Overview

• Florida Atlantic University’s Harbor Branch published a 40-year synthesis in Harmful Algae detailing how pelagic Sargassum expanded into the Great Atlantic Sargassum Belt.
• In May 2025 the belt reached a record 37.5 million tons of biomass, not counting an estimated 7.3 million tons in the Sargasso Sea baseline, after annual appearances since 2011 except 2013.
• Tissue chemistry shows nitrogen rose by more than 50% since the 1980s while phosphorus dipped slightly, a shift the authors link to agricultural runoff, wastewater and atmospheric deposition.
• Observations and models indicate biomass is supplied from nutrient-rich coasts, particularly the western Gulf of Mexico, and transported offshore via the Loop Current and Gulf Stream, with Amazon outflow and local recycling also contributing.
• Mexico designated Caribbean sargassum a fishery resource with a precautionary annual harvest cap of up to 945,000 tons, permitting open-sea collection under oversight as the region pursues a coordinated plan for monitoring, prevention and sustainable use.