In May of this year, satellites captured a staggering view from space: 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 didn’t even exist 15 years ago.

And it’s growing.

“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, confined to the nutrient-poor gyre of the Sargasso Sea, adrift in a blue desert.

But today, scientists say that view is obsolete.

The Great Sargassum Escape

In a sweeping new study 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

Once confined to the warm but nutrient-poor Sargasso Sea, sargassum is now flourishing in areas it never dominated before, especially nutrient-rich coastal waters. These waters, loaded with runoff from agriculture, sewage, and atmospheric pollutants, have turned out to be a kind of steroid for the seaweed.

The Great Bloom: Origins and Consequences

The GASB made its first truly dramatic appearance in 2011. It has returned nearly every year since (except in 2013), growing larger each time. By 2025, the GASB reached its largest recorded size, forming a continuous mat over 8,850 kilometers long. That’s more than twice the length of the continental United States.

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.

Either way, what followed was unprecedented.

The GASB now represents a five-fold increase in biomass compared to historical levels in the Sargasso Sea. 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, Growth in the Sea

Sargassum’s success essentially comes down to access to nutrients, especially nitrogen (N) and phosphorus (P). Historically, growth was thought to be limited in the open ocean. But field experiments and lab studies dating back to the 1980s turned that idea on its head.

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.

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.

The Amazon River plays a central role. When the basin floods, nutrient-rich water flows into the Atlantic, feeding sargassum blooms. In dry years, the bloom subsides. That tight correlation strengthens the idea that land-based nutrients, and not just ocean currents, are now a major force in the ocean’s biological systems.

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.

“These nutrient-rich waters fueled high biomass events along the Gulf Coast, resulting in mass strandings, costly beach cleanups and even the emergency shutdown of a Florida nuclear power plant in 1991,” Lapointe told Gizmodo.

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.

Why This Matters

Sargassum provides an important habitat. It supports over 100 species of fish, invertebrates, and even sea turtles. The National Oceanic and Atmospheric Administration has designated it as an “Essential Fish Habitat.”

But too much of a good thing can become a marine hazard. When sargassum reaches shorelines, it decomposes rapidly, creating dead zones, damaging coral reefs, and overwhelming local waste management systems.

The massive influxes of biomass are also raising concerns about climate feedbacks. As the seaweed dies and decays, it releases methane and other gases. Scientists are now trying to quantify the carbon cycle implications of such large-scale algal blooms.

What Can Be Done?

The authors call for coordinated international monitoring systems, better forecasting models, and long-term strategies to reduce nutrient runoff.

The study’s findings echo a growing theme in marine science: eutrophication—the enrichment of ecosystems with nutrients—is no longer just a coastal problem. It’s now reshaping the open ocean.

And sargassum may be just the beginning.

As the ocean warms and human-driven nutrient flows continue, similar transformations may arise in other parts of the world. Already, macroalgal blooms are being observed in the Pacific and Indian oceans. The boundary between local pollution and global ocean health is blurring.