Carbon and nitrogen uptake in the South Pacific Ocean: evidence for efficient dinitrogen fixation and regenerated production leading to large accumulation of dissolved organic matter in nitrogen-depleted waters

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Abstract
A major goal of the BIOSOPE cruise on the R/V Atalante to the South Pacific Ocean (conducted in October–November 2004) was to establish rate of productivity along a longitudinal section across the oligotrophic South Pacific Gyre (SPG), and compared these measurements with those obtained in nutrient–repleted waters from Chilean upwelling and around Marquesas Islands. A dual 13C/15N isotopic technique was used to estimate rates of carbon fixation, inorganic nitrogen uptake (including dinitrogen fixation), ammonium (NH4) and nitrate (NO3) regeneration, and dissolved organic nitrogen (DON) release resulting from both NH4 and NO3 uptake. The SPG had revealed the lowest rates of primary production (0.1 gC.m−2.d−1), while rates were 7 to 20 fold higher around the Marquesas Islands and in the Chilean upwelling, respectively. In this very low productive area, most of primary production was sustained by active regeneration processes which fuelled up to 95% of the biological nitrogen demand. Since nitrification was very active in the surface layer and often balanced the biological demand of nitrate, dinitrogen fixation, although acting at low daily rate (≈1–2 nmoles l−1d−1), sustained the main part of new production. Then, new production in the SPG (0.008±0.007 gC m−2.d−1) was two orders of magnitude lower than this measured in the upwelling where it essentially sustained by nitrate (0.69±0.49 gC.m−2.d−1). In the whole investigated region, the percentage of nitrogen release as DON represented a large part of the inorganic nitrogen uptake (13–15% in average), and reaching 26–41% in the SPG where the production of DON appeared to be a major part of the nitrogen cycle. Due to the lack of annual vertical mixing and very low lateral advection, the high release rates could explain the large accumulation of dissolved organic matter observed in the nitrogen-depleted and low productive waters of the South Pacific Gyre.
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