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. The Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise tookplace in the western tropical South Pacific (WTSP) during the austral summer(March–April 2015). The aim of the OUTPACE project was to investigate alongitudinal gradient of biological and biogeochemical features in the WTSP,and especially the role of N2 fixation in the C, N, and P cycles. Twocontrasted regions were considered in this study: the Western MelanesianArchipelago (WMA), characterized by high N2 fixation rates,significant surface production and low dissolved inorganic phosphorus (DIP)concentrations, and the South Pacific Gyre (WGY), characterized by very low N2 fixation rates, surface production and high DIP concentrations.Since physical forcings and mixed layer dynamics in both regions weresimilar, it was considered that the gradient of oligotrophy observed insitu between the WMA and WGY was not explained by differences in physicalprocesses, but rather by differences in biogeochemical processes. Aone-dimensional physical–biogeochemical coupled model was used toinvestigate the role of N2 fixation in the WTSP by running twoidentical simulations, only differing by the presence (sim WMA ) orabsence (sim WGY ) of diazotrophs. We showed that the nitraclineand the phosphacline had to be, respectively, deeper and shallower than themixed layer depth (MLD) to bring N-depleted and P-repleted waters to thesurface during winter mixing, thereby creating favorable conditions for thedevelopment of diazotrophs. We also concluded that a preferentialregeneration of the detrital phosphorus (P) matter was necessary to obtainthis gap between the nitracline and phosphacline depths, as the nutriclinedepths significantly depend on the regeneration of organic matter in thewater column. Moreover, the model enabled us to highlight the presence ofseasonal variations in primary production and P availability in the uppersurface waters in simWMA, where diazotrophs provided a new source of nitrogen(N) to the ecosystem, whereas no seasonal variations were obtained in simWGY,in the absence of diazotrophs. These main results emphasized the fact thatsurface production dynamics in the WTSP is based on a complex and sensitivesystem which depends on the one hand on physical processes (vertical mixing,sinking of detrital particles), and on the other hand on biogeochemicalprocesses ( N2 fixation, remineralization).

biogeosciences

Diazotrophy as the main driver of the oligotrophy gradient in the western tropical South Pacific Ocean: results from a one-dimensional biogeochemical–physical coupled model