Since 2011, massive mats of golden-brown seaweed—pelagic Sargassum—have repeatedly swamped the shores of the Caribbean, West Africa, and parts of Central and South America. These sprawling blooms have suffocated coral reefs, crippled tourism, and disrupted coastal life.
What caused this sudden explosion of seaweed in regions that had rarely experienced it before?
A modeling study published earlier this year in Nature Communications Earth & Environment offers one possible explanation. It links the start of this phenomenon to the 2009–2010 North Atlantic Oscillation (NAO)—a rare climatic event involving stronger-than-usual Westerlies and altered ocean currents. According to the study, NAO conditions transported Sargassum from its historic home in the Sargasso Sea in the western North Atlantic into tropical waters farther south, where nutrient-rich upwellings and warm temperatures triggered the algae’s explosive growth.
Migrating Macroalgae
Julien Jouanno, senior scientist at the Institut de Recherche pour le Développement and head of the Dynamics of Tropical Oceans team at Laboratoire d’Etudes en Géophysique et Océanographie Spatiales in Toulouse, France, led the modeling work behind the study.
“Our simulations, which combine satellite observations with a coupled ocean-biogeochemical model, suggest that ocean mixing—not river discharge—is the main nutrient source fueling this proliferation,” Jouanno explained. The model incorporates both ocean circulation and biological processes like growth and decay, enabling the team to test various scenarios involving inputs such as ocean fertilization by rivers (such as the Amazon) or influxes of nutrients from the atmosphere (such as dust from the Sahara).
“Turning off river nutrients in the model only reduced biomass by around 15%,” said Jouanno. “But eliminating deep-ocean mixing caused the blooms to collapse completely. That’s a clear indicator of what’s actually driving the system.”
“When we exclude the ocean current anomaly linked to the NAO, Sargassum stays mostly confined to the Sargasso Sea,” Jouanno said. “But once it’s included, we start to see the early formation of what is now known as the Great Atlantic Sargassum Belt.”
But not all scientists are convinced by the study. Some argue the truth is more complex, and more grounded in historic ecological patterns.
Was the Seaweed Already There?
Amy N. S. Siuda, an associate professor of marine science at Eckerd College in Florida and an expert in Sargassum ecology, critiqued the study’s core assumptions. “The idea that the 2011 bloom was seeded from the Sargasso Sea doesn’t hold up under scrutiny,” she said.
The dominant form of Sargassum present in the early blooms in the Caribbean and elsewhere (Sargassum natans var. wingei), she explained, “hasn’t been documented in the north Sargasso Sea at all, and only scarcely in the south.”
Historical records suggest that the variety had long existed in the Caribbean and tropical Atlantic, however—just at such low concentrations that it was easily missed, Siuda said. She also cited research on population genetics that show little physical mixing between S. natans var. wingei and other morphotypes through at least 2018.
“We were simply not looking closely enough,” she noted. “Early blooms on Caribbean beaches were misidentified. What we thought was S. fluitans var. fluitans, another common morphotype, turned out to be something else entirely.”
A Sargassum bloom can be difficult to model, Siuda explained. Models “can’t distinguish whether Sargassum is blooming or simply aggregating due to currents. Field data, shipboard observations, and genetic studies tell a much more complex story,” she said.
Donald Johnson, a senior research scientist at the University of Southern Mississippi, offered a different perspective. While he agreed that Sargassum has long existed in the tropical Atlantic, he believes the NAO may have also played a catalytic role in the blooms—just not in the way the original study claims.
“Holopelagic Sargassum has always been in the region—from the Gulf of Guinea to Dakar—as evidenced by earlier observations stretching back to Gabon,” Johnson explained. “What changed in 2010 was the strength of the Westerlies. Drifting buoys without drogues showed unusual eastward movement, possibly carrying Sargassum from the North Atlantic toward West Africa.”
He offered a crucial caveat, however: “There was never any clear satellite or coastal evidence of a massive influx [of Sargassum]. If the NAO did contribute, it may have done so gradually—adding to existing Sargassum in the region and pushing it over the threshold into a full-scale bloom.”
In this view, the 2011 event was less about transport and more about amplification, described as an environmental tipping point triggered by a convergence of factors already present in the system.
More Than Just Climate
Both Siuda and Johnson agreed that multiple nutrient sources in the tropical Atlantic are likely playing a major role in the ongoing blooms:
- Riverine discharge from the Amazon, Congo, and Niger basins
- Saharan dust, rich in iron and phosphates, blown westward each year
- Seasonal upwelling and wind-driven mixing, particularly off West Africa and along the equator.
“Modeling surface currents in the tropical Atlantic is extremely difficult.”
And, Johnson pointed out, persistent gaps in satellite coverage—due to cloud cover and the South Atlantic Anomaly—mean we’re still missing key pieces of the puzzle. “Modeling surface currents in the tropical Atlantic is extremely difficult,” he said. “First-mode long waves and incomplete data make it impossible to fully visualize how Sargassum is moving and growing.”
Ultimately, both researchers said that understanding these golden tides requires reconciling models with fieldwork, as well as recognizing the distinct morphotypes of Sargassum. “Each variety reacts differently to environmental conditions,” Siuda explained. “If we don’t account for that, we risk oversimplifying the entire phenomenon.”
“There’s a danger in leaning too heavily on satellite models,” Johnson cautioned. “They measure aggregation, not growth. Without field validation, assumptions about bloom dynamics could mislead management efforts.”
Jouanno, too, acknowledged the study’s limitations. The model does not differentiate between Sargassum morphotypes and struggles with interannual variability, particularly in peak bloom years like 2016 and 2019. “This was likely a regime shift—possibly amplified by climate change—and while we can simulate broad patterns, there’s still much we don’t know about how each bloom evolves year to year.”
“We’re still learning,” Jouanno said. “Our understanding of vertical mixing, surface stratification, and nutrient cycling in the tropics is incomplete—and the biology of different Sargassum types is another critical gap.”
Ultimately, Jouanno said, “This is climate-driven. The NAO was a catalyst, and ongoing warming may be sustaining it. But without better field data and biological detail, we can’t fully predict what comes next.”
—Sarah Nelson (@SarahPeter3), Science Writer
