sargassoa type of floating brown algae, has changed its range in recent decades, declining in the North Atlantic Sargasso Sea while proliferating in the tropical Atlantic. That trend, which has been underway since 2011, continued in 2026 when the algae, commonly known as a type of seaweed, reached its annual peak in June in a stretch of the ocean known as the Great Atlantic. sargasso Belt.

the belt sargasso The abundance in June 2026 made it the second highest. sargasso year in the satellite record, slightly behind 2025, according to scientists at the University of South Florida (USF) College of Marine Sciences. Regionally, the Caribbean Sea and the Gulf of America (Gulf of Mexico) reached record levels, according to the June 2026 USF report. sargasso perspective. The Western and Eastern Caribbean recorded 3.6 and 9 million metric tons, respectively, while the Gulf recorded 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 USF’s Optical Oceanography Laboratory. “The monitoring carried out by our laboratory helps communities know the current extent of sargassum and prepare for what is to come.”

In moderate quantities in the open sea, sargasso It provides habitat for turtles, invertebrates, fish and birds, and adds oxygen to the water through photosynthesis. But too much near shore can entangle and suffocate marine life, and sinking mats can smother corals and seagrass beds. On the beaches, decomposing. sargasso It releases hydrogen sulfide, a gas with a rotten egg smell that is a potential problem for both ecosystems and tourism.

The map above shows sargasso density in the tropical Atlantic Ocean in June 2026. The red and orange areas are where sargasso The densities were the highest. Please note that although the “belt” appears continuous, discrete sargasso The mats are scattered across the ocean surface. The map is based on satellite measurements of the amount of ocean surface covered by algae, 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 sargasso belt, which, despite the irregularity, extends almost continuously from West Africa to the Gulf and maintains a fairly constant “width” from the western tropical Atlantic westward, explained Chuanmin Hu, also an optical oceanographer at USF. Ocean currents have also prevented flooding on Florida’s west coast this summer, while bringing large amounts of algae to the Florida Keys and the state’s east coast. The bulk of the sargassoHowever, it is visible in the Caribbean Sea, as shown in detail above, where problems associated with flooding have been most severe, Hu said.

Data for the maps was provided by Lin Qi, an oceanographer at NOAA’s Satellite Research and Applications Center, who has been working to generate sargasso maps based on PACE data, which was released in February 2024. The work expands that of Qi and his colleagues at the USF Optical Oceanography Laboratory. This team developed for the first time sargasso detection techniques using MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA’s long-duration Terra and Aqua satellites and VIIRS (Visible Infrared Imaging Radiometer) on the NOAA-20 satellite, data that have been a key component of the USF study. sargasso Watch System and research into long-term trends in seaweed.

Satellites detect sargasso by their signals in reflected sunlight. Due to its plant structure and chlorophyll pigments, sargasso reflects more near-infrared light than water. Scientists pinpoint pixels where reflectance increases above levels produced by seawater and then use the strength of this peak to estimate sargasso density, which refers to the fraction of the ocean surface covered by algae in each pixel. Density estimates can then be converted to biomass, or the total weight of sargasso present within one pixel, which is how long-term trends are tracked in the graph below.

The graph above uses the continuous MODIS record since March 2000 to show how sargasso biomass across the Great Atlantic sargasso The belt has changed through June 2026. Note the rebound that started around 2011, when the belt was first developing, and the seasonal dips in winter and spikes in spring and summer. The historical maximum reached in July 2025 stands out, followed by the rapid increase in early 2026 (especially in the first four months of the year) that culminated in the peak of the year in June. More recent observations, not yet reflected in the graph, indicate sargasso biomass decreased during the following July.

“Since the initial appearance of the Great Atlantic sargasso Belt in 2011, the total sargasso 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 is possibly related to warming oceans, multiple nutrient sources and the fact that large sargasso The mats attract other organisms, such as nitrogen-fixing bacteria, which can supply additional nutrients to support further growth.

In addition to MODIS and VIIRS data, PACE OCI data is now incorporated into the sargasso Near real-time daily and weekly composite maps from Watch System. A recent study of the western central Atlantic led by Qi, spanning from May to August 2024, found that OCI offers several advantages over its predecessors, observing more of the ocean and detecting sargasso with greater sensitivity.

Hu noted that OCI’s added pixels can improve near-real-time monitoring and analysis of short-term fluctuations. And its increased sensitivity, he said, will also lead to improved maps during the winter months, “thus helping to understand sargasso “it changes over time.”

Additionally, the study authors found that OCI’s hyperspectral capability makes it the only sensor capable of spectrally discriminating sargasso pixels across the Atlantic Ocean “unambiguously,” adding confidence to the interpretation of detected image characteristics, especially in parts of the Atlantic where other types of floating algae, Trichodesmiumhas 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 advances come to life,” said Jeremy Werdell, PACE project scientist at NASA’s Goddard Space Flight Center. “The OIC has initiated a true renaissance in monitoring aquatic ecosystems from space.”

NASA Earth Observatory maps and graphs 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.

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