The so-called “brown ribbon” across the Atlantic is real, but it is not a continuous carpet that has suddenly swallowed the ocean. It is a seasonally changing distribution of scattered rafts of Sargassum—floating seaweed that can nurture marine life offshore and create serious problems when it washes ashore in overwhelming quantities.
What exactly is floating out there?
Sargassum is a genus of brown macroalgae. Two free-floating species, Sargassum natans and Sargassum fluitans, make up much of the Great Atlantic Sargassum Belt (GASB). Unlike seaweeds anchored to rock, these forms spend their lives drifting at the surface. Their gas-filled structures help keep them afloat, while winds and currents gather them into patches and long windrows.
Historically, floating Sargassum was especially associated with the Sargasso Sea in the subtropical North Atlantic. Since 2011, a recurring large bloom population has also been documented in the tropical Atlantic, with seaweed transported toward the Caribbean and Gulf of Mexico.
How big has the belt become?
A landmark 2019 Science study documented an approximately 8,850-kilometer-wide belt in June 2018, with more than 20 million metric tons of biomass. A 2026 Nature Communications analysis reports that tropical Atlantic blooms exceeded 30 million tons in 2025. These are estimates for particular periods, not a fixed amount that stays in place year-round.
What the latest 2026 outlook says: The University of South Florida's September 30, 2026 satellite bulletin reported that overall Sargassum quantity had continued its expected seasonal decline. The estimated amount across the belt was about half the August total, itself about half the July total. Yet quantities in most regions remained high compared with past Septembers, and some coastal beaching events were still expected. In other words, a large long-term problem can coexist with a seasonal decrease.
Explore the University of South Florida Sargassum Watch System and monthly satellite outlooks →
Why is it increasing?
There is no single proven cause. Ocean circulation, changes in wind patterns, nutrient supply, water-column mixing, and the biology of the seaweed all interact. The U.S. Environmental Protection Agency identifies nitrogen and phosphorus supplied by rivers, ocean upwelling, vertical mixing, and atmospheric deposition as potential contributors. A river may matter strongly in one region or season without explaining the entire belt.
A 2026 Nature Communications study offers an additional insight. Its modeling suggests that shifts in North Atlantic atmospheric circulation helped establish the tropical population and that winds and mixed-layer changes initially enhanced growth. Over time, nutrients recycled within aging Sargassum mats and the organisms living among them may have become an increasingly important source supporting recurring blooms.
Ocean warming and other climate changes may influence the system, but it would be misleading to attribute the whole phenomenon solely to higher temperatures or a single source of pollution. Researchers are still testing how much each driver matters in different places and years.
A floating nursery—until it reaches shore
In open water, Sargassum creates a three-dimensional habitat where juvenile fish, invertebrates, sea turtles, and other organisms can feed, shelter, and travel. NOAA recognizes its ecological value and the importance of conserving offshore mats.
Nearshore, very large arrivals can have almost the opposite effect. Dense, decaying accumulations may smother seagrass and coral, reduce local oxygen availability, interfere with fishing and navigation, obstruct water intakes, and impose expensive beach-cleanup demands on coastal communities. Decomposition can release hydrogen sulfide, which smells of rotten eggs and can irritate the respiratory system. Seaweed can also accumulate arsenic and other contaminants, complicating disposal and proposed commercial uses.
Can the seaweed be turned into a resource?
Researchers and entrepreneurs are examining Sargassum for composting, soil amendments, fuels, and construction materials. Some approaches could reduce waste and cleanup costs, but the practical hurdles are substantial: salt, moisture, transport costs, inconsistent supply, and potential accumulation of arsenic or other substances. Any product intended for agriculture, food, or close human contact needs appropriate safety testing.
What should scientists—and coastal managers—watch next?
Satellite measurements can track broad distributions, while nearshore observations are needed to understand where and when beach landings will occur. Important questions include how currents steer different floating populations; whether nutrient recycling can keep blooms large even as outside inputs change; how seasonal forecasts can be improved; and how to remove stranded seaweed without causing further ecological damage.
For the public, the most useful approach is to follow regularly updated monitoring programs rather than treating each dramatic map or headline as proof of a new, ocean-wide emergency.
Sources & further reading
- NOAA CoastWatch — Sargassum FAQ: habitat, patchy distribution and coastal effects.
- NOAA Ocean Service — Sargassum: From Sea to Shore.
- U.S. EPA — Origin and Drivers of the Great Atlantic Sargassum Belt.
- Wang and colleagues (2019), Science — The Great Atlantic Sargassum Belt.
- Zhou and colleagues (2026), Nature Communications — Changing drivers of the Great Atlantic Sargassum Belt.
- USF/CARICOOS — September 30, 2026 Sargassum outlook.
- Futura-Sciences (October 12, 2026) — news article that prompted this MKP explainer.