A new review by researchers at Florida Atlantic University’s Harbor Branch Oceanographic Institute has revealed that the unprecedented expansion of free-floating sargassum seaweed across the Atlantic Ocean is directly linked to human-driven nutrient pollution. The study, published in the journal Harmful Algae, combines four decades of data to explain how this once-confined algae has grown into the Great Atlantic Sargassum Belt, a massive seasonal bloom that stretches from West Africa to the Gulf of Mexico.
Record Biomass and a Shift in Nutrients
In May 2025, the Sargassum Belt reached a record biomass of 37.5 million tons, a staggering figure that does not even include the seaweed typically found in the Sargasso Sea. Lead author and research professor Dr. Brian Lapointe noted that the analysis of the seaweed’s chemical composition showed a significant change over time. From the 1980s to the 2020s, the nitrogen content of sargassum increased by more than 50%, a clear indicator of a shift from natural nutrient sources to land-based inputs like agricultural runoff, wastewater discharge, and atmospheric pollution.
“Our review takes a deep dive into the changing story of sargassum how it’s growing, what’s fueling that growth, and why we’re seeing such a dramatic increase in biomass,” said Dr. Lapointe.
From Ocean to Shore: A Growing Threat
Once a benign part of the ocean ecosystem, the massive blooms are now causing significant problems for coastal communities. When they wash ashore, they can clog beaches, damage fisheries, and negatively impact the tourism industry. The researchers also confirmed that sargassum grows much more quickly in nutrient-rich coastal waters, with some species able to double their biomass in just 11 days under optimal conditions.
The study also links variations in the seaweed’s biomass to flood and drought cycles in the Amazon River basin, suggesting that nutrient outflow from the river plays a significant role in the belt’s development. Scientists caution that understanding these complex connections between land-based pollution and ocean processes is crucial for managing the growing impacts of these seaweed blooms on coastal communities and marine ecosystems.