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 (N-2) fixation. Our results show a significant relationship between the backscattering coefficient at 700 nm (b(b)(p)) and PON, especially when the latter is measured using the wet oxidation method (R-2 =0.87). b(bp) may be used to estimate PON concentrations (PONopt) between 0.02 and 0.95 mu M, allowing for unprecedented monitoring using autonomous profiling floats. The b(bp) vs PON relationship can be used to study phytoplanktonic biomass dynamics at relevant seasonal temporal scales, with clear evidence of PONopt as a proxy of phytoplanktonic biomass, at least for this specific area. Temporal analyses of PONopt show significant increases (from 0.16 to 0.80 mu M) likely related to new production associated to N-2 fixation events measured during stratification periods in the Melanesian Archipelago. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement