Productivity, growth, and biogeochemistry of pelagic Sargassum in a changing world

This review examines the changing distribution of pelagic Sargassum in the North Atlantic over the past four decades and how boundary current circulation enhances nutrient supply, productivity, and growth. Early explorers first described Sargassum in the Sargasso Sea and recognized the physical connectivity of "Gulfweed" to neritic populations in the Gulf of Mexico/America. In the 1980s, studies showed that Sargassum is more productive in neritic waters where it has lower carbon to nitrogen (C:N) and C to phosphorus (C:P) ratios. Sargassum productivity and growth are limited by both N and P, although P is often the primary limiting nutrient. The origin of the Great Atlantic Sargassum Belt (GASB) in 2011 was north the Amazon River mouth, suggesting this riverine nutrient source contributed to its development. This is supported by Sargassum tissue analysis and biomass increases/decreases between extreme flood/drought years in the Amazon basin. Comparison of the nutrient composition of Sargassum tissue (n= 849) from the 1980s with post-2010 and post-2020 showed increased %C (28 %), %N (55 % overall; 95 % in the Sargasso Sea), and N:P (50 %) but decreased %P (6 %) over the past four decades. Measurement of stable N isotopes (δ¹⁵N) in Sargassum revealed enriched δ¹⁵N correlated with increasing %N, pointing to the importance of terrestrial runoff, wastewater discharges, and coastal upwelling as potential N sources. We additionally report enriched levels of N and P in the western tropical region of the GASB, illustrating the importance of recent nutrient enrichment, especially P, to the excessive Sargassum biomass in the GASB.