Massive Atlantic sargassum blooms traced to west Africa

Massive blooms of Sargassum seaweed that have inundated coastlines across the Atlantic since 2011 likely originate off the coast of West Africa—forming years before they are visible and overturning long-standing assumptions about where these events begin

News Release 20-Apr-2026

Peer-Reviewed Publication

University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science

 Massive blooms of Sargassum seaweed that have inundated coastlines across the Atlantic since 2011 likely originate off the coast of West Africa—forming years before they are visible and overturning long-standing assumptions about where these events begin.

MIAMI-- Scientists at the University of Miami Rosenstiel School of Marine, Atmospheric and Earth Science and collaborators used a new combination of physics-based modeling and probabilistic analysis to trace the origin of the first major bloom. Their findings, published in PNAS Nexus, show the bloom can be tracked back to coastal waters near the Gulf of Guinea up to two years before it was detected by satellites in the western Atlantic.

“Sargassum blooms have had enormous ecological and economic impacts across the Caribbean, Gulf of Mexico, South Florida and West Africa,” said lead author Francisco Beron-Vera, a research professor in the Department of Atmospheric Sciences at the Rosenstiel School. Beron-Vera studies the transport and mixing processes in ocean and atmospheric systems. “Our results provide strong evidence that these blooms begin in the eastern tropical Atlantic, not in the Sargasso Sea as previously thought.” 

A new explanation for a decade-long mystery

Since 2011, vast accumulations of floating Sargassum known as the Great Atlantic Sargassum Belt have appeared across the tropical Atlantic, affecting coastal ecosystems, fisheries and tourism. Their origin has remained a subject of debate.

The new study identifies West Africa, rather than the Sargasso Sea, as the primary source. Researchers used two independent analytical approaches: Bayesian inversion, which estimates the most likely source based on where and when the bloom was observed, and transition path theory, which identifies the most efficient transport pathways feeding the bloom. Both methods pointed the same origin region near coastal West Africa, particularly the Gulf of Guinea.

The findings align with reports of Sargassum washing ashore in Ghana in 2009, supporting an African coastal origin. In contrast, the study shows that transport pathways from the Sargasso Sea to the tropics are relatively weak, challenging earlier hypotheses that linked the blooms to subtropical waters.

Modeling how seaweed moves across an ocean

To uncover the bloom’s origin, researchers developed a new way to simulate how Sargassum travels.

“Instead of treating the seaweed as a passive drift, the model represents it as clusters of floating ‘rafts’ influenced by ocean currents, winds and interactions between clumps,” Beron-Vera said. “Thousands of simulated trajectories were then translated into a probabilistic framework using a time-varying Markov chain, allowing our team to reconstruct where the bloom most likely began.”

Environmental conditions set the stage

The study also links the timing of the bloom to unusual environmental conditions in 2009–2010.

A strong Dakar Niña-like event marked by cooler sea surface temperatures and enhanced nutrient-rich upwelling off West Africa likely created favorable conditions for rapid Sargassum growth. Additional nutrient inputs from Saharan dust and increased river runoff may have further fueled the bloom.

“These conditions appear to have provided the right combination of nutrients and ocean dynamics to trigger large-scale growth,” said co-author María Josefina Olascoaga, a professor in the Department of Ocean Sciences.

Evidence from biology

The findings are also supported by biological evidence. The dominant Sargassum type observed during the initial years of the tropical bloom differs from the most commonly type found in the Sargasso Sea, reinforcing the conclusion that the bloom did not originate there.

Instead, the researchers suggest that low background concentrations of Sargassum already present in the tropical Atlantic expanded rapidly once environmental conditions became favorable.

The findings provide a clearer understanding of where and how Atlantic Sargassum blooms begin—an essential step for improving forecasts and response strategies. Recurring blooms continue to affect communities across the Caribbean, Gulf of Mexico, South Florida and West Africa, with impacts ranging from ecosystem disruption to economic losses.

“Understanding the origin of these blooms gives us a much stronger foundation for predicting future events,” Olascoaga said.

Funding for the study was provided by the National Science Foundation.

The study, “Tracing the origin of tropical North Atlantic Sargassum blooms to West Africa,” was published on March 2, 2026 in the journal PNAS Nexus. The authors include Francisco Javier Beron-Vera,a Maria Josefina Olascoaga,b Philippe Mironc and Gage Bonnerd

a. Department of Atmospheric Sciences, University of Miami Rosenstiel School of Marine, Atmospheric & Earth Science, b. Department of Ocean Sciences, Rosenstiel School.  c. Center for Ocean–Atmospheric Prediction Studies, Florida State University, and d. Morgridge Institute for Research.

About the University of Miami and Rosenstiel School of Marine, Atmospheric and Earth Science

 The University of Miami is a private research university and academic health system with a distinct geographic capacity to connect institutions, individuals, and ideas across the hemisphere and around the world. The University’s vibrant academic community comprises 12 schools and colleges serving more than 19,000 undergraduate and graduate students in more than 180 majors and programs. Located within one of the most dynamic and multicultural cities in the world, the University is building new bridges across geographic, cultural, and intellectual borders, bringing a passion for scholarly excellence, a spirit of innovation, and a commitment to tackling the challenges facing our world. The University of Miami is a member of the prestigious Association of American Universities (AAU).

 Founded in 1943, the Rosenstiel School of Marine, Atmospheric, and Earth Science is one of the world’s premier research institutions in the continental United States. The school’s basic and applied research programs seek to improve understanding and prediction of Earth’s geological, oceanic, and atmospheric systems by focusing on four key pillars:

*Saving lives through better forecasting of extreme weather and seismic events. 

*Feeding the world by developing sustainable wild fisheries and aquaculture programs. 

*Unlocking ocean secrets through research on climate, weather, energy and medicine. 

*Preserving marine species, including endangered sharks and other fish, as well as protecting and restoring threatened coral reefs. www.earth.miami.edu.