SerpApi.Org
Stretching nearly 5,000 miles across the open ocean, an enormous band of brown floating seaweed is once again moving through the Atlantic, and researchers tracking it say its most recent measurements place it at record-high levels.
The formation, known as the Great Atlantic Sargassum Belt, is not a single solid mass but a series of dense, drifting mats that follow prevailing ocean currents from the tropical Atlantic through the Caribbean Sea and into the Gulf of Mexico. What was once a periodic curiosity has become a fixed feature of the seasonal cycle, one with measurable consequences for coastlines, reefs, and island economies across the western Atlantic.
The belt carries more than 20 million metric tons of biomass at its seasonal peak. Under the right conditions, the seaweed doubles in mass in under 11 days, driven by warm water temperatures and elevated nutrient concentrations. Satellite monitoring by the University of South Florida and the National Oceanic and Atmospheric Administration shows the belt remained at record-high levels through the most recent July observation period.
A New Seaweed Population Established Itself in the Equatorial Atlantic
For most of recorded history, large concentrations of Sargassum floated in the Sargasso Sea, a relatively calm zone in the North Atlantic bounded by ocean currents rather than coastlines. Small amounts would drift south and west, but they rarely caused significant problems. Around 2011, that pattern changed. A secondary population of the seaweed established itself in the Equatorial Atlantic, in the warm waters between Africa and South America, and it proved far more productive than scientists had previously observed.
Optical oceanographer Chuanmin Hu of the University of South Florida identified this shift using NASA satellite imagery and named the resulting structure the Great Atlantic Sargassum Belt. His tracking data showed the belt had become a recurring seasonal feature rather than an isolated event. Each spring and summer, the population blooms rapidly, accumulating biomass before the North Equatorial Current carries it westward toward the Caribbean.
A study published in the journal Harmful Algae by Julien Jouanno of the Institut de Recherche pour le Développement identifies elevated nutrient levels as the primary driver of this growth. One major nutrient source is discharge from the Amazon River, which carries nitrogen and phosphorus from the South American interior into the tropical Atlantic, effectively fertilizing the water column where the belt develops. Ocean chemistry in this region has shifted enough, according to the research, that large-scale annual blooms are now the expected outcome rather than an exception.
Decaying Seaweed Releases Toxic Gas and Cuts Off Light Below the Surface
The consequences of the belt become most acute when the mats reach shore. On Caribbean beaches, the seaweed piles up in mounds several feet high. As it decomposes, it releases hydrogen sulfide, a gas that smells like rotten eggs and can cause respiratory problems for people in the area. The decomposition also strips oxygen from the surrounding water, creating dead zones where fish and other marine organisms cannot survive.
Beneath the surface, the mats block sunlight from reaching the seafloor. Seagrass beds and coral reefs that depend on light to survive are particularly vulnerable. Extended coverage can kill the photosynthetic organisms these ecosystems rely on, disrupting food webs and reducing habitat for juvenile fish, sea turtles, and other marine life that depend on shallow coastal environments.
When the seaweed remains offshore in the open ocean, it serves a different function, providing temporary shelter and food for small fish, sea turtles, and other species that use the floating mats as refuge. At the scale now observed, however, the coastal accumulation outweighs those open-water benefits for the communities living alongside the affected shores.
Caribbean Tourism and Coastal Infrastructure Face Recurring Disruption
The economic pressure from annual inundations is substantial. In the Caribbean, some hotels have been forced to close beach access during peak arrival periods. Physically removing the seaweed is both expensive and logistically difficult: heavy machinery deployed to clear beaches can erode shorelines and disturb sea turtle nesting sites, creating a second layer of environmental damage in the process of managing the first.
Island nations that depend on clear water and healthy reef systems for tourism revenue bear a disproportionate share of the burden. The cost of cleanup, lost bookings, and reef degradation compounds year after year without a reliable way to reduce the volume reaching the coast.
Researchers have explored whether the seaweed could be repurposed, with some companies examining its potential as fertilizer, building bricks, or biofuel. High salt content and the presence of heavy metals including arsenic complicate processing at any useful scale, and no commercial solution has moved beyond early development.
NOAA and University Partners Track the Belt Week by Week
Monitoring the belt has become a coordinated research effort. Scientists at NOAA’s Atlantic Oceanographic and Meteorological Laboratory work alongside researchers at NOAA’s National Environmental Satellite Data, Information Service and the University of South Florida to publish weekly Sargassum Inundation Reports. These reports use satellite sensor data to estimate the extent and density of the belt and assess the risk of seaweed reaching specific coastal areas, including the Caribbean, southeast Florida, and the Gulf of Mexico.
The belt’s behavior is also influenced by a climate pattern called the Atlantic Equatorial Mode, which affects sea surface temperatures and wind strength across the tropical Atlantic. When winds weaken, mats tend to consolidate into larger, denser patches. Stronger wind conditions scatter the seaweed more broadly.
NOAA’s AOML research program also includes field experiments using satellite tracking devices and surface drifters to monitor actual seaweed movement, complementing the satellite imagery with real-world trajectory data. The University of South Florida continues to update its tracking system ahead of the 2026 bloom season.
