Escape from Sargasso Sea: Tremendous Sargassum Blooms Challenge Caribbean and Atlantic Communities

Abstract

The spring of 2023 brought headlines about "giant blobs of seaweed"--a brown alga called Sargassum--hitting Atlantic Ocean beaches from Florida through the Caribbean and South America. But this was not the first time Sargassum has confounded beach-goers. During the spring and summer of 2011, coastlines on both sides of the tropical Atlantic were inundated with Sargassum, which can accumulate into piles up to several feet high. "We had never seen anything like it," says James Franks, a senior research scientist at the University of Southern Mississippi's (USM) Gulf Coast Research Lab in Ocean Springs. This pelagic Sargassum ordinarily spends its entire existence floating on the ocean. Small amounts would wash up on the shore from time to time, but these mass strandings were unprecedented. It would prove to be far from a one-time event. Since 2011, a floating aggregation of Sargassu.

The spring of 2023 brought headlines about "giant blobs of seaweed"1-a brown alga called Sargassum-hitting Atlantic Ocean beaches from Florida through the Caribbean and South America. But this was not the first time Sargassum has confounded beach-goers. During the spring and summer of 2011, coastlines on both sides of the tropical Atlantic were inundated with Sargassum,2^ which can accumulate into piles up to several feet high.4

"We had never seen anything like it," says James Franks, a senior research scientist at the University of Southern Mississippi's (USM) Gulf Coast Research Lab in Ocean Springs. This pelagic Sargassum ordinarily spends its entire existence floating on the ocean.5 Small amounts would wash up on the shore from time to time, but these mass strandings were unprecedented.3

It would prove to be far from a one-time event. Since 2011, a floating aggregation of Sargassum patches, or "rafts," has been growing in the Atlantic Ocean.3'6 Called the Great Atlantic Sargassum Belt (GASB), this floating mass can stretch nearly

9,000 km across, extending from West Africa into the Caribbean Sea and the Gulf of Mexico.3 For coastal communities in its path, the seaweed is now an unwelcome fact of life.

Within 48 hours of stranding, Sargassum begins to rot; if trapped in water, it degrades anaerobically (i.e., with insufficient oxygen) and releases a noxious plume of hydrogen sulfide (H2S) and ammonia.7'8 At certain concentrations, these gaseous emissions have negative impacts on both marine and human health, causing respiratory, ocular, and possibly neurological effects; moreover, the revolting stench travels long distances, permeating the air where people live and work.8'9 Sargassum emissions are especially concerning for people "who are unable to avoid chronic and repeated exposure," says Andrea Boggild, medical director of the Tropical Disease Unit at Toronto General Hospital. Those at greatest risk, she says, include coastal residents, people with underlying respiratory conditions (such as asthma), cleanup crews, pregnant women, and children.

Because Sargassum takes up and concentrates arsenic and other metals that occur naturally in seawater,10 disposal or use of the stranded biomass poses a tricky challenge.11 Sargassum also wreaks havoc on coastal ecosystems when driven ashore by ocean currents and winds, smothering coral reefs, nearshore seagrass beds, and beaches. It can even block newly hatched sea turtles as they scramble toward the ocean.12'13 The seaweed harms fisheries and tourism, which are vital to Caribbean economies.13

"Sargassum is a chronic environmental problem for us," says Dabor Resiere, a physician and clinical toxicologist at University Hospital of Martinique (UHM). "And we need a way to solve it."

Cause Unknown

Scientists have documented more than 300 species of Sargassum, most of them benthic varieties that live fastened to the seafloor in shallow waters.14 The recent strandings involve two free-floating pelagic species-S. fluitans and S. natans-that occur throughout the temperate and tropical oceans of the world but historically accumulated in a vast part of the northwest Atlantic called the Sargasso Sea.2 Christopher Columbus provided the first known written account of the Sargasso Sea after his flotilla became mired in it for days during a windless calm in 1492.15

In its natural environment, the seaweed "is comparable to a rainforest or an upside-down coral reef," says Hazel Oxenford, a marine and fisheries ecologist at the University of West Indies Cave Hill campus in Barbados. Sargassum rafts provide protection and habitat for a stunning array of marine life, from flatworms, amphipods, and snails, to crabs, shrimps, turtles, and many species of fish, including commercially important species that roam the Atlantic, she adds.

Sargassum in nearshore waters is protected by some state and national laws. The South Atlantic Fishery Management Council, for instance, which manages U.S. fisheries from North Carolina to Florida, limits annual removals to no more than 5,000 pounds per year to protect sea turtles.16 But when the seaweed piles up on the shore, it switches from being an ecological treasure to an environmental nemesis, for example by stranding turtle hatchlings.12'13

Using satellite images, researchers pinpoint 2011 as the year of the first Sargassum bloom in the North Equatorial Atlantic (south of the Sargasso Sea), which previously saw only small amounts of the seaweed.3 As to what gave rise to the GASB and the masses of Sargassum washing up on coasts from West Africa to Florida to Central and South America, scientists still do not agree.2

Donald Johnson, a senior scientist at the USM Gulf Coast Research Lab, says shifting wind patterns may have blown alarge patch from the Sargasso Sea toward the southward-flowing African coastal currents and into the nutrient-rich tropical Atlantic, where the seaweed flourished. From there, he says, the seasonal equatorial current system carries it westward into the Caribbean and, eventually back to the North Atlantic, with a large enough quantity remaining in the tropics to maintain the bloom.

Other hypotheses implicate a combination of factors, including warming seas, changing ocean currents, and the fertilizing effects of iron in airborne dust and high nutrient loads flowing from major rivers such as the Amazon, Congo, and Mississippi.2'3 When exposed to elevated nitrogen levels, "Sargassum biomass can double within two weeks," says Brian LaPointe, a researcher at Florida Atlantic University in Boca Raton. The strandings, he says, "could be a reflection of how we're altering global nutrient cycles."

Ajit Subramaniam, an oceanographer at Columbia University in New York, agrees that elevated nutrient levels could be driving the blooms but emphasizes that more research is needed. Some amount of Sargassum "clearly escaped from the Sargasso Sea and found a new and ideal niche to occupy," he says. "There are theories about how this came about-none completely answers the question."

Remi Neviere, a pathophysiologist at UHM, points out that Sargassum reproduces by both sexual and vegetative means. Vegetative reproduction takes place when fragments separate and develop into new rafts. Neviere says this property must be considered when floating Sargassum is removed by mechanical means, because any remaining fragments may multiply.

Meanwhile, the GASB has been growing steadily. In the summer of 2018, the belt hit a record of 20 million tons (estimated from satellite measurements), creating major problems for shorelines in the tropical Atlantic.3 More recently, March 2023satellite data were evaluated by researchers at the University of South Florida's College of Marine Sciences in St. Petersburg, who estimated the GASB's weight at 13 million tons-a record for that time of year.17 However, in an unexpected twist, the mass of Sargassum that appeared on course to overwhelm the Gulf of Mexico in 2023 fell by 75% in June,18 even though blooms typically diminish gradually after peaking later in the season.3 Chuanmin Hu, a professor of optical oceanography at the University of South Florida, was quoted by CNN saying such a drastic decrease has "never happened in history at this time of year."19 In more good news, at the end of July, the quantity of Sargassum across the entire GASB was reported to have "decreased substantially from June" levels.20

Potential Health Impacts

The 2023 event brought alarming reports21-23 of "flesh-eating bacteria" trapped in the Sargassum. These reports were based on a new genetic sequencing study by scientists at Florida Atlantic University showing that Sargassum samples contained several species of Vibrio bacteria adhered to microplastic particles.24 However, those authors did not detect Vibrio vulnificus, the waterborne bacteria that can cause severe wound infections, including necrotizing fasciitis ("flesh-eating disease"), points out Gisele Galoustian, senior media relations director for research and health at Florida Atlantic University. The species they found simply share certain genes with V. vulnificus and other pathogenic Vibrio species.

Spurious news reports aside, for affected communities, these strandings do raise serious health concerns. Oxenford says Caribbean islands have declared states of emergency over Sargassum strandings and enlisted help from their armed forces to clean it up. More than 8,000 people in Guadalupe and Martiniquesought medical care for Sargassum exposures over an 8-month period in 2018; four required intensive care.25 Resiere and colleagues reviewed health data from 154 cases and reported that the most common symptoms were headache, dizziness, abdominal pain, cough, rashes, eye disorders, and effects on mood.25 "All the patients complained of irritability," says Resiere, adding that it is not unusual for patients to weep when describing how the exposures are affecting their quality of life.

Wherever the strandings occur, locals with modest means tend to suffer the most, Neviere says. He adds that Sargassum in Martinique frequently accumulates on the windward east and southeastern coasts, which are home to roughly a third of the island's ~ 380,000 residents.26 "Many of them are quite poor and not traveling a lot," Neviere says. "They live in the same area with the Sargassum day in and day out."

Neviere is now leading SargaCare, a long-term study of the stranded seaweed's chronic health effects. The project was launched in 2020 with the goal of recruiting 1,000 people, including a small number of control subjects from Martinique's leeward west coast, where he says residents have less Sargassum exposure. Neviere and his colleagues are assessing pulmonary function and measuring exhaled nitric oxide, a marker of airway inflammation. The team plans to study additional end points such as cognitive functioning and sleep.

In a 2021 case report of two individuals in Martinique, the UHM team tied Sargassum exposure to keratoconjunctivitis, an inflammatory eye condition.27 The two patients, who lived within 50 m of the beach, developed the condition within 48 hours of the seaweed washing ashore. As the H2S odor dissipated, the condition likewise began clearing up. In a subsequent study of more than 3,000 pregnant Martinican women,28 the UHM team associated Sargassum emissions with earlier onset of preeclampsia among those diagnosed with the condition. Women who lived within 2 km of Sargassum stranding sites developed preeclampsia an averageof 3 weeks sooner than women who lived farther away. The team found that distance had no association with preeclampsia incidence, however.

These studies support the notion that health risks from exposure grow with increasing proximity to stranded Sargassum, where H2S levels are sometimes remarkably high. In 2018, UHM researchers measured levels of 5 ppm or higher in 16 coastal sampling sites. At one site, hourly readings higher than 5 ppm were recorded on more than 50 days of the year.25 Although individuals vary widely in their threshold for smelling odors, some are able to detect H2S at concentrations as low as 0.0005 ppm.29 H2S is known to cause headaches, difficulty breathing, irritation to eyes, nose, and throat, and other symptoms at air concentrations as low as 2 ppm.29

"The odor from H2S is very offensive, like rotten eggs," says Susan Schiffman, a clinical psychologist at North Carolina State University in Raleigh, who specializes in disorders of smell and taste. She explains that H2S triggers olfactory and cranial nerves that govern fear and emotional responses. "Odor signals from H2S activate a part of the brain stem called the vomit center, so you want to retch when you smell it," Schiffman says. "And when malodorous air containing H2S stimulates the trigeminal nerve, you get a headache."

Bethia Thomas, a Ph.D. candidate at the University of West Indies' Cave Hill campus, filmed schoolteachers in St. Lucia talking about how Sargassum odors affect children in the classroom. One teacher described how, when the Sargassum appears, the children become unsettled and complain they do not feel well- that they have a headache or feel nauseated. Sick children are sent to the school nurse or the bathroom. By the time everyone is settled, she says, the period is over, and "I have to plan to revisit this topic again because little to no learning has taken place."30

Sargassum's gas emissions are not the only problem. Its ability to concentrate arsenic could pose health risks for peopleexposed to contaminated shellfish or the seaweed's leachates in groundwater.31 Sargassum accumulates arsenic from the low ambient levels in sea water, Oxenford says.32

Therefore, even though geologic arsenic levels in the Caribbean region tend to be low,31 Sargassum strandings could be an emerging source of regional contamination,33 cautions Damien Devault, a lecturer in ecotoxicology at the University of Mayotte. Mayotte is an island group in the Indian Ocean that, like Martinique, is an overseas department of France.

Rosa Rodriguez-Martinez, a biologist at the National Autonomous University of Mexico, also worries that the arsenic in the seaweed might contaminate groundwater supplies in Mexico, where much of the Sargassum removed from Caribbean beaches is dumped into abandoned limestone quarries near the coast. More studies are needed to assess how arsenic and Sargassum-derived metals move in the groundwater, but in the meantime, Rodriguez-Martinez calls for improving Sargassum management to limit aquifer contamination.

Solutions Urgently Needed

Jurisdiction over the seaweed in Mexico is split between different levels of government, Rodriguez-Martinez explains. Sargassum in the water is subject to federal jurisdiction and requires permits for removal even if it's going to be dumped, she says. After Sargassum reaches land, responsibility shifts to municipal governments. But once the seaweed starts rotting, she notes, "it's considered a dangerous residue that should be taken to a special garbage dump." Meanwhile, hotels clear the Sargassum in front of their own beaches, "but no one checks to see where it goes," she says.

Removing Sargassum from beaches strains both workers and coastal environments. The French Agency for Food, Environmental and Occupational Health and Safety recommends that workers equip themselves with portable H2S detectors and then wear personal protective equipment when concentrations exceed lOppm.34 Boggild describes watching work crews use pitchforks, wheelbarrows, and large tractors to remove the seaweed. "Despite the tropical heat, all crew members donned knee-high rubber boots, thick pants, long-sleeved shirts, work gloves, and all wore bandanas to cover their mouths and noses," she says. "Clearly, they were trying to reduce skin contact with the Sargassum and inhalation of the odors." At the same time, heavy machinery may destroy turtle nesting sites and habitat for crabs and other sea creatures.

Better solutions are urgently needed, and proposed options include using Sargassum as a raw material for commercial products such as animal feed,35 cosmetics,36 drugs,37 plastics,38 and biogas.39 In many places, Sargassum has long been used to supplement nutrient-deficient soils.40 The challenge for all these applications is to ensure that arsenic and other toxic constituents do not pose unacceptable risks, according to Devault, who is first author on an extensive review of Sargassum contamination and its consequences for downstream uses.40

Sand also complicates the seaweed's commercial potential, adds Luke Gray, the cofounder of SOS Carbon SRL, a Sargassum removal company based in the Dominican Republic. "Once Sargassum gets into the sand, it's hard to use for anything," he says. "You can't use it for compost, because you generally don't want to plant your produce in sandy soils. Or imagine companies that have an interest in using Sargassum for biogas-they have very specific ingredient standards, and sand just ruins the Sargassum in this case." Coastal barriers can keep the seaweed offshore, Gray adds, but these barriers are expensive and typically placed only at large beach resorts where hotels cover the cost.

Subramaniam points to yet another option: Sargassum removes tons of carbon dioxide from the air and locks the greenhouse gas away as organic carbon in its own tissues. At least one company has proposed sinking Sargassum in the deep ocean to sequester that carbon and keep it out of the atmosphere until the seaweed decomposes41-"at least 100 years," Subramaniam says. The eastern Caribbean is advantageous in this respect, "because you can reach depths of over 2,000 meters when you're only 8 kilometers offshore."

In the end, Subramaniam says, "Sargassum really is an urgent problem in the Caribbean that people live with every day. We need to put more attention on it."

Boggild agrees, adding that many questions remain about how the strandings are affecting public health, particularly regarding adverse effects of chronic low-level exposures to H2S that occur when the odor becomes detectable. She calls for greater monitoring of shoreline H2 S levels and surveillance of associated health effects in exposed people. "And we need to better understand how low levels of arsenic and other toxic elements in the Sargassum contribute to clinical illness, and whether they act synergistically with exposure to H2S and ammonia," she says. "It's important to raise awareness about the potential adverse effects of this emerging and evolving phenomenon."

Charles Schmidt is a freelance writer based in Portland, Maine.

References

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