Evidence for efficient regenerated production and dinitrogen fixation in nitrogen-deficient waters of the South Pacific Ocean: impact on new and export production estimates

P. Raimbault,N. Garcia

doi:10.5194/bg-5-323-2008

Abstract

Abstract. One of the major objectives of the BIOSOPE cruise, carried out on the R/V Atalante from October-November 2004 in the South Pacific Ocean, was to establish productivity rates along a zonal section traversing the oligotrophic South Pacific Gyre (SPG). These results were then compared to measurements obtained from the nutrient – replete waters in the Chilean upwelling and around the Marquesas Islands. A dual 13C/15N isotope technique was used to estimate the carbon fixation rates, inorganic nitrogen uptake (including dinitrogen fixation), ammonium (NH4) and nitrate (NO3) regeneration and release of dissolved organic nitrogen (DON). The SPG exhibited the lowest primary production rates (0.15 g C m−2 d−1), while rates were 7 to 20 times higher around the Marquesas Islands and in the Chilean upwelling, respectively. In the very low productive area of the SPG, most of the primary production was sustained by active regeneration processes that fuelled up to 95% of the biological nitrogen demand. Nitrification was active in the surface layer and often balanced the biological demand for nitrate, especially in the SPG. The percentage of nitrogen released as DON represented a large proportion of the inorganic nitrogen uptake (13–15% in average), reaching 26–41% in the SPG, where DON production played a major role in nitrogen cycling. Dinitrogen fixation was detectable over the whole study area; even in the Chilean upwelling, where rates as high as 3 nmoles l−1 d−1 were measured. In these nutrient-replete waters new production was very high (0.69±0.49 g C m−2 d−1) and essentially sustained by nitrate levels. In the SPG, dinitrogen fixation, although occurring at much lower daily rates (≈1–2 nmoles l−1 d−1), sustained up to 100% of the new production (0.008±0.007 g C m−2 d−1) which was two orders of magnitude lower than that measured in the upwelling. The annual N2-fixation of the South Pacific is estimated to 21×1012g, of which 1.34×1012g is for the SPG only. Even if our "snapshot" estimates of N2-fixation rates were lower than that expected from a recent ocean circulation model, these data confirm that the N-deficiency South Pacific Ocean would provide an ideal ecological niche for the proliferation of N2-fixers which are not yet identified.

Highlights

The nitrogen cycle in the oceanic gyres has been studied ever since the pioneering work of Menzel and Ryther in the Sargasso Sea (Menzel and Ryther, 1960; Ryther and Menzel, 1961)
Twenty-four short-stay stations were sampled along an 8000-km transect crossing different oceanic regimes. These were the mesotrophic area associated with the plume off the Marquesas Island (141◦ W–134◦ W), the adjacent high nitrate – low chlorophyll waters (132◦–123◦ W), the ultraoligotrophic waters associated with the central part of the south pacific gyre (123◦ W–101◦ W), the oligotrophic eastern side of the gyre (101◦ W–81◦ W) and the Chilean upwelling (80◦ W–72◦ W)
Other regions (Marquesas Islands and Chilean upwelling) showed significant nitrate concentrations (>0.5 μmoles l−1) in the surface. Chlorophyll biomass followed this general trend with very low values in the center of the South Pacific Gyre (SPG) (0.023 μg l−1), with levels reaching 0.3 μg l−1 near the Marquesas Islands and 1 μg l−1 in the Chilean upwelling

Summary

Introduction

The nitrogen cycle in the oceanic gyres has been studied ever since the pioneering work of Menzel and Ryther in the Sargasso Sea (Menzel and Ryther, 1960; Ryther and Menzel, 1961). Regenerated production, on the other hand, is fuelled by autochthonous Nsources (mainly NH+4 ), derived from biological processes (Harrison et al, 1987). The fraction of primary production derived from “new nutrients” is termed the f-ratio (Eppley and Peterson, 1979) and when in a steady state, accounts for the proportion of production available for export. Measurements of the nitrogen cycle were primarily based on 15N tracer techniques. New production estimates, computed in terms of carbon using the f-ratio and primary production, have shown considerable variations (Aufdenkampe et al, 2002), due to inaccuracies in estimating f and NO−3 assimilation (Priscu and Downes, 1985; Ward et al, 1989; Gentilhomme and Raimbault, 1994; Raimbault et al, 1999; Diaz and Raimbault, 2000; Aufdenkampe et al, 2001)

Methods

Results

Conclusion