Sizing Up the Sargassum Belt

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 reflectance spikes above the levels produced by plain seawater, and then they use the strength of this spike to estimate Sargassum density, which refers to the fraction of ocean surface covered by the seaweed in each pixel. Density estimates can then be converted into biomass, or the total weight of Sargassum present within a pixel, which is how the longer-term trends in the chart below are tracked.
The chart above uses the continuous MODIS record since March 2000 to show how Sargassum biomass across the Great Atlantic Sargassum Belt has changed through June 2026. Notice the uptick beginning around 2011, when the belt was first developing, and the seasonal dips in winter and peaks in spring and summer. The record high in July 2025 stands out, followed by the quick rise in early 2026—especially in the first four months of the year—that culminated in the year’s peak in June. More recent observations, not yet reflected in the chart, indicate Sargassum biomass declined through the following month of July.
“Since the initial appearance of the Great Atlantic Sargassum Belt in 2011, the total Sargassum amount in the Atlantic Ocean has increased substantially, more than doubling every five years,” Hu said. He added that the exact mechanism is still being investigated, but it’s possibly related to ocean warming, multiple nutrient sources, and the fact that large Sargassum mats attract other organisms—such as nitrogen-fixing bacteria—that can supply additional nutrients to sustain further growth.
Alongside data from MODIS and VIIRS, OCI data from PACE now feeds into the Sargassum Watch System’s near-real-time daily and weekly composite maps. A recent study of the central-west Atlantic led by Qi, spanning May through August 2024, found that OCI offers several advantages over its predecessors, observing more of the ocean and detecting Sargassum with greater sensitivity.
Hu noted that the added pixels from OCI can improve near-real-time monitoring and analyses of short-term fluctuations. And its higher sensitivity, he said, will also lead to improved maps during winter months, “thus helping understand Sargassum changes over time.”
Additionally, the study’s authors found that OCI’s hyperspectral capability makes it the only sensor able to spectrally discriminate Sargassum pixels across the Atlantic Ocean “without ambiguity,” adding confidence to the interpretation of detected image features—especially in parts of the Atlantic where another type of floating algae, Trichodesmium, has been reported.
“I think I can speak for all project members, past and present, in sharing how rewarding it is to see the promise of PACE’s advancements come to life,” said Jeremy Werdell, PACE project scientist at NASA’s Goddard Space Flight Center. “OCI has started a true renaissance in aquatic ecosystem monitoring from space.”
NASA Earth Observatory maps and chart by Lauren Dauphin, using PACE and MODIS data courtesy of Lin Qi (NOAA), and Brian Barnes and Chuanmin Hu (University of South Florida, Optical Oceanography Laboratory). Story by Kathryn Hansen.
References & Resources
- NASA Earth Observatory (2023, April 8) A Massive Seaweed Bloom in the Atlantic. Accessed July 28, 2026.
- NASA Earth Observatory (2019, July 9) Scientists Discover the Biggest Seaweed Bloom in the World. Accessed July 28, 2026.
- Qi, L., et al. (2026) Hyperspectral OCI/PACE observations of the Atlantic Sargassum. Remote Sensing of Environment, 334, 115185.
- University of South Florida, College of Marine Science (2026, June 30) Outlook of 2026 Sargassum blooms. Accessed July 28, 2026.
- University of South Florida, Optical Oceanography Laboratory (2026, July) Satellite-based Sargassum Watch System (SaWS). Accessed July 28, 2026.
- Zhang, Y., et al. (2025) Dramatic decline of Sargassum in the north Sargasso Sea since 2015. Nature Geoscience, 18, 1266–1272.