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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →The 2023 warning about a “largest ever” Sargassum bloom is now historical: satellite observations through June 2026 put 2025 first in the record and 2026 second. That basin-wide abundance does not tell us how much seaweed will reach any particular beach, or when. Sargassum travels in scattered floating mats, and local winds, currents, tides and waves shape where it comes ashore.
How the record changed after the 2023 warning
The Great Atlantic Sargassum Belt is monitored through satellite observations. NASA’s account of University of South Florida estimates says the series uses continuous MODIS observations from March 2000 through June 2026; the recurring tropical belt trend began around 2011. In that record, 2025 was the most abundant year, while June 2026 ranked second through the latest observation cited here.
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The figures below refer to different periods and geographic scopes, so they should not be treated as a like-for-like comparison. Regional monthly estimates are not amounts expected to wash onto shore.
| Period and area | Reported estimate | What it describes |
|---|---|---|
| June 2026, western Caribbean | 3.6 million metric tons | University of South Florida estimate reported by NASA |
| June 2026, eastern Caribbean | 9 million metric tons | University of South Florida estimate reported by NASA |
| June 2026, Gulf | 5 million metric tons | University of South Florida estimate reported by NASA; nearly double the previous Gulf record, set in 2025 |
| July 2025, Atlantic including the Caribbean and Gulf | 38 million metric tons | University of South Florida Optical Oceanography Laboratory estimate for that month and combined region |
The 2025 July estimate is a monthly total across a broad region; the June 2026 figures are regional estimates. Neither tells us how much biomass was headed for a particular coastline. The July 2025 bulletin attributed increases in the Gulf and western and eastern Caribbean mostly to physical transport.
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What the Great Atlantic Sargassum Belt is
Sargassum is a floating brown algae. The belt is a broad distribution of patches across the tropical Atlantic, Caribbean and Gulf—not a single solid island or an unbroken blanket over the sea. NOAA describes patches ranging from a few centimeters to hundreds of meters across and a belt spanning about 5,000 kilometers. NASA describes its broad extent as nearly continuous from West Africa toward the Gulf, while noting that the individual mats are scattered.
That distinction matters: a very large satellite estimate describes sea-surface abundance over an ocean basin, not a wall of seaweed moving intact toward North America. The University of South Florida’s Brian Barnes has described it as “a basin-scale phenomenon that can have devastating local-scale impacts throughout the Caribbean and Gulf.”
How Sargassum reaches the Caribbean, Gulf and North American coasts
Major accumulations have recurred annually in the tropical Atlantic, Caribbean and Gulf since 2011, but their amounts vary substantially from year to year. NOAA says aggregation typically begins around January or February north of the Equator near the convergence of the trade winds. Seasonal winds and currents carry mats northward, and some enter the Caribbean and Gulf through the North Equatorial and Caribbean current systems.
Gulf pathways are not one-way or sourced from only one place. A peer-reviewed study hosted by NOAA, using satellite observations and ocean-current data from 2000–2023, found that Gulf Sargassum can originate in the northwestern Gulf or arrive from the northwestern Caribbean. The Loop Current System and its eddies influence distribution, with pathways that can carry material north, east or west.
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The 2026 pattern shows why basin-wide totals cannot predict which coast will be affected most: NASA reported that currents spared Florida’s west coast while carrying substantial Sargassum to the Keys and Florida’s east coast.
What satellite monitoring can—and cannot—tell you
Satellites detect light reflected from the ocean surface to map Sargassum coverage; those observations can be converted into biomass estimates. NASA describes PACE/OCI maps for 2026 and the use of MODIS and VIIRS observations in the longer record. These tools help track broad distribution, but they do not establish how much material will reach a specific beach.
NOAA’s experimental Sargassum Inundation Report shows current patch extent and flags coastlines within 50 kilometers of patches as potentially at risk. It is not a forecast of beaching. Local currents, wind, tides and waves affect whether mats reach shore, and NOAA CoastWatch says the timing of beaching cannot currently be predicted. A named beach’s likely arrival date therefore requires a current local official bulletin; the basin-scale record alone cannot provide one.
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Why Sargassum matters to marine animals—and when it becomes a problem
Floating mats provide habitat offshore
In normal amounts in open water, Sargassum provides habitat, food, shelter and breeding grounds for marine life, including fish and sea turtles. The ecological role of floating mats is different from the effects of a heavy accumulation in shallow coastal water.
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Large coastal accumulations can damage habitats and wildlife
Near shore, excessive Sargassum can smother coral and seagrass, entangle or suffocate marine animals, and decompose in water or on beaches. Decomposition can lower oxygen in nearshore water, adding stress for aquatic life.
Odor, health uncertainty and contaminant sampling
Decomposing Sargassum releases hydrogen sulfide, which is associated with a rotten-egg odor and can pose respiratory concerns. NOAA says research on human-health effects is recent and needs more time; it also notes that Sargassum can contain heavy metals. Neither point establishes that every beach pile is contaminated or unsafe.
In March 2026, NOAA’s National Centers for Coastal Ocean Science reported that scientists were sampling Sargassum from southeastern Florida, Puerto Rico and the U.S. Virgin Islands for pollutants and trace metals. That is an active investigation, not a finding that all Sargassum in those places contains harmful levels.
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Beach cleanup can be costly, and heavy accumulations can disrupt tourism and fisheries. A 2026 Woods Hole Oceanographic Institution announcement describing a WHOI–University of Rhode Island study reported multi-million-dollar annual losses in Puerto Rico and the U.S. Virgin Islands and potentially billion-dollar impacts along Florida’s Atlantic coast. These are modeled or estimated impacts for the places studied, not a universal bill for every Sargassum season.
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