June 2026 Sargassum Belt Near-Record as Caribbean and Gulf See Record June Accumulations

Satellite observations show the Great Atlantic Sargassum Belt swelled to near-record levels in June 2026, with the Caribbean Sea and Gulf of Mexico posting record amounts for the month, according to a NASA Earth Observatory report published Tuesday summarizing University of South Florida monitoring.

Why that matters is not just the size of the bloom offshore, but where it ended up. NASA and the University of South Florida said winds and currents helped spare much of Florida’s west coast while pushing large amounts of the floating brown seaweed toward the Florida Keys and Florida’s east coast. Severe beaching events were reported along Florida’s southeast coast and continued around the Caribbean and Lesser Antilles, where heavy accumulations can foul beaches, disrupt tourism and strain coastal ecosystems.

The University of South Florida’s Optical Oceanography Lab, in its June 30 Sargassum Watch System bulletin, estimated total basinwide wet biomass at 33.6 million metric tons in June. Regionally, it put the Gulf of Mexico at 5 million metric tons, the western Caribbean at 3.6 million, the eastern Caribbean at 9 million, the western Atlantic at 14.6 million and the east Atlantic at 4.3 million.

The distinction in the data is important. Scientists did not say the entire Atlantic basin set an all-time record. Rather, basinwide June 2026 levels were near-record, and 2026 ranked as the second-highest year in the satellite record through June, slightly behind 2025. The regional highs were sharper: The Caribbean Sea “continued to show record-high Sargassum amount for the month of June,” the South Florida lab said, and the Gulf of Mexico total “reached 5 million metric tons,” nearly doubling its previous June record from 2025.

NASA’s July 29 Earth Observatory article, “Sizing Up the Sargassum Belt,” said the June maps drew on the Ocean Color Instrument aboard NASA’s PACE satellite, launched in February 2024, along with older sensors including MODIS and VIIRS. NASA said the newer instrument offers improved sensitivity and coverage, helping scientists detect and track Sargassum more effectively.

“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,” Brian Barnes, a marine scientist at the University of South Florida’s Optical Oceanography Laboratory, said in the NASA report.

Jeremy Werdell, PACE project scientist at NASA’s Goddard Space Flight Center, said in the same report that “OCI has started a true renaissance in aquatic ecosystem monitoring from space.”

The Great Atlantic Sargassum Belt is a recurring mass of floating seaweed that has appeared at large scale across the tropical Atlantic since 2011. In the open ocean, Sargassum can provide habitat for marine life. But when large mats pile up near shore, they can smother seagrasses and corals, deplete oxygen as they sink and make beaches unusable. As the seaweed decomposes on land, it can release hydrogen sulfide and ammonia, gases linked in past outbreaks to respiratory, eye and neurological irritation.

The longer-term trend has also been upward. “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,” Chuanmin Hu of the University of South Florida said in the NASA story.

Even so, the June peak may not define the rest of the season. NASA said more recent observations showed biomass declining through July 2026, though that drop was not yet reflected in the longer MODIS-based chart. That makes June a significant high point in an already intense year, especially for coastal communities dealing with what washed ashore.