A Brown Tide Threatens Coastal Ecosystems

Sargassum beaching events may be a new normal in the Caribbean Sea.

The nearshore meadows of sea turtle grass and manatee grass were the first to perish. In 2015, a vast quantity of Sargassum seaweed inundated the Caribbean coast of Mexico, depleting the water of oxygen near the shore as it decomposed. A die-off ensued, transforming thriving marine habitats into murky dead zones.

Sargassum offshore along the Gulf Coast
A Sargassum line offshore along the Gulf Coast. The so-called Great Atlantic Sargassum Belt (GASB) is a sprawling, novel, seasonally-recurring seaweed bloom, not yet fully documented or understood. It reached a peak mass of more than 20 million metric tons in July 2018. With large blooms erupting again in 2019 and 2020, experts fear that the GASB heralds a long-term and irreversible shift in the ecology of the Atlantic Ocean. Photo by Gulf Coast Research Library/University of Southern Mississippi.

“The first thing we saw was the mortality of seagrass beds close to the beach,” said Rosa Rodríguez-Martínez, a marine biologist and coral specialist at the Universidad Nacional Autónoma de México.

“In some areas, soft corals and sponges, and other types of animals and plants that are typical of reefs, we observed them dying too,” Rodríguez-Martínez explained. “Then, in 2018, we received even more Sargassum ... and we began to see the mortality of motile fauna such as fish and crustaceans and echinoderms and some octopuses and other molluscs, all along the Mexican Caribbean coast.”

According to surveys of the touristic zones of Quintana Roo, at least 78 species of fauna were impacted by the event. Hypoxic (low oxygen) conditions combined with toxic levels of ammonia and hydrogen sulphide are thought to have been the cause.

Unfortunately, the die-off in Mexico was not an isolated occurrence. Coastal communities everywhere from Nigeria to Sierra Leone to Barbados and Guadeloupe have been afflicted with the same ruinous brown tides for a decade now.

The GASB is composed of two pelagic species of the brown macroalga Sargassum: S. natans (pictured above) and S. fluitans. Pelagic Sargassum is dense and bushy. It has no roots, but it does have gas-filled bladders that keep it buoyant, and it reproduces asexually by breaking apart and regrowing. Photo by James St. John.

Drifting Sargassum mats are in fact rich and complex habitats that provide nourishment and protection to a diversity of marine animals, including fish, crustaceans, turtles and birds. However, the tides that are now washing up on tropical Atlantic shores are neither golden nor lively. They are dark, brown, and soupy — and a major threat to coastal ecosystems.

Photo by Jonathan Wilkins​.

Spanning nearly 9,000 kilometers from the coast of West Africa to the shores of South America and north to the Caribbean Sea, the so-called Great Atlantic Sargassum Belt (GASB) is a sprawling, novel, seasonally-recurring seaweed bloom, not yet fully documented or understood. A true leviathan, it reached a peak mass of more than 20 million metric tons in July 2018.

With large blooms erupting again in 2019 and 2020, experts fear that the GASB heralds a long-term and irreversible shift in the ecology of the Atlantic Ocean.

The GASB is composed of two pelagic species of the brown macroalga Sargassum: S. natans and S. fluitans. Pelagic Sargassum is dense and bushy. It has no roots, but it does have gas-filled bladders that keep it buoyant, and it reproduces asexually by breaking apart and regrowing. A single, sprawling, behemoth life form, Sargassum aggregates into enormous free-floating mats that migrate along global currents.

Drifting Sargassum mats are in fact rich and complex habitats that provide nourishment and protection to a diversity of marine animals, including fish, crustaceans, turtles and birds. Sargassum forms the bulk of biomass in the Sargasso Sea — the only shoreless sea in the world. First documented by Christopher Columbus in 1492, the sea harbors an ecosystem so unique and lively that the esteemed marine biologist Sylvia Earle once described it as “the golden rainforest of the ocean.”

However, the tides that are now washing up on tropical Atlantic shores are neither golden nor lively. They are dark, brown, and soupy — and a major threat to coastal ecosystems.

In nearshore waters, Sargassum clogs boat engines, snags fishing lines, and severely reduces diving visibility. On land, it obstructs sea access for both fishermen and turtle hatchlings. And as it starts to rot, it releases foul-smelling hydrogen sulphide and ammonia, which can cause respiratory illness in humans and, when dissolved in water, the death of marine organisms.

Mengqiu Wang of the University of South Florida, who coined the term “Great Atlantic Sargassum Belt” in a landmark article in Science in 2019, thinks the Sargassum blooms may become a new norm. “From 2011 to 2020, including the most recent data, almost every year the Sargassum belt occurs,” she said.

Although small amounts of Sargassum had previously washed up on tropical Atlantic beaches, the GASB first appeared in 2011. Since then, it has formed every year except 2013. The jury is still out on the exact tipping point or mechanism that caused the bloom to proliferate, but one theory put forward by NOAA oceanographer Elizabeth Johns and her colleagues in an article for Progress in Oceanography is that additional “seed” populations were transported from the Sargasso Sea during a negative phase of the North Atlantic Oscillation in 2010. (The North Atlantic Oscillation is an irregular fluctuation of atmospheric pressure over the North Atlantic Ocean that has a strong effect on winter weather in Europe, Greenland, northeastern North America, North Africa, and northern Asia.)

“I think right now it’s very hard to eradicate so much Sargassum abundance,” said Wang. “And even if we didn’t see much from the satellite, it’s still possible for the Sargassum to grow back, to form massive abundance when conditions are optimal.”

Identifying those optimal conditions is one of the main challenges facing Wang and her colleagues at USF’s Optical Oceanography Laboratory. Led by oceanographer Chuanmin Hu, the group uses a range of spectrum resolution sensors to study algal blooms, red tides, and other marine phenomena. “We measure the light intensity at different wavelengths and we try to quantify that signal and link it to the relative abundance of Sargassum,” explained Wang.

Numerous factors are known to influence the growth and distribution of Sargassum blooms, including sea surface temperature, wind, currents, and salinity. The blooms could be fed by upwellings of nutrients and by vast discharge plumes of freshwater and organic matter — sediments, minerals, ground leaves, twigs, fungi and other suspended particles — at the mouths of the Amazon and Congo Rivers, as well as by iron-rich Saharan dust, sewage outflows, and fertilizer runoff, particularly in Brazil, where rainforest is being rapidly converted to industrial-scale soy plantations.

Understanding how Sargassum mixes and interacts with global systems will enable Wang and her colleagues to develop better prediction tools.

“We need to make short-term predictions for coastal regions,” she said. “We need to know how much Sargassum is coming and when to expect the bloom. We would then be able to utilize that information to help management agencies to allocate their resources to handle the problem. I think that’s a major task.”

To guide short-term predictions, the USF team currently publishes near real-time imagery of the GASB on its satellite-based Sargassum Watch System website. Hu also publishes a monthly Sargassum outlook bulletin with longer term forecasts. He has done so every month since February 2018, when he correctly predicted a bumper bloom for that summer.

Elsewhere in the research community, there has been a robust response involving numerous working groups and organizations, both public and private.

Organized by the United Nations Environment Programme (UNEP), the world’s first international Sargassum conference was held in Guadeloupe in October 2019. The event sought to enhance public knowledge of brown tides and to identify a common political strategy for the impacted regions. The UNEP followed up in May 2020 with a two-hour-long webinar with expert contributors on both sides of the Atlantic. Several other groups have broadcast webinars during the Covid-19 pandemic and further meetings are planned.

Naturally, one recurring theme in most Sargassum discussions is the need for further research, as the full ecological and socio-economic implications of the GASB have yet to be determined.

In Mexico, for example, cases of stony coral tissue loss disease were detected during Sargassum arrivals in 2018. Identified for the first time in Florida in 2014, the disease is novel and highly lethal to corals. Researchers do not yet know if its arrival in Mexico is connected to the massive influx of Sargassum, but they suspect that bacteria living in Sargassum mats may be involved.

Meanwhile, the region’s water chemistry appears to be undergoing profound transformations.

“What we do know is the water quality of this whole area is changing from one that is very low in nutrients to a eutrophic [overly-enriched with minerals and nutrients] one,” said Rodríguez-Martínez. “Because when the Sargassum dies and goes back into the ocean, it provides a lot of nitrogen, phosphorus, and carbon. So the water is changing. Before it used to be transparent. Now the visibility has been reduced a lot in areas close to shore.”

The toxicity of brown tides is another area requiring urgent research. Sargassum contains alginates, naturally occurring anionic polymers, which readily absorb heavy metals. As such, dangerous levels of cadmium, lead, and other industrial pollutants have been found in Sargassum tissue in Nigeria and Ghana.

In Mexico, Rodríguez-Martínez detected high levels of arsenic in samples from eight localities along a 370-kilometer stretch of the Caribbean coast. Inorganic arsenic has the potential to accumulate to dangerous levels with successive tides. If improperly dumped after clean-up operations, as has been alleged, it might leach into the region’s extensive and poorly understood limestone aquifer, which is the only source of fresh water in the Yucatán Peninsula.

Heavy metal pollution also rules out Sargassum’s use as an edible crop, a fertilizer, or as an ingredient in myriad beauty products. For example, under European Union regulations, 86 percent of the samples analysed by Rodríguez-Martínez contained arsenic levels above the maximum allowable for animal fodder. Moreover, Mexico’s political response to Sargassum appears to highlight a challenging gap between research and policy.

Rodríguez-Martínez said that progress on Sargassum had been hampered by political squabbles over jurisdictional responsibilities. Municipal, state, and federal authorities “played tennis” with the issue, she said, and there was “zero” transparency concerning how much Sargassum was being removed from the coast, where to, and at what cost.

The Mexican federal government has in fact issued guidelines for managing Sargassum, but it has yet to provide firm legislation for its removal and disposal. Hoteliers and municipal authorities in the state of Quintana Roo have installed floating nearshore barriers to steer Sargassum away from the beach, but these cover less than 0.5 percent of the state’s 1,176-kilometer coastline. The authorities have also commissioned boats to clean up the water, but these appear to capture just a small fraction of the inundations.

Meanwhile, local responses and research efforts continue to be constrained by inadequate budgets and overbearing bureaucratic demands.

Responding to the GASB at a global level will require expertise across a wide range of fields, not least because it impacts stakeholders in dozens of countries in three continents. The GASB connects the deforestation of the Amazon with the death of a seagrass meadow in Mexico. It links fishermen in the Ivory Coast with holidaymakers in Barbados. And it joins the fate of untold nonhuman communities with political decision makers at every level.

Disentangling from Sargassum — if such a thing is possible — will require extraordinary effort, insight, and international cooperation.

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