TAMPA, Fla. (March 13, 2025) – The Great Atlantic Sargassum Belt has puzzled researchers since 2011. A recent study published in Nature Communications may have identified what drove a tipping point that established the phenomenon in the tropical Atlantic Ocean.
Using computer modeling, a team of international researchers demonstrated that sargassum blooms were brought to the tropics by strong ocean currents and wind and thrived in ideal growing conditions.
Specifically, two consecutive years of a strong negative North Atlantic Oscillation (NAO), a shift in atmospheric pressure over the Atlantic that changes circulation and wind patterns, pushed sargassum into the tropics starting in 2009. There it found warm, nutrient-rich waters and lots of sunlight all year-round.
"At first, we saw just a few patches of sargassum being pushed south by the NAO," said co-author Frank Muller-Karger, Distinguished University Professor and biological oceanographer at the USF College of Marine Science. "But these algae patches were met with the right conditions to grow and perpetuate blooms."
Sargassum is floating macroalgae that has inundated beaches in the Caribbean since 2011, impacting tourism, harming the health of humans and marine life, and costing local governments millions of dollars per year to clean up. Scientists have been divided on the causes of the Great Atlantic Sargassum Belt.
Authors of the recent paper used a computer model that built on a prior study, which identified the influence of the NAO on sargassum. In both papers, researchers used models to simulate the transport of sargassum from the northern to the southern part of the North Atlantic, testing whether the NAO was the root cause of the first bloom that occurred in the tropical Atlantic in 2011.
"Both models showed that some patches of the sargassum were swept up by the wind and currents from the Sargasso Sea toward Europe, then moved southward, and from there were injected into the tropical Atlantic. There, this population of algae, now separated from the Sargasso Sea, forms new blooms every year thanks to having enough light, nutrients and warmer temperatures," said Muller-Karger, who contributed to both studies.
Still, the question remained: What provided the nutrients to promote the growth of sargassum in the tropical Atlantic?
To determine nutrient sources, the group again turned to computer models to analyze decades-worth of wind, currents and three-dimensional nutrient measurements collected in the Atlantic Ocean. These models successfully reproduced the annual blooms.
The models showed that nutrients were supplied via an ocean process known as vertical mixing, in which water masses mix on a seasonal basis due to shifting winds. This brings deeper water that has higher nutrient concentrations to the surface. In this sunlit surface layer, photosynthesis occurs and sargassum grows, thus fueling the massive blooms that eventually end up on the beaches of the Caribbean Sea and Gulf Coast.
"This was a surprising result," Muller-Karger said. "We had posed the hypothesis before that it is not the rivers that feed the formation of the sargassum blooms in the tropical Atlantic. This model supports the idea that nutrients from slightly deeper layers in the ocean feed the blooms."
The publication was an international collaboration between University of Toulouse, the Center for Scientific Research and Higher Education, Sorbonne University and the University of South Florida.
