OUTPACE - Oligotrophy to Ultra-Oligotrophy, South Pacific Experiment

The OUTPACE project aimed to give a zonal description of the biogeochemical functioning and biological diversity of the western tropical South Pacific (WTSP) toward a gradient of macro- and micro-nutrients availability, and to produce a detailed study of the biological production and its subsequent fate in three contrasting sites, with a specific emphasis on the production sustained by nitrogen fixation and carbon export via the biological carbon pump. More specifically, the purpose was to obtain a precise representation of the complex interactions existing between planktonic organisms and the cycle of biogenic elements (C,N,P,Si,Metals) in the oligotrophic ocean, considering a variety of scales, from single-cell processes to the WSTP.

 

Most of the results have been published in a special issue from the European Biogeosciences revue: Interactions between planktonic organisms and biogeochemical cycles across trophic and N2 fixation gradients in the western tropical South Pacific Ocean: a multidisciplinary approach (OUTPACE experiment).

Editor(s): T. Moutin, S. Bonnet, K. Richards, D. G. Capone, E. Marañón, and L. Mémery

 

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Biological pump

One of our objective was to study the production and fate of organic matter across trophic and N2 fixation gradients.

We deployed drifting mooring lines equipped with sediment traps, and performed nitrogen budgets using a lagrangian strategy. We show that N2 fixation contributed to more than 90 % to the new nitrogen input in the photic layer, before nitrate input by vertical diffusion (1-8 %) and atmospheric deposition (< 1.5 %). N2 fixation sustained almost all of the new production in the region characterized by high N2 fixation rates (~600 µmol N m-2 d-1) and only 3 % in the South Pacific gyre, where N2 fixation rates were low (~50 µmol N m-2 d-1).

The e-ratios (particulate carbon export / primary production) measured in the first region (up to 9.7 %) are far higher than the one measured in other regions oligotrophic impacted by diazotrophy, revealing the high efficiency of the WTSP to export carbon relative to primary production.

Diazotrophs in the water column were poorly efficiently exported, suggesting an indirect export after transfer of fixed nitrogen through non-diazotrophic planktonic organisms, which were subsequently exported.

Off the coast of Fiji, our researchers collect particles from the atmosphere, some of which contain nutrients essential for the growth of microalgae...