The mystery behind massive Sargassum blooms has finally been unraveled — but the debate about what fuels these astonishing events is far from over. By early June, this year, an estimated 38 million tons of Sargassum, a type of brown algae,washed up along the shores of the Caribbean islands, the Gulf of Mexico, and parts of northern South America — setting a new, alarming record. During the summer months, these floating mats accumulate in large quantities on beaches, where they decompose and emit an unpleasant, foul smell. Not only does this discourage tourists and beachgoers, but it also puts significant stress on delicate coastal ecosystems. Yet, surprisingly, the same surface-floating Sargassum, far from shore, offers vital food and shelter for many marine species — highlighting its complex role in ocean life.
Originating from the Sargasso Sea located east of Florida, these algae have been intensively studied since 2011 when scientists identified the recurring phenomenon known as the Great Atlantic Sargassum Belt. This massive band of floating gulfweed travels from the equator toward the Caribbean during periods of strong easterly winds. For years, the main mystery was what nutrients—specifically phosphorus (P) and nitrogen (N)—were fueling this rapid proliferation. Some scientists speculated that agricultural runoff or nutrients resulting from rainforest deforestation were responsible, but these hypotheses failed to fully explain the significant and steady increases in Sargassum biomass observed over recent years.
Uncovering the driving forces behind these blooms has been a challenge — until now. A team of international researchers, led by the Max Planck Institute for Chemistry, has pinpointed the key processes behind these vast algal growths. They also identified climate patterns that set the stage for these blooms, paving the way toward future prediction models.
In a recent study published in Nature Geoscience, the researchers detailed how powerful wind-driven upwelling near the equator brings nutrient-rich, phosphorus-laden water from deep in the ocean up to the surface. This deep water then travels northward into the Caribbean. The surge in phosphorus availability fosters an increase in cyanobacteria — tiny microorganisms that thrive on the surface of Sargassum. These bacteria can capture atmospheric nitrogen (N₂) and convert it into forms readily usable by the algae, a process called nitrogen fixation. Cyanobacteria often colonize Sargassum, creating a symbiotic relationship that provides the algae with a supplementary nitrogen source — a competitive edge over other types of algae in the Atlantic. This partnership helps explain why Sargassum has been increasing so dramatically in recent years.
Coral skeletons tell a long, detailed story — over a century’s worth — about nitrogen fixation and ocean health. The team studied coral cores, which serve as natural environmental archives. As corals grow, their skeletons incorporate chemical traces from seawater, recording changes over long periods—much like the rings of a tree mark annual growth. By analyzing the nitrogen isotopic ratios within these skeletons, the scientists could infer patterns of nitrogen fixation stretching back around 120 years. Specifically, lower ratios of nitrogen isotopes (15N to 14N) in coral tissue indicate periods when bacteria were actively fixing nitrogen from the atmosphere, enriching the water with this nutrient.
To validate their findings, the team compared coral data with water samples collected aboard the research vessel Eugen Seibold. These confirm that the isotopic signatures reliably reflect past nitrogen fixation activity, linking it directly to events of increased Sargassum growth.
Since 2011, a fascinating pattern has emerged — a close correlation between nitrogen fixation and Sargassum biomass. PhD student Jonathan Jung, the lead author of the study, explained, “Our initial measurements highlighted notable increases in nitrogen fixation around 2015 and 2018, both years with record-breaking Sargassum blooms. When we compared coral-based reconstructions with biomass data, they matched remarkably well. However, establishing a true cause-and-effect relationship took more work.”
A comprehensive comparison revealed that fluctuations in algal biomass and nitrogen fixation have been consistently linked since 2011, including both peaks and declines. Interestingly, the first major transportation of Sargassum from the Sargasso Sea into the tropical Atlantic, driven by strong winds, occurred around 2010 and likely played a role in setting this pattern.
The researchers ruled out other potential nutrient sources — casting doubt on previous theories. For example, the idea that Saharan dust enriched the ocean with iron, thereby stimulating algae growth, did not align with biomass records. Similarly, nutrient input from the Amazon or Orinoco rivers showed no clear connection to the timing or magnitude of Sargassum blooms.
This new understanding reveals a clear mechanism: the combination of phosphorus supplied by upwelling deep water and nitrogen fixed by bacteria fuels the algae’s explosion in growth over recent years. Geochemist Jung notes, “Our model offers a much better explanation for the variability of Sargassum blooms than any prior hypotheses. Still, there’s ongoing uncertainty about the roles that other factors might play.”
The process hinges on how climatic conditions—specifically sea surface temperatures—affect wind patterns and upwelling. Cooler waters in the tropical North Atlantic and warmer regions in the south create pressure differences that alter wind strength and direction, enabling the deep, nutrient-rich water to rise to the surface.
Looking ahead, the team suggests that monitoring these key climate variables could improve forecasts of Sargassum events. Alfredo Martínez-García, senior author and group leader at the Max Planck Institute, emphasizes, “The future of Sargassum blooms depends heavily on how climate change influences the processes that deliver excess phosphorus to the equatorial Atlantic.” They plan to expand their research by analyzing coral records from various Caribbean locations to refine their understanding. Ultimately, these insights aim to support both the conservation of coral reefs and help coastal communities better manage the economic and ecological impacts of increasing Sargassum blooms. But here's where it gets controversial — as climate patterns shift, will our current models hold, or will unanticipated factors drive even more unpredictable blooms? What do you think? Are we truly prepared for the future of Sargassum in the Atlantic? Share your thoughts below.
