Long term decline of the planktonic biomass in a hotspot of nitrogen fixation

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.