Since 2011, massive blooms of sargassum seaweed have increasingly disrupted beaches and coastal economies in South Florida, the Caribbean, and the Gulf of Mexico. Now, scientists from the Georgia Institute of Technology suggest that these vast mats of smelly brown seaweed may have entered a self-sustaining cycle, effectively feeding their own expansion and creating a chronic disaster with little sign of ending.
What is sargassum and where does it come from?
Pelagic sargassum is a type of brown seaweed that floats on the ocean surface and is naturally abundant in the Sargasso Sea, a region of the subtropical North Atlantic. The seaweed takes its name from sargaço, the Portuguese word for grape, due to its small, grapelike, air-filled bladders that keep it afloat. Christopher Columbus wrote about encountering vast mats of it during his voyage to the Americas.
However, in 2011, unusually large mats of sargassum were noticed in the tropical Atlantic, between the coasts of Africa and Brazil. Since then, blooms have become more frequent, lasted longer, and grown much larger, with vast amounts washing ashore across Central America, the Caribbean, and the Gulf of Mexico. In this new area, now called the Great Atlantic Sargassum Belt, the amount of sargassum surpassed an estimated 37,500 million tons in 2025. Forecasts suggest summer 2026 could break records again.
What drives the blooms?
For years, scientists have searched for the causes of the Great Atlantic Sargassum Belt. They explored changes in ocean nitrogen and phosphorus from fertilizer and other human activities in the Amazon and Congo river basins, increasing amounts of Saharan dust, and changes in ocean temperatures, winds, and currents. None of these hypotheses could fully explain year-to-year variations.
New research suggests the main cause has changed. In the winter of 2010, strong winds brought some sargassum southward from the Sargasso Sea to the tropical Atlantic. For a few years, winds stayed strong in winter and spring, acting like a giant spoon and mixing deeper, nutrient-rich water up to the sunlit surface, giving the algae the resources it needed to bloom and grow.
According to the scientists, a floating sargassum mat is much more than just seaweed. It is like a floating city, home to fish, shrimp, crabs, and countless tiny living organisms, all food for larger fish. Together, they form an ecosystem called the sargassumsphere. These animals collect food around the floating mats and recycle nutrients through their waste and the breakdown of older sargassum. In this way, nutrients are gathered from a much larger area and concentrated around the seaweed, helping the next bloom to grow. This process can make sargassum blooms very likely to persist in the years ahead.
Model predicts blooms
To test this theory, the researchers created a mathematical model that accounts for the mechanisms controlling the blooms. Using measurements of nitrogen content and isotopic composition in sargassum collected around the U.S. Virgin Islands, the model not only reproduced the blooms from 2011 to 2022 but also predicted ahead of time the size and evolution of blooms that occurred in 2023 and 2024, suggesting the theory is correct.
Turning nuisance into resource
The research shows that the Great Atlantic Sargassum Belt is likely here to stay. Predicting when blooms will occur and understanding what drives them can help create early warning systems that give coastal communities time to prepare. It can also support long-term efforts to turn sargassum from a costly nuisance into a valuable resource.
Sargassum biomass has attracted growing interest as a renewable feedstock for biofuel production. Unlike bioenergy crops grown on land, sargassum does not need land, freshwater or fertilizers, reducing competition with food production. The seaweed is rich in carbohydrates that can be converted into bioethanol, biogas, bio-oil, or hydrogen through processes such as fermentation, anaerobic digestion, hydrothermal liquefaction, and gasification.
Scientists at a university in Barbados developed a way to produce biofuel from a combination of the excessive sargassum on the island's shores and wastewater from the local rum industry. The island is also encouraging farms to use sargassum as a fertilizer and soil amendment, providing a sustainable alternative to synthetic fertilizers. Sargassum is rich in organic matter, potassium, calcium, magnesium, and trace elements. After careful processing to remove the salt and excessive metals, it can be used to improve soil structure, enhance water retention, and stimulate plant growth.
Sargassum is increasingly also being explored as a sustainable construction material, particularly in Mexico and Brazil. The biomass can be incorporated into building products, such as compressed panels similar to drywall developed by researchers at the National Autonomous University of Mexico, and low-cost bricks developed by entrepreneur Omar Vázquez Sánchez in Mexico using about 40% sargassum with limestone and other organic materials.
Conclusion
By learning to live with and make use of sargassum, affected regions may be able to transform a growing challenge into a new opportunity. This research was conducted by Xing Zhou, Annalisa Bracco, and Joseph Montoya of the Georgia Institute of Technology and published in The Conversation.
