In 2025, satellite observations revealed a sprawling brown band of sargassum extending an estimated 8,800 kilometers from West Africa toward the Gulf of Mexico. Known as the Great Atlantic Sargassum Belt, it is a recurring tropical-Atlantic phenomenon, distinct from the Sargasso Sea. The floating algae is made up primarily of species such as Sargassum natans and Sargassum fluitans. Research on the belt, including work published in Harmful Algae, has tracked its growth and effects on ocean and coastal environments.
By May 2025, estimates put the floating biomass at 37.5 million tons, making it the largest year in the satellite record described by researchers. The scale is extraordinary, but huge accumulations are not new: a June 2018 estimate put wet sargassum biomass above 20 million metric tons. The belt’s seasonal reach is commonly described as about 8,850 kilometers, spanning waters between West Africa, the Caribbean and the Gulf of Mexico. Satellite-derived biomass estimates depend on how floating algae is measured and whether totals refer to wet or dry mass. Still, the overall trend is clear: the belt has become a major recurring feature of the Atlantic.
From Anomaly to Annual Crisis
Teams at the Harbor Branch Oceanographic Institute, part of Florida Atlantic University, have tracked sargassum’s rise using satellite records alongside field observations and chemical analyses. Major blooms were widely documented from 2011, and arrivals have recurred almost every year since, with 2013 identified as an exception. By 2026, satellite-based reporting described the belt as near historic highs again, second in size only to 2025 in the record cited. What began as an unusual event has become a seasonal management challenge for coastal communities.
Research reported by Futura-Sciences, citing a study in PNAS Nexus, suggests that some blooms may begin near the Gulf of Guinea years before they become detectable farther west. That finding puts West African waters among the areas researchers are examining to understand how the belt develops and spreads across the tropical Atlantic.
Sargassum: Impressive Habitat or Coastal Threat?
Floating sargassum is not inherently harmful. Offshore mats provide habitat and food for marine life, including juvenile fish and turtles, as well as crabs and other organisms. The ecological balance changes when dense accumulations drift into shallow coastal waters or ashore. There, mats can reduce light reaching seagrass beds and reefs, smother habitat and contribute to oxygen depletion as the algae breaks down.
Decomposition also releases hydrogen sulfide and ammonia, producing a strong rotten-egg odor around heavily affected beaches. Dense deposits can disrupt coastal ecosystems, restrict swimming and require repeated removal. For tourism-dependent communities, the effects are immediate: beach conditions deteriorate while local authorities and businesses face the cost of cleanup and disposal.
Nutrient Overload: The Engine of Growth
Researchers are investigating how nutrient supply, warmer ocean conditions and circulation patterns may combine to support larger blooms. Nitrogen and phosphorus are among the nutrients under study. Experiments cited in the research have found that sargassum biomass can double in 11 days under nutrient-rich conditions. Between 1980 and 2020, nitrogen content in sargassum tissue rose by 55%, while the nitrogen-to-phosphorus ratio increased by 50%, changes consistent with a shift in the nutrient environment of the Atlantic.
Agricultural runoff and wastewater can carry nutrients from land into rivers and coastal waters. Atmospheric deposition is another potential pathway. Nutrient enrichment, or eutrophication, can encourage algal growth. Researchers consider its contribution to the belt alongside other factors, rather than treating it as a single explanation for the belt’s expansion.
The Role of Rivers and Ocean Currents
The Amazon is one of the major river systems researchers are examining for its potential role in delivering nutrients to the Atlantic. Seasonal runoff can carry nitrogen and other nutrients offshore, while winds and currents influence where floating mats travel. The Congo is also being considered as a possible source of nutrient input. Ocean circulation, including the Loop Current and Gulf Stream, helps transport sargassum and shape which coastlines receive the largest accumulations. An extensive offshore belt does not mean every beach will be affected equally: local arrivals depend on shoreline shape and cleanup capacity, as well as winds and currents.
Large sargassum accumulations have been documented in the Gulf of Mexico since at least 2004. The region’s changing flows connect the belt’s offshore movement with the risks facing coastal waters and infrastructure.
Risks for Beaches, Ecosystems, and Tourism
When large quantities wash ashore, decomposing algae can lower oxygen levels in coastal waters and put pressure on coral reefs and seagrass habitats. The same deposits can make beaches difficult to use and deter visitors, while cleanup operations require sustained labor and equipment. Offshore, but, floating mats can support marine life. Coastal managers must weigh sargassum’s ecological value at sea against the damage and disruption caused by excessive nearshore accumulations.
What Can Be Done?
Research recommendations include stronger international monitoring, improved forecasting and sustained efforts to reduce nutrient pollution. Satellite tracking can help map the belt offshore, while landfall forecasts can give vulnerable communities more time to prepare cleanup teams. Measures to reduce nitrogen and phosphorus runoff, alongside better wastewater management, address potential nutrient pathways without treating them as the sole cause of the blooms.
These steps cannot eliminate the phenomenon, but they can help communities respond earlier and limit local impacts. Since 2011, recurring blooms have made sargassum management a continuing seasonal concern across the Caribbean and parts of the Gulf of Mexico, calling for coordination between researchers and coastal authorities.
Climate Questions and the Carbon Cycle
The belt’s implications extend beyond beaches and tourism. As sargassum decomposes, it can release methane and other gases, raising questions about how large accumulations affect the marine carbon cycle. Researchers are studying warmer ocean conditions and nutrient inputs alongside circulation patterns as possible contributors to the belt’s changing scale and distribution. Its record growth has made the Atlantic’s brown ribbon a visible sign of a rapidly changing marine environment.
