposing algae, namely high levels of the flesh-eating Vibrio bacteria lurking in the vegetation.
The alarming discovery by marine biologists at Florida Atlantic University (FAU) lends a dangerous new aspect to the brown seaweed onslaught, which is already threatening to spoil the state’s busy summer tourism season as coatings of decaying goop exude a pungent aroma akin to that of rotting eggs.
Even more worrying, the researchers say, is the role of ocean pollution in the proliferation of the bacteria, which can cause disease and death if a person gets infected. Samples tested from the Caribbean and Sargasso Sea within the Atlantic were abundant with plastic debris, which interacted with the algae and bacteria to create a “perfect pathogen storm [with] implications for both marine life and public health”.
“Our lab work showed that these Vibrio are extremely aggressive and can seek out and stick to plastic within minutes,” said Tracy Mincer, assistant professor of biology at FAU’s Harbor Branch Oceanographic Institute and Harriet L Wilkes Honors College.
He said the seaweed belt stretching from the Gulf of Mexico to the African coast provided the perfect breeding ground for “omnivorous” strains of the bacteria that target both plant and animal life, and associated “microbial flora” potentially harboring potent levels of pathogens.
“We really want to make the public aware of these associated risks. In particular, caution should be exercised regarding the harvest and processing of sargassum biomass until the risks are explored more thoroughly,” he said.
That’s become a worry for many, from municipal crews charged with clearing the washed-up seaweed from Florida’s beaches to make them more attractive for vacationers, to the tourists themselves and teams of environmentally conscious volunteers who fill trash sacks with washed up detritus.
“It’s very alarming in the first place to see it on the beaches, and alarming to see all the plastic that is entangled in it. And now even more than that, there’s harmful bacteria too. That’s so scary,” said Sophie Ringel, founder of the non-profit Clean Miami Beach.
The group is hosting a beach cleanup on Saturday to mark next week’s World Ocean Day, and recruits will be taking precautions including thick gloves, sanitizers and long-handled grabbers to avoid direct contact with the materials they remove.
“We’ll be paying extra attention and making sure everybody washes their hands, and doesn’t touch their faces after the cleanup. But I wonder what happens if we ingest it or come in contact with it? Is it transferable? And when it rains, does it end up in our drinking water?” Ringel said.
Florida’s department of health is advising residents and visitors to avoid sargassum and warns that Vibrio vulnificus infections “can be severe for people who have weakened immune systems, such as those with chronic liver disease”.
The state’s department of environmental protection (DEP), meanwhile, says it’s working with the Florida fish and wildlife commission and municipalities to monitor the seaweed belt, and notes that the Florida legislature has budgeted $5m to assist local governments with cleanup efforts.
“This is not a new phenomenon and many local governments, particularly in south Florida, are experienced in managing it on their beaches and already have management plans and the necessary authorizations in place to respond,” a DEP spokesperson, Jon Moore, told the Guardian.
“We’re ready to work with any impacted local government … as well as expedite necessary authorizations so that cleanup activities can be conducted in an efficient and protective manner.”
Crews with heavy machinery remove sargassum from 15 miles of shoreline on Miami Beach, and two more on Key Biscayne, early most mornings, after surveyors check for turtle nests, Tom Morgan, chief of operations for Miami-Dade county’s parks, recreation and open spaces, said.
It ensures the beaches of the popular tourist destination remain clean and attractive, and helps to remove the threat of infection, or respiratory distress from hydrogen sulfide, the source of the “rotten eggs” smell that comes from rotting algae.
“We’re aware of the report, and our beach maintenance crews are instructed to wear gloves if they’re removing anything from the water’s edge and the sargassum related to plastics, or any other type of debris, pieces of wood or anything like that,” he said.
“That’s to protect them while they’re working, and that’s been standard practice even before this report came about.”
Beaches were packed with visitors over the Memorial Day weekend, and Michael Zimmer, director of marketing and development for Miami-Dade parks, said tourism was “so far so good”.
“We get pictures every morning and afternoon and I gotta tell you, the beaches look really good,” he said.
“The team does an incredible job cleaning it up every morning and we just haven’t seen any effects on tourism yet.”
The county expects to spend about $6m on seaweed removal this year, but pulling sargassum from the sea before it washes ashore is neither legal nor desirable.
It’s a crucial habitat for crabs, shrimps and other marine invertebrates, which in turn provide a rich floating “buffet” for seabirds including gulls, terns and plovers. Unfortunately, the toxicity of the sargassum belt can simultaneously be harmful.
“The amount of plastic we find entangled in the seaweed on a daily basis, and every tide that comes in brings more, is shocking. And the animals out in the ocean who live on it, they try to get nutrition from it and automatically ingest the plastic,” Ringel said.
“They just can’t help it. It’s so, so sad.”
One bright spot is that scientists at the University of South Florida (USF), who have tracked the sargassum using satellite imaging, say the amount in the Atlantic unexpectedly decreased by about 15% in May, and is forecast to drop in the Gulf of Mexico this month.
“[That] is good news for many coastal residents of Florida,” the university’s optical oceanography laboratory said on its website.
Overall, though, the researchers have recorded huge increases in sargassum over the last decade, and expect it to continue.
“The plausible theory was that in 2010 there was a long distance transport from the Sargasso sea to the tropical Atlantic. That was an usual event,” said Chuanmin Hu, professor of optical oceanography at USF.
“The tropical Atlantic has a lot more seed populations of sargassum, and warm water, and sunshine, enough nutrients … all the conditions are favorable for sargassum to grow.”