Vast mats of sargassum seaweed, which have been affecting beaches in South Florida and the Caribbean since 2011, may now be in a self-sustaining cycle, according to new research. This process could be feeding the seaweed's own expansion, creating a chronic issue for affected coastal areas.
Historically, pelagic sargassum is found in the Sargasso Sea. However, since 2011, unusually large mats have been observed in the tropical Atlantic, forming what is now known as the Great Atlantic Sargassum Belt. In 2025, the amount of sargassum in this belt exceeded an estimated 37,500 million tons, with forecasts suggesting 2026 could see new record levels.
Scientists propose that while strong winds initially brought sargassum southward from the Sargasso Sea and mixed nutrient-rich water to the surface, the blooms are now sustained by a 'sargassumsphere' ecosystem. This ecosystem, comprising fish, shrimp, crabs, and other organisms, collects and recycles nutrients around the floating mats, supporting further growth.
A mathematical model, tested with nitrogen content measurements from sargassum around the U.S. Virgin Islands, reproduced past blooms and predicted the size and evolution of blooms in 2023 and 2024, lending support to this theory. The research indicates that the Great Atlantic Sargassum Belt is likely to persist.
Efforts are underway to transform sargassum from a nuisance into a resource. It is being investigated as a renewable feedstock for biofuel production, a fertiliser, and a sustainable construction material in regions like Mexico and Brazil.