A Massive Seaweed Belt Crosses the Atlantic. Scientists Are Rethinking What Feeds It

New studies point to ocean nutrients and recycling within the floating ecosystem.

In May of 2025, satellites captured a staggering view from space: an estimated 37.5 million metric tons of floating brown seaweed stretched across the Atlantic Ocean, from the coast of West Africa to the Gulf of Mexico. This is the Great Atlantic Sargassum Belt (GASB), a recurring mass of pelagic algae that first appeared in satellite records in 2011.

But what keeps feeding it?

“Floating Sargassum, though intriguing in its own right, is no more than a surface outcrop of a great oceanic phenomenon,” wrote oceanographer John Ryther in 1956. At the time, sargassum was largely seen as an ecological curiosity, associated with the nutrient-poor gyre of the Sargasso Sea, adrift in a blue desert.

The Great Sargassum Escape

In a sweeping 2025 review published in Harmful Algae, researchers at Florida Atlantic University’s Harbor Branch Oceanographic Institute compiled four decades of data—including historical oceanographic observations, satellite imagery, field experiments, and chemical analyses—to understand one of the most dramatic marine transformations in recent memory: the rapid rise and explosive spread of pelagic sargassum.

“Our review takes a deep dive into the changing story of sargassum—how it’s growing, what’s fueling that growth, and why we’re seeing such a dramatic increase in biomass across the North Atlantic,” said Brian Lapointe, Ph.D., lead author and research professor at FAU Harbor Branch, in the university’s August 2025 announcement.

The GASB made its first truly dramatic appearance in 2011. It has returned nearly every year since (except in 2013), although its size varies from year to year. The belt spans thousands of kilometers, but it consists of scattered clumps and mats, with open water between them. It is not a continuous carpet.

The belt appears to have been seeded, at least in part, by an atmospheric anomaly: the negative phase of the North Atlantic Oscillation in 2009–2010. This may have nudged sargassum southward from the Sargasso Sea into tropical waters. But genetic and morphological data suggest that some sargassum, especially Sargassum natans var. wingei, was already living in the tropical Atlantic before 2011.

The consequences are visible—and costly. Coastal areas in the Caribbean, Gulf of Mexico, and West Africa have reported waves of rotting sargassum choking beaches, emitting noxious hydrogen sulfide gas, and disrupting local economies.

Nutrients from the Land and the Ocean

Sargassum’s success essentially comes down to access to nutrients, especially nitrogen (N) and phosphorus (P). Experiments dating back to the 1980s showed how quickly the seaweed can grow when those nutrients are available.

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Controlled experiments show that under nutrient-rich conditions, Sargassum natans and Sargassum fluitans can double their biomass in just 11 days. Growth is even faster in “neritic” waters—shallow, coastal zones—than in the open ocean.

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Between the 1980s and 2020s, the nitrogen content in sargassum tissue increased by 55%. Phosphorus content dropped slightly. As a result, the nitrogen-to-phosphorus (N:P) ratio jumped by 50%.

“These changes reflect a shift away from natural oceanic nutrient sources like upwelling and vertical mixing, and toward land-based inputs such as agricultural runoff, wastewater discharge and atmospheric deposition,” Lapointe added.

His team linked some bloom variations to Amazon floods and droughts. But that interpretation is not settled, and a correlation does not establish how much of the belt’s growth river nutrients actually sustain.

A November 2025 study in Nature Geoscience offered a different explanation. Using chemical records preserved in corals, researchers connected bloom variability to phosphorus brought toward the surface by equatorial upwelling, the rise of deeper ocean water near the equator. This supply can also support microbes that turn dissolved nitrogen gas into forms living things can use. Together, the two processes could provide much of the seaweed’s nourishment.

Experiments reported in Marine Environmental Research in 2026 added another piece. During a 2024 research cruise, scientists grew sargassum with different nutrient sources. Water collected below the ocean’s surface mixed layer produced the strongest growth; Amazon-plume water and Saharan dust produced limited growth under the tested conditions.

The experiment shows that sargassum can rapidly exploit subsurface nutrients. It does not establish their contribution across the whole Atlantic, or rule out important river inputs elsewhere.

The Physics of Drift

Satellite imagery and ocean circulation models reveal that the Loop Current and Gulf Stream ferry sargassum from the Gulf of Mexico into the Atlantic basin. Back in 2004 and 2005, researchers observed huge mats, called “windrows” in the western Gulf, where nutrient levels had spiked from rivers like the Mississippi and Atchafalaya.

Yet the seaweed is not increasing everywhere. A December 2025 study found a sharp decline in the north Sargasso Sea since 2015. The authors proposed that reduced delivery from the Gulf of Mexico, possibly linked to warming and marine heatwaves there, could help explain it. The Atlantic’s sargassum habitat appears to be shifting as well as expanding.

The sargassum itself can recycle nutrients. Microbial breakdown and animal excretions within the mats help maintain localized nutrient supplies, allowing sargassum to persist even in nutrient-poor areas of the ocean. This ability helps the GASB survive and expand far from shore.

An April 2026 study in Nature Communications suggests this recycling has become increasingly important. Fish, shrimp and other residents gather food from surrounding waters and release nutrients around the mats. A model combining ocean mixing with this ecological contribution reproduced much of the observed bloom variability, including in 2023 and 2024, which were excluded from its training data. Chemical measurements supported the proposed recycling mechanism, although its estimated dominance remains a model-based interpretation.

Why This Matters

Sargassum provides an important habitat. Its floating rafts shelter and feed hundreds of marine species, including young sea turtles. NOAA designates sargassum areas as Essential Fish Habitat in several U.S. regions.

But too much of a good thing can become a marine hazard. Large coastal accumulations can smother corals and seagrasses. As the seaweed decomposes, it can deplete oxygen in surrounding water and release hydrogen sulfide, the rotten-egg-smelling gas that can irritate people’s airways.

The USF bulletin covering September 2026 illustrates the seasonal rhythm. Estimated biomass across the belt fell by roughly half from August, yet most monitored regions still held unusually large amounts for September. A seasonal decline does not mean the recurring problem has disappeared.

The 2025 review’s authors called for coordinated international monitoring systems, better forecasting models, and long-term strategies to reduce nutrient runoff.

The newer evidence makes the task more complicated. Cutting pollution can protect coastal waters, but it should not be presented as a demonstrated way to switch off the entire Atlantic bloom. Managing these influxes will also require understanding how currents, deeper-water nutrients and the organisms living among the seaweed interact.

The belt is changing the ocean. Scientists are still working out which changes keep the belt alive.