- Brazil’s northern beaches recently suffered from arrivals of sargassum blooms, a phenomenon affecting Caribbean nations that most scientists so far have associated with nutrients coming from the Amazon River plume into the Atlantic Ocean.
- A recent study suggests that ocean changes are the primary nutrient source for sargassum blooms since 2011, challenging previous hypotheses.
- Sargassum is causing considerable health and economic concerns as large amounts of this brown macroalgae arrive and accumulate in coastal ecosystems of western Africa and the greater Caribbean Sea every year.
- Brazilian authorities are learning from Caribbean countries how to manage sargassum blooms best, and experts think they should keep monitoring possible ocean current changes.
The macroalgae pelagic sargassum is abundant in the Sargasso Sea, but since 2011, a recurrent Great Atlantic Sargassum Belt (GASB) has been observed on beaches and in satellite imagery, often extending from West Africa to the Gulf of Mexico.
Millions of tons of sargassum make their way across the Caribbean each year, pushing coastal ecosystems toward collapse. This brownish wave of macroalgae covers the region’s white beaches and blue waters, mostly from May to October, mainly coming from the GASB — a bloom that stretches more than 8,000 kilometers (4,971 miles). In June 2022, it reached a record peak of 6,989 square km (2,699 square mi) of coverage.
This equates to an estimated wet biomass of more than 23.3 million metric tons of seaweed, but recent forecasts suggest that 2025 might break a new record for sargassum blooms. As the algae accumulate in the calm Caribbean basins, they begin to decompose, switching colors from orange to brown, while releasing heavy toxic metals and gases that are harmful to human health. The stench of rotten sargassum is tourism’s worst nightmare, as visitors cannot enjoy white sand beaches and pristine turquoise waters anymore.
At the UNOC3 oceans summit, in June, the Dominican Republic President, Luis Abinader, reiterated his call to world leaders to urgently recognize and address the proliferation of sargassum in the Caribbean, calling it an environmental, economic, and health crisis that requires a global and coordinated response. While recalling that the island nations of the Caribbean are being hit hard by the effects of climate change, Abinader warned that the sargassum phenomenon has ceased to be seasonal and has become a continuous threat.
Until recently, experts in oceanography believed the massive bloom might result from Amazon River discharge during spring and summer in previous years. They also argued that recent increases and interannual variability after 2011 appeared to be driven by rising tides off West Africa during boreal winter. For the past 15 years, this scenario has brought recurrent blooms in the tropical Atlantic and Caribbean Sea and turned it into the new norm.
The Amazon River is the largest river in the world and accounts for 20% of the world’s total river discharges. Previous scientific literature stated that GASB can be fed by nutrients, such as nitrates and phosphates, from various sources, including the Congo, Amazon and Mississippi rivers, all of which are affected by nearby crops. Research also stated that other sources could be at play, such as the upwelling off the coast of Africa, vertical mixing, equatorial upwelling, atmospheric deposition from Saharan dust and biomass burning of vegetation in central and Southern Africa.
The Amazon River and the sargassum blooms
However, recent research has found evidence of a new explanation for the sargassum bloom. It may have been seeded by an extreme negative phase of the North Atlantic Oscillation in 2009 to 2010, which allowed for sargassum from the Sargasso Sea to reach the North Equatorial Current and central tropical Atlantic Ocean.
“While deforestation and pollution in the Amazon are pressing and important environmental concerns, our results support the hypothesis that hydrological changes on land are not the primary drivers of sargassum proliferation,” oceanographer Julien Jouanno told Mongabay. He is the lead author of a recent article published in Nature Communications Earth & Environment in February.
Since 2021, Jouanno and his team have developed simulations of sargassum distribution that integrate both transport and growth. By being able to turn different nutrient sources on and off, they were able to test how sargassum responds. They estimate that the contribution of the Amazon-Orinoco rivers’ discharge accounts for approximately 10% of the total sargassum biomass, including both natural and anthropogenic sources of riverine nutrients — not enough to explain recent blooms.
“Hence, our conclusions are that the Amazon River is not responsible for the proliferation,” Jouanno said. “The exceptional North Atlantic Oscillation [NAO] anomaly in 2009-10 has been instrumental in driving the regime shift and the last 15 years’ proliferation, but we still do not understand the whole process, and in particular if this NAO exceptional event is the result of climate change — and if other factors such as the warming of the surface water in the Atlantic could have contributed to the regime shift.”
Sargassum now proliferates in a region where light-temperature conditions and nutrient availability are naturally more favorable to its growth than in the original Sargasso Sea. Jouanno’s team also identified that most nutrient inputs (nitrogen and phosphorus) are brought through vertical exchanges between the subsurface (about 50 meters or 164 feet) and the surface, driven by typical oceanic mechanisms, mixing processes such as wind-driven turbulence and vertical convection.
The French oceanographer also said that additional inputs also come from atmospheric sources, notably through the capacity of sargassum to fix nitrogen present in the atmosphere and through iron deposition from Saharan dust. Observations over the past 20 years do not indicate any significant changes in these various sources.
In March, beaches in the Brazilian state of Pará were overrun by tons of sargassum, making them inappropriate for swimming and recreational use due to the piles and strong stench. The phenomenon had not occurred in Pará since 2015. Jouanno said remote sensing observations and sargassum forecasts indicate that the Brazilian bloom has been caused by a southwestward transport of sargassum originating from the north equatorial region.
Chuanmin Hu, professor of optical oceanography at the University of South Florida and the discoverer of the GASB, said that the Jouanno et al. study conducted a numerical modeling experiment to reinforce the two major points of the Johns et al. (2020) paper.
“Jouanno’s model showed that the negative phase of NAO in 2010 induced a long-distance transport from the Sargasso Sea to the tropical Atlantic to provide sargassum seed populations, and that open-ocean vertical mixing is a major nutrient source,” Hu said.
“This is certainly a nice way to examine what factors can drive the recent Sargassum blooms in the tropical Atlantic,” Hu said. “On the other hand, the bloom is a complex phenomenon, and the theory of long-distance transport still requires direct evidence to verify, for example, through observations of sargassum during the transport.”
Additionally, Hu explains that the major role of open-ocean vertical mixing does not exclude other factors — for example, river discharge and coastal upwelling — as local drivers to stimulate sargassum growth in coastal regions.
Coastal communities’ worst nightmare
Sargassum can be easily spotted from the sky: Looking out from an airplane window, one can see idle dark strips of algae adrift getting close to the shore. But diving into a patch of algae is an invitation to contemplate sea life in suspended animation mode: When large quantities of sargassum flood the coastline, the light struggles to penetrate the water surface, causing fish and turtles to swim away from it.
The arrival of the algae to the shorelines of the Riviera Maya in the Mexican state of Quintana Roo caused an annual decrease in tourism revenue of 11.6% between 2016 and 2019. This also severely affects the hotel industry in destinations in the Dominican Republic and Barbados.
Caribbean entrepreneurs and scientists are actively seeking solutions to tackle the sargassum crisis and turn it into a business opportunity: from deploying anti-algae barriers to hold back the sargassum to experimenting with possible applications for sargassum such as biostimulant for agriculture or renewable methane gas.
The threat of this macroalgae to marine life is severe: In Mahahual, a small coastal town in southeastern Mexico, turtles face the challenge of breeding on beaches covered in sargassum; they cannot detect the white sand of the shores, affecting their nesting sites and their reproduction rates.
Despite the Caribbean being the region most affected by sargassum, Brazilian experts are also participating in the debate. One of the leading South American experts on the matter is José Eduardo Martinelli Filho, researcher at the Federal University of Pará in Brazil. He became involved with this issue when he was called by the local government to dig into the first sargassum bloom in the country, in 2014. But the episode in Pará in March was the worst of all so far, despite being a rare event.
Speaking with Mongabay from his lab in Belém do Pará, Martinelli explains that Jouanno’s paper resumes a hypothesis that dates back to 2020, referring to vertical mixing from deep waters rich in nutrients as the leading cause for sargassum blooms in the equatorial Atlantic.
“Additionally, warmer waters facilitate the reproduction of macroalgae species like Sargassum fluitans and Sargassum natans,” Martinelli says. “This novel paper is of geopolitical significance, since it finally shows that Brazil and the Amazon River’s runoff are probably not related to the enlargement of the GASB.”
According to him, sargassum blooms in this region of Brazil pose a particular risk to both tourism and fishing — the two main economic drivers of the area — given that due to the specific tide and wind conditions, contentions nets like the ones used in the Caribbean wouldn’t work to hold back the algae from shores.
“The hydrodynamics on the coast of Pará is complex, hence it is difficult to produce forecasts of sargassum arrival. The peak of tourism on the coast of Salinópolis during the months of January and July – when the population tenfold from 40,000 inhabitants to 400,000 — demands further research and work on preparedness with drones to reduce the impact of sargassum on tourism and local fishing communities,” he said.
A public concern ahead of COP30
Last March, about 572 trucks were needed to collect the sargassum from Atalaia, the main beach of Salinópolis in Pará, one of the northernmost sea towns of Brazil, located just below the line of the Equator.
Brazilian marine scientists and policymakers are working on a project to increase local preparedness, which aims to reduce the negative impacts of sargassum through the participatory creation of a management system with the communities.
Josie Figuereido, a biologist working for the Environment Secretariat of Salinópolis, says that during the floods of 2018, they found six turtles stranded in sargassum, two of which suffocated and died.
“This coastal town is expecting about 600,00 tourists for next July, and COP30 is driving tourism even further: due to the lack of accommodation infrastructure in the hosting city of Belem, some delegations are planning on staying in Salinópolis and simply take short domestic flights to attend the climate conference,” Figueiredo tells Mongabay.
The Brazilian government expects to welcome some 100,000 visitors over the two weeks of the climate conference in November.
Banner image: Abundant in the Sargasso Sea (above), sargassum has been blossoming in the Caribbean and on the Amazon shore. Image © Deirdre Leowinata / Greenpeace.
This story was supported by the Pulitzer Center.
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Citations:
Jouanno, J., Berthet, S., Muller-Karger, F., Aumont, O., Sheinbaum, J., (2025). An extreme North Atlantic Oscillation event drove the pelagic Sargassum tipping point. Communications Earth & Environment 6, 95. https://doi.org/10.1038/s43247-025-02074-x
Lapointe, B. E., Brewton, R. A., Herren, L. W., Wang, M., Hu, C., McGillicuddy, D. J., … Morton, P. L. (2021). Nutrient content and stoichiometry of pelagic sargassum reflects increasing nitrogen availability in the Atlantic basin. Nature Communications, 12(1). doi:10.1038/s41467-021-23135-7
Wang, M., Hu, C., Barnes, B. B., Mitchum, G., Lapointe, B., & Montoya, J. P. (2019). The great Atlantic Sargassum belt. Science, 365(6448). doi:10.1126/science.aaw7912
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