Journal ArticleEquatorial upwelling of phosphorus drives Atlantic N2 fixation and Sargassum bloomsJung, Jonathan (ORCID:0000000227399926); Duprey, Nicolas N (ORCID:0000000211090772); Foreman, Alan D (ORCID:0000000250825786); D’Olivo, Juan Pablo (ORCID:0000000232917484); Pellio, Carolin; Ryu, Yeongjun (ORCID:0000000235098082); Murphy, Erin L; Romshoo, Baseerat; Kersting, Diego K; Cardoso, Gabriel O (ORCID:0000000271024726); Wald, Tanja (ORCID:0000000217744749); Fripiat, François (ORCID:0000000225916301); Jimenez, Carlos (ORCID:0000000334136662); Gischler, Eberhard; Montagna, Paolo; Alonso-Hernández, Carlos; Gomez-Batista, Miguel; Treinen-Crespo, Christina (ORCID:0000000244370877); Carriquiry, José; Ong, Maria Rosabelle (ORCID:0000000192714083); Goodkin, Nathalie F (ORCID:0000000196975520); Guppy, Reia; Aardema, Hedy (ORCID:0000000232021194); Slagter, Hans (ORCID:0000000242439869); Heins, Lena; de_Angelis, Isabella Hrabe; Bieler, Aaron L; Yehudai, Maayan; Noël, Trevor P; James, Kendon; Scholz, Denis (ORCID:0000000200558915); Hu, Chuanmin (ORCID:0000000339496560); Barnes, Brian B (ORCID:0000000300563500); Pozzer, Andrea (ORCID:0000000324406104); Pöhlker, Christopher (ORCID:000000016958425X); Lelieveld, Jos (ORCID:0000000163073846); Pöschl, Ulrich (ORCID:0000000314123557); Vonhof, Hubert (ORCID:0000000208978244); Haug, Gerald H; Schiebel, Ralf; Sigman, Daniel M (ORCID:0000000279231973); Martínez-García, Alfredo (ORCID:0000000272065079)<title>Abstract</title> <p>The Great Atlantic<italic>Sargassum</italic>Belt first appeared in 2011 and quickly became the largest interconnected floating biome on Earth. In recent years,<italic>Sargassum</italic>stranding events have caused substantial ecological and socio-economic impacts in coastal communities.<italic>Sargassum</italic>requires both phosphorus (P) and nitrogen (N) for growth, yet the primary sources of these nutrients fuelling the extensive<italic>Sargassum</italic>blooms remain unclear. Here we use coral-bound N isotopes to reconstruct N₂fixation, the ultimate source of the ocean’s bioavailable N, across the Caribbean over the past 120 years. Our data indicate that changes in N₂fixation were primarily controlled by multidecadal and interannual changes in equatorial Atlantic upwelling of ‘excess P’, that is, P in stoichiometric excess relative to fixed N. We show that the supply of excess P from equatorial upwelling and N from the N₂fixation response can account for the majority of<italic>Sargassum</italic>variability since 2011.<italic>Sargassum</italic>dynamics are best explained by their symbiosis with N₂-fixing epiphytes, which render the macroalgae highly competitive during strong equatorial upwelling of excess P. Thus, the future of<italic>Sargassum</italic>in the tropical Atlantic will depend on how global warming affects equatorial Atlantic upwelling and the climatic modes that control it.</p>Nature Geoscience2025-11-0510646217Nature Geoscience1752-0894https://doi.org/10.1038/s41561-025-01812-21903586National Science Foundation