<p><strong>Abstract.</strong> The Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise took place in the Western Tropical South Pacific (WTSP) during the austral summer (March&amp;ndash;April 2015). The aim of the OUTPACE project is to investigate a longitudinal gradient of biological and biogeochemical features in the WTSP, and especially the role of N₂ fixation on the C, N, P cycles. Two contrasted regions were considered: the Western Melanesian Archipelago (WMA),

more »

... aracterized by high N₂ fixation rates, significant surface production and low dissolved inorganic phosphorus (DIP) concentrations, and the Western south Pacific GYre (WGY), characterized by very low N₂ fixation rates, low surface production rates and high DIP concentrations. A one-dimensional biogeochemical &amp;ndash; physical coupled model was used to investigate the role of N₂ fixation in the WTSP by running two identical simulations, only differing by the presence or not of diazotrophs. We evidenced that the nitracline and the phosphacline had to be respectively deeper and shallower than the Mixed-Layer Depth (MLD) to bring N-depleted and P-repleted waters to the surface during winter mixing, thereby creating favorable conditions for the development of diazotrophs. We also concluded that a preferential regeneration of the detrital phosphorus (P) matter was necessary to obtain this gap between the nitracline and the phosphacline depths, as the nutricline depths significantly depend on the regeneration of organic matter in the water column. Moreover, the model enabled us to highlight the presence of seasonal variations in upper surface waters in the simulation standing for WMA, where diazotrophs provided a new source of nitrogen (N) to their ecosystem, whereas no seasonal variations were obtained in the simulation standing for WGY, in absence of diazotrophs. These main results emphasized the fact that surface production dynamics in the WTSP is based on a complex and sensitive system which depends on N₂ fixation in a crucial way.</p>