Local scientists identify massive seaweed belt’s origins

Researchers at the University of South Florida’s St. Petersburg campus have identified a “tipping point” in atmospheric conditions that caused the formation of a floating seaweed mass once visible from space.

The Great Atlantic Sargassum Belt, comprised of brown algae, made global headlines in March 2023 when it spanned 5,000 miles in length- twice the length of the U.S. The blobs were nearly nonexistent until 2011, and national agencies tasked researchers at the USF’s College of Marine Science (CMS) with tracking the ever-growing accumulation in 2016.

Stumped scientists previously posited that harmful nutrient runoff from the Congo, Amazon and Mississippi Rivers caused the potentially toxic belt’s formation. A new study showed that the primary causes of the tourism-disrupting phenomenon were extreme wind and ocean current changes.

A significant shift in atmospheric pressure from 2009 through 2010 pushed the sargassum into the tropics, where it feasted on warm, nutrient-rich waters and year-round sunlight. Those conditions, known as the North Atlantic Oscillation, were so anomalous that study co-author Dr. Frank Muller-Karger, a biological oceanographer at the CMS, believes the cycle will continue indefinitely.

“What the winds did is bring it there, and now it’s established,” Muller-Karger told the Catalyst. “Now you have a permanent population that I don’t think is going away anytime soon.”

Sargassum often washes ashore in the Florida Keys. Photo by Keara McGraw.

Muller-Karger credited advanced computer modeling for the scientific breakthrough. He noted that Christopher Columbus named the North Atlantic’s Sargasso Sea after the naturally occurring algae in the 1400s.

However, scientists struggled to discern why massive clumps started washing ashore in the Caribbean, Florida and Mexico during the spring and summer. The annual inundation impacts tourism, and local officials must remove the seaweed before it decomposes.

Rotting sargassum releases hydrogen sulfide, which smells like rotting eggs and can exacerbate respiratory issues. A continuous blob can trap boats by tangling around propellers and clogging intake valves.

In March 2023, a Florida Atlantic University professor warned that sargassum and accompanying floating plastic could carry Vibrio vulnificus – “flesh-eating bacteria” – ashore. However, Muller-Karger noted that currents typically keep the blobs away from Florida’s Gulf Coast.

He noted the importance of understanding life on Earth and how it behaves under atypical circumstances. Muller-Karger said the research team’s findings, published in Nature Communications, also have economic implications.

The study could have opened the door for litigation if it found that intensive farming along the Mississippi and deforestation in the Amazon provided the harmful nutrients needed for the algae to flourish and subsequently impact tourism industries.

“To me, that was a real possibility,” Muller-Karger added. “I’m surprised it never happened. Now there’s better evidence about what is really happening.”

A USF St. Petersburg Optical Oceanography Laboratory map highlights the Great Atlantic Sargassum Belt’s size in February 2023. Image provided.

An ocean process known as vertical mixing brought deeper water with higher concentrations of nutrients, like nitrogen and phosphorous, to the surface. The sargassum then had the ideal location and conditions for explosive growth.

The researchers used sophisticated computer models to analyze decades of wind, current and three-dimensional nutrient measurements and virtually recreated the annual blooms. “We turned off the nutrients in the rivers, and the plumes kept happening the same way,” Muller-Karger explained.

“This finding shows that it’s not the rivers,” he continued. “The rivers can play a role, but it’s minor compared to nutrients that pumped up from deeper in the ocean. To me, the surprise was that some people were proposing this was from rivers when it didn’t make any sense.”

However, computer modeling still has limitations. Forecasting remains challenging, and it is impossible to know when the sargassum belt will again balloon to a record size.

Muller-Karger could not definitively blame human-caused climate change or global warming for the belt. “If there’s anything we can say with certainty, it’s that we have more extremes happening more frequently,” he said.

While questions regarding the biology and ecology of sargassum remain unanswered, the study’s authors remain hopeful that collaborative efforts can continue shedding new light on how it reacts to environmental changes. The international research team included scientists from the University of Toulouse and Sorbonne in France and Mexico’s Center for Scientific Research and Higher Education (CICESE).

“This analysis and the publication highlight the importance of international cooperation in studying and dealing with the ‘Sargassum problem’ in the Atlantic,” said Julio Sheinbaum, a physical oceanographer at CICESE, in a prepared statement.