NASA Earth Observatory reports that the Great Atlantic Sargassum Belt reached near-record levels in June 2026, based on observations from the PACE satellite. The result is significant at two scales: the belt as a whole recorded its second-highest abundance in the satellite record, while the Caribbean and Gulf of America reached their own record levels.
The findings come from NASA's Earth Observatory analysis of data collected by the Ocean Color Instrument, or OCI, aboard PACE. The satellite maps the fraction of the ocean surface covered by floating Sargassum, allowing scientists to follow a phenomenon that stretches across a large part of the tropical Atlantic but can create highly concentrated impacts near individual coastlines.
A second-highest year for the Atlantic belt
Sargassum is a brown floating alga that forms drifting mats in the open ocean. NASA's analysis shows that the Great Atlantic Sargassum Belt reached its annual peak in June. Its total abundance was second only to the record set in 2025, according to scientists at the University of South Florida College of Marine Science.
Regional measurements were even more pronounced. NASA reports estimates of 3.6 million metric tons in the western Caribbean and 9 million metric tons in the eastern Caribbean. The Gulf recorded about 5 million metric tons, nearly twice its previous regional record, which was also set in 2025. Those figures describe biomass within the monitored regions; they do not mean that every part of the belt was covered by a continuous, solid layer of seaweed.
The belt is patchy even when it appears continuous on a map. Ocean currents and winds shape its distribution from West Africa toward the Gulf, moving mats toward some shorelines while leaving others with much less material. NASA notes that Florida's west coast avoided major inundation during the reported period, while the Florida Keys and the state's east coast received larger amounts.
Why the satellite view matters
Local observations can show what is arriving on a beach, but they cannot easily capture the full scale of a system that spans an ocean basin. Satellite measurements provide that wider view and can also be repeated frequently. Brian Barnes, a marine scientist at the University of South Florida's Optical Oceanography Laboratory, told NASA that the tracking helps communities understand the belt's current extent and prepare for what may follow.
PACE adds a newer layer to a monitoring record that already includes data from NASA's Terra and Aqua satellites and the NOAA-20 satellite. The OCI instrument detects Sargassum through the way its plant structure and chlorophyll reflect sunlight. In particular, the algae produces a stronger near-infrared signal than ordinary seawater. Scientists use that contrast to identify likely Sargassum pixels, estimate the fraction of each pixel covered by the algae, and convert those density estimates into biomass.
The PACE instrument is also hyperspectral, meaning it measures reflected light across many closely spaced wavelengths. NASA's report says this gives OCI greater sensitivity and helps distinguish Sargassum from other floating material in parts of the Atlantic. Its observations now contribute to near-real-time daily and weekly maps used by the Sargassum Watch System.
High biomass does not mean every effect is negative
In moderate amounts offshore, Sargassum supports a floating habitat for turtles, invertebrates, fish, and birds. Its photosynthesis also adds oxygen to the surrounding water. The ecological balance changes when large quantities accumulate near shore. Dense mats can entangle or suffocate marine life, while sinking material can cover coral and seagrass. On beaches, decomposing Sargassum releases hydrogen sulfide, a gas associated with the familiar rotten-egg smell and potential concerns for ecosystems and tourism.
That contrast is why a basin-wide satellite total cannot be read as a single measure of harm. A high total may be distributed across open water, concentrated in particular coastal areas, or shifted by currents before it reaches land. NASA's observations through July indicated that biomass had begun to decline after the June peak, although later measurements were not included in the chart presented in the report.
The cause is still being investigated
The longer record shows that the Great Atlantic Sargassum Belt began developing around 2011 and that total Sargassum in the Atlantic has increased substantially since then, more than doubling every five years according to Chuanmin Hu of the University of South Florida. The report does not identify one confirmed cause for the pattern.
Researchers are considering several possibilities, including warmer ocean conditions, multiple nutrient sources, and biological feedbacks within large mats. Nitrogen-fixing bacteria and other organisms can add nutrients that help sustain further growth, but NASA presents these mechanisms as possibilities under investigation rather than a settled explanation.
The clearest conclusion from the latest observations is therefore about measurement as much as magnitude. PACE is making it easier to compare the belt's basin-scale changes with the local coastal conditions that matter to ecosystems and communities. June 2026 was an exceptional month in the record, but the satellite data also show why the event must be tracked as a changing, patchy ocean system rather than treated as one uniform bloom.
