Optical proxy for particulate organic nitrogen from BGC-Argo floats

Using biogeochemical-Argo float measurements, we propose, for the first time, an optical proxy for particulate organic nitrogen concentration (PON) in the Western Tropical South Pacific, an area influenced by dinitrogen (N₂) fixation. Our results show a significant relationship between the backscattering coefficient at 700 nm (b_bp) and PON, especially when the latter is measured using the wet oxidation method (R²=0.87). b_bp may be used to estimate PON concentrations (PON^opt) between 0.02 and 0.95 µM, allowing for unprecedented monitoring using autonomous profiling floats. The b_bpvs PON relationship can be used to study phytoplanktonic biomass dynamics at relevant seasonal temporal scales, with clear evidence of PON^opt as a proxy of phytoplanktonic biomass, at least for this specific area. Temporal analyses of PON^opt show significant increases (from 0.16 to 0.80 µM) likely related to new production associated to N₂ fixation events measured during stratification periods in the Melanesian Archipelago.