Since 2011, large mats of smelly brown seaweed called sargassum have increasingly washed ashore on beaches in South Florida and throughout the Caribbean, clogging harbours, interfering with fishing and boating, and in some cases harming marine life. As the seaweed decomposes, it releases gases including hydrogen sulfide, ammonia and methane, raising concerns for public health and the environment.
Why sargassum blooms keep growing
Scientists from The Conversation study the oceans, marine life and algae blooms. Their research suggests that these vast mats of seaweed may have entered a self-sustaining cycle, effectively feeding their own expansion. Pelagic sargassum is a type of brown seaweed that floats on the ocean surface and was naturally abundant only in the Sargasso Sea. But in 2011, unusually large mats appeared in the tropical Atlantic, between 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. This new area is now called the Great Atlantic Sargassum Belt, where the amount of sargassum surpassed an estimated 37,500 million tons in 2025. Forecasts suggest summer 2026 could break records again.
The role of the 'sargassumsphere'
For years, scientists searched for the causes, exploring changes in ocean nitrogen and phosphorus from fertiliser, Saharan dust, and changes in ocean temperatures and currents. But none fully explained year-to-year variations. New research suggests the main cause has changed. In winter 2010, strong winds brought sargassum southward from the Sargasso Sea. For a few years, strong winds in winter and spring mixed deeper, nutrient-rich water to the surface, giving algae resources to bloom. A floating sargassum mat is more than seaweed; it is a floating ecosystem home to fish, shrimp, crabs and tiny organisms, called the ‘sargassumsphere’. These animals collect food and recycle nutrients through their waste, concentrating nutrients around the seaweed and helping the next bloom grow. This makes blooms very likely to persist.
Model predicts blooms accurately
To test this theory, scientists created a mathematical model that accounts for the mechanisms controlling blooms. Using measurements of nitrogen content and isotopic composition in sargassum collected around the US Virgin Islands, the model not only reproduced blooms from 2011 to 2022 but also predicted ahead of time the size and evolution of blooms in 2023 and 2024, suggesting the theory is correct.
Turning nuisance into resource
Predicting blooms can help create early warning systems for coastal communities. It can also support efforts to turn sargassum into a valuable resource. Sargassum biomass is being explored as a renewable feedstock for biofuel production – it does not need land, freshwater or fertilisers. Scientists at a university in Barbados developed a way to produce biofuel from a combination of sargassum and wastewater from the local rum industry. The island also encourages using sargassum as a fertiliser and soil amendment after processing to remove salt and metals. Sargassum is increasingly explored as a sustainable construction material, particularly in Mexico and Brazil, 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. Using sargassum as a mortar additive also helps reduce dependence on carbon-intensive materials.
