// NASA BREAKING NEWS — SPAZIO & SCIENZA
Sizing Up the Sargassum Belt
Seaweed composing the Great Atlantic Sargassum Belt hit near-record levels in June 2026, with the Caribbean Sea and Gulf of America both reaching all-time highs.
Sargassum, a type of brown floating algae, has shifted its range in recent decades, thinning out in the North Atlantic’s Sargasso Sea while proliferating in the tropical Atlantic. That trend, underway since 2011, continued in 2026 as the algae, commonly known as a type of seaweed, reached its annual peak in June across a stretch of ocean known as the Great Atlantic Sargassum Belt.
The belt’s Sargassum abundance in June 2026 made it the second-highest Sargassum year in the satellite record, slightly behind 2025, according to scientists at the University of South Florida (USF) College of Marine Science. Regionally, the Caribbean Sea and the Gulf of America (Gulf of Mexico) both hit record highs, according to USF’s June 2026 Sargassum outlook. The western and eastern Caribbean saw 3.6 and 9 million metric tons, respectively, while the Gulf saw 5 million metric tons—nearly double its previous record, also set in 2025.
“The belt is a basin-scale phenomenon that can have devastating local-scale impacts throughout the Caribbean and Gulf, and satellite observations are the only method that captures both scales on a daily basis,” said Brian Barnes, a marine scientist at the Optical Oceanography Laboratory at USF. “The tracking done by our lab helps communities know the current extent of Sargassum and prepare for what's to come.”
In moderate amounts in the open ocean, Sargassum provides habitat for turtles, invertebrates, fish, and birds, and adds oxygen to the water through photosynthesis. But too much of it near shore can tangle and suffocate marine life, and mats that sink can smother corals and seagrasses. On beaches, decomposing Sargassum releases hydrogen sulfide, a rotten-egg-smelling gas that’s a potential problem for both ecosystems and tourism.
The map above shows Sargassum density in the tropical Atlantic Ocean in June 2026. Red and orange areas are where Sargassum densities were the highest. Note that although the “belt” appears continuous, discrete Sargassum mats are scattered across the ocean surface. The map is based on satellite measurements of how much of the ocean surface was covered by the seaweed, averaged per pixel across all observations made in June by the OCI (Ocean Color Instrument) on NASA’s PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) satellite.
Ocean currents and winds shape the Sargassum belt, which, despite the patchiness, stretches nearly continuously from West Africa to the Gulf and holds a fairly steady “width” from the western tropical Atlantic westward, explained Chuanmin Hu, also an optical oceanographer at USF. The ocean currents have also spared Florida's west coast from inundation this summer, while delivering large amounts of seaweed to the Florida Keys and the state's east coast. The bulk of the Sargassum, however, is visible in the Caribbean Sea, shown in detail above, where problems associated with inundation have been more severe, Hu said.
Data for the maps were provided by Lin Qi, an oceanographer at NOAA's Center for Satellite Applications and Research, who has been working to generate Sargassum maps based on data from PACE, which was launched in February 2024. The work extends that of Qi and colleagues at USF’s Optical Oceanography Laboratory. This team first developed Sargassum detection techniques using MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA’s long-running Terra and Aqua satellites and VIIRS (Visible Infrared Imaging Radiometer Suite) on the NOAA-20 satellite—data that have been a key component of USF’s Sargassum Watch System and of research into the seaweed’s longer-term trends.
Satellites detect Sargassum by its signals in reflected sunlight. Because of its plant structure and chlorophyll pigments, Sargassum reflects more near-infrared light than water. Scientists flag pixels where the reflect