Multidecadal variability in the abiotic environment regulates Sargassum biomass maintenance in the Sargasso Sea

Historically, the subtropical, saline, and oligotrophic waters of the Sargasso Sea (SS) have sustained moderate Sargassum proliferation. A pronounced decline in Sargassum abundance has been reported in the SS since 2015, which has been attributed to a disruption of the historical fall/winter Sargassum supply from the Gulf of Mexico and to the consolidation and intensification of the Great Atlantic Sargassum Belt (GASB). Studies concerning how changes in the abiotic environment regulate Sargassum maintenance and dynamics in this region are scarce and temporally limited. This study evaluated multidecadal changes (1994-2025) in key abiotic drivers for Sargassum growth, including seawater surface temperature (SST), seawater surface salinity (SSS), and the availability of limiting nutrients (phosphorus, nitrogen, and iron), to assess their implications for Sargassum biomass dynamics in the SS. The results showed that the maximum biomass observed for 2006-2015 period, coincided with episodic anomalous pulses of phosphorus and nitrogen availability; whereas in the most recent decade (2016-2025), Sargassum biomass declined during fall-winter, coinciding with reduced phosphorus and iron availability, as well as warming and salinization processes, besides the recently reported reductions in Sargassum supply from the Gulf of Mexico. Only for this last decade, Sargassum biomass increased during spring-summer, consistent with the recent spring-summer intensification of the GASB. The long-term data suggest that multidecadal variability in the abiotic environment of the SS, together with changes in Sargassum supply from different source regions, has regulated Sargassum biomass maintenance and seasonality in the SS.