Nutrient sources driving Sargassum growth in the northern tropical Atlantic: shipboard experiments simulating Amazon plume, Saharan dust, and vertical mixing.

The North Equatorial Recirculation Region (NERR) in the northern tropical Atlantic functions as the region of origin of the recurring large-scale blooms of pelagic Sargassum spp. that have occurred since 2011, creating the Great Atlantic Sargassum Belt (GASB), with Sargassum rafts extending from West Africa into the Gulf of Mexico. Various nutrient sources are hypothesized to force this bloom. We tested the effects of different nutrient sources on the growth and physiology of Sargassum in controlled on-board experiments during a research expedition through the northern tropical Atlantic (including the NERR) during the summer of 2024. The nutrient sources were Amazon plume, Saharan dust addition, simulated vertical mixing (addition of nutrient-rich water collected from below the mixed layer) and control (ocean surface water) waters as treatments. Pelagic Sargassum species and genotypes exhibited distinct physiological responses when exposed to different nutrient sources. The highest growth rates were observed in the vertical mixing treatment (0.08 ± 0.02 doublings·d-1), accompanied by elevated tissue chlorophyll a, chlorophyll c, and carotenoid concentrations, as well as increased photosynthetic efficiency (Y(II)). Nitrate (∼11-15 μmol g-1·h-1) and phosphate (∼0.6 μmol g-1·h-1) uptake rates were also substantially higher in this treatment. The Amazon plume and Saharan dust treatments resulted in weak or inconsistent nutrient absorption and limited growth. Only the vertical mixing treatment led to balanced stoichiometric ratios (C:N:P), whereas these ratios indicated P deficiency for the other treatments. Multivariate analysis confirmed that deep-sea water conditions led to improved physiological responses whereas dust and control treatments produced high stress responses. Collectively, these results demonstrate that pelagic Sargassum spp. can rapidly exploit nutrients supplied from subsurface waters when they become available and suggest that subsurface nutrient reservoirs may represent an important nutrient source capable of supporting bloom development.