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

Abstract : The Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise took place in the western tropical South Pacific (WTSP) during the austral summer (March-April 2015). The aim of the OUTPACE project was to investigate a longitudinal gradient of biological and bio-geochemical features in the WTSP, and especially the role of N 2 fixation in the C, N, and P cycles. Two contrasted regions were considered in this study: the Western Melane-sian Archipelago (WMA), characterized by high N 2 fixation rates, significant surface production and low dissolved inorganic phosphorus (DIP) concentrations, and the South Pacific Gyre (WGY), characterized by very low N 2 fixation rates, surface production and high DIP concentrations. Since physical forcings and mixed layer dynamics in both regions were similar, it was considered that the gradient of oligotro-phy observed in situ between the WMA and WGY was not explained by differences in physical processes, but rather by differences in biogeochemical processes. A one-dimensional physical-biogeochemical coupled model was used to investigate the role of N 2 fixation in the WTSP by running two identical simulations, only differing by the presence (sim WMA) or absence (sim WGY) of diazotrophs. We showed that the nitr-acline 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 phos-phacline 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 primary production and P availability in the upper surface 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 that surface production dynamics in the WTSP is based on a complex and sensitive system which depends on the one hand on physical processes (vertical mixing, sinking of detrital particles), and on the other hand on biogeochemi-cal processes (N 2 fixation, remineralization).
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Audrey Gimenez, Melika Baklouti, Thibaut Wagener, Thierry Moutin. 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. Biogeosciences, European Geosciences Union, 2018, 15 (21), pp.6573-6589. ⟨10.5194/bg-15-6573-2018⟩. ⟨hal-01988473⟩

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