Long term decline of the planktonic biomass in a hotspot of nitrogen fixation Fumenia, A. Loisel, H. Karl, D. M. /Vantrepotte, Vincent Petrenko, A. /Bonnet, Sophie Tran-Duy, M. Bretagnon, M. Mangin, A. Moutin, T. Oceanic N2 fixation by diazotrophic microorganisms is the primary external source of new nitrogen to the surface ocean sustaining net production of organic matter. Studying long-term trends in biomass within N2 fixation hotspots is crucial for understanding and predicting the response of N2 fixation to global climate change. Here we developed a bio-optical model based on the spectral phytoplankton absorption coefficient derived from satellite ocean color observations to estimate a proxy for particulate organic nitrogen in the western tropical South Pacific. We demonstrate the existence of a seasonal new biomass production annually over the past 20 years, likely driven by recurrent N2 fixation. Importantly, our results also reveal a gradual decline in biomass within this N2 fixation hotspot over the last two decades. This decline indicates that seasonal nitrogen inputs via N2 fixation are decreasing. This trend inevitably could lead to a decline in the efficiency of the biological carbon pump, with potential implications for global biogeochemical cycles and climate regulation. 2025 text https://www.documentation.ird.fr/hor/fdi:010096012 fdi:010096012 Fumenia A., Loisel H., Karl D. M., Vantrepotte Vincent, Petrenko A., Bonnet Sophie, Tran-Duy M., Bretagnon M., Mangin A., Moutin T.. Long term decline of the planktonic biomass in a hotspot of nitrogen fixation. 2025, 16 (1), 11697 [12 p.] EN PACIFIQUE SUD OUEST ZONE TROPICALE