Researchers have identified a key nutrient partnership fueling massive sargassum blooms in the Atlantic: phosphorus from equatorial upwelling and nitrogen fixed by cyanobacteria on the algae. This climate-driven process has intensified over the past decade, aligning with surges in sargassum since 2011. The discovery, detailed in Nature Geoscience, could improve predictions of these ecologically disruptive events.
Massive sargassum blooms have plagued the Caribbean and Atlantic since 2011, forming the Great Atlantic Sargassum Belt that originates in the Sargasso Sea east of Florida. By early June this year, an estimated 38 million tons of the brown algae drifted toward coastlines in the Caribbean islands, Gulf of Mexico, and northern South America, releasing unpleasant odors upon decay and stressing coastal ecosystems. While providing habitat for marine life at sea, these mats discourage beach visitors and pose economic challenges.
A team led by the Max Planck Institute for Chemistry has pinpointed the drivers of this rapid growth. Strong easterly winds cause upwelling near the equator, bringing phosphorus-rich deep water to the surface and northward into the Caribbean. This phosphorus boosts cyanobacteria living on sargassum, which fix atmospheric nitrogen gas into a usable form through nitrogen fixation. This symbiosis gives sargassum a competitive edge over other algae.
To trace this process, researchers analyzed coral cores from across the Caribbean, which record environmental changes in their growth layers over 120 years. By measuring nitrogen isotopic ratios (15N to 14N), they detected increased fixation since 2011. Calibration with modern seawater samples confirmed the corals' reliability as archives.
Lead author Jonathan Jung, a PhD student at the Max Planck Institute, noted, "In the first set of measurements we noticed two significant increases in nitrogen fixation in 2015 and 2018, two years of record Sargassum blooms. So we compared our coral reconstruction with annual Sargassum biomass data, and the two records aligned perfectly! At that time, however, it was not at all clear whether there was a causal link."
The study rules out prior theories like Saharan dust or river runoff from the Amazon and Orinoco, as they do not correlate with bloom patterns. Instead, cooler sea surface temperatures in the tropical North Atlantic and warmer southern conditions drive wind shifts that enhance upwelling.
Senior author Alfredo Martínez-García emphasized the climate link: "Ultimately, the future of Sargassum in the tropical Atlantic will depend upon how global warming affects the processes that drive the supply of excess phosphorous to the equatorial Atlantic." The team plans further coral analysis to refine predictions, aiding coral reef protection and coastal management.