Abstract
Abstract. Excess nitrogen gas (N2xs) was measured in samples collected at six locations in the eastern and western sub-basins of the Cariaco Basin, Venezuela, in September 2008 (non-upwelling conditions) and March 2009 (upwelling conditions). During both sampling periods, N2xs concentrations were below detection in surface waters, increasing to ~ 22 μmol N kg−1 at the oxic–anoxic interface ([O2] < ~ 4 μmol kg−1, ~ 250 m). Below the oxic–anoxic interface (300–400 m), the average concentration of N2xs was 24.7 ± 1.9 μmol N kg−1 in September 2008 and 27.5 ± 2.0 μmol N kg−1 in March 2009, i.e., N2xs concentrations within this depth interval were ~ 3 μmol N kg−1 higher (p < 0.001) during the upwelling season compared to the non-upwelling period. These results suggest that N-loss in the Cariaco Basin may vary seasonally in response to changes in the flux of sinking particulate organic matter. We attribute the increase in N2xs concentrations, or N-loss, observed during upwelling to: (1) higher availability of fixed nitrogen derived from suspended and sinking particles at the oxic–anoxic interface and/or (2) enhanced ventilation at the oxic–anoxic interface during upwelling.
Highlights
Dissolved inorganic nitrogen (DIN or “fixed” nitrogen), includes nitrate, nitrite, and ammonium
A plot of salinity (S) versus potential temperature (θ) for the six stations selected for excess N2 measurements during the two cruises indicates that water masses below ∼ 150 m in the Cariaco Basin during the two sampling periods were similar (Fig. 3)
In this study we evaluated changes in Nx2s in the Cariaco Basin under contrasting seasonal conditions. [Nx2s] were below detection at the surface, and increased to a mean value of ∼ 22 μmol N kg−1 at the oxic–anoxic interface (250–300 m) and deeper
Summary
Dissolved inorganic nitrogen (DIN or “fixed” nitrogen), includes nitrate, nitrite, and ammonium. The microbial processes that result in the conversion of DIN back to dinitrogen gas (N-loss) are the primary oceanic sinks for fixed nitrogen (Chang et al, 2010; Codispoti, 2007; Devol et al, 2006). These include water column and sedimentary denitrification (NO−3 → NO−2 → N2O → N2), and anammox (anaerobic ammonium oxidation; NO−2 + NH+4 → N2). Large-scale water column N-loss in the global ocean occurs mainly in the three major oceanic oxygen minimum zones (eastern tropical North Pacific, eastern tropical South Pacific, and the Arabian Sea; Codispoti et al, 2001; Gruber and Sarmiento, 1997)
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