A landmark 40-year review by Florida Atlantic University researchers reveals how human-caused pollution is fueling the massive seaweed blooms that are increasingly choking Atlantic coastlines. The study, published in the journal Harmful Algae, traces the dramatic expansion of pelagic, or free-floating, Sargassum seaweed from its historical home in the Sargasso Sea to a sprawling oceanic phenomenon. By analyzing four decades of data, scientists have connected the explosive growth to significant changes in the seaweed’s internal chemistry, pointing directly to land-based pollution as a primary cause. This research offers a comprehensive understanding of the forces driving the Great Atlantic Sargassum Belt, a recurring bloom that now spans from West Africa to the Caribbean.

The review highlights a critical shift in the nutrients feeding Sargassum. Historically, the seaweed’s growth was limited by the naturally low levels of nitrogen and phosphorus in the open ocean. However, by comparing tissue samples from the 1980s to those from today, researchers found that the nitrogen content in Sargassum has increased by a stunning 55%. This change in its biogeochemistry—the way chemical elements cycle through the ecosystem—signals a move away from natural nutrient sources toward anthropogenic, or human-caused, inputs. These sources include agricultural fertilizer runoff, wastewater discharge, and atmospheric pollution, which load the ocean with excessive nutrients and promote uncontrollable growth.

First appearing in 2011, the Great Atlantic Sargassum Belt is believed to have originated near the nutrient-rich mouth of the Amazon River, whose outflows contribute significantly to its development. Combining satellite imagery with direct analysis of the seaweed, the study supports the theory that nutrients from major river systems are seeding these massive blooms. In May, the belt reached a record biomass of 37.5 million tons, a substantial amount that poses a significant threat to coastal economies. The research resolves a long-standing paradox of how Sargassum could thrive in nutrient-poor waters by showing it is transported from richer coastal zones into the open ocean.

The consequences of this expansion are severe, impacting tourism, fisheries, and coastal ecosystems as tons of seaweed wash ashore. The review underscores the urgent need for international cooperation to manage the nutrient pollution that is transforming ocean ecology. By connecting the dots between land-based activities and the unprecedented growth of Sargassum, scientists provide a clear warning about the far-reaching effects of human pollution on our oceans. Understanding what fuels these blooms is the first step toward mitigating their costly and damaging impacts on communities across the Atlantic.

References

• Florida Atlantic University. (2025, August 28). Telling the story of the Atlantic’s sargassum surge with 40 years of data. Phys.Org; Florida Atlantic University. https://phys.org/news/2025-08-story-atlantic-sargassum-surge-years.html

• Lapointe, B. E., Webber, D. F., & Brewton, R. A. (2025). Productivity, growth, and biogeochemistry of pelagic Sargassum in a changing world. Harmful Algae, 102940. https://doi.org/10.1016/j.hal.2025.102940