Biogenic nitrogen gas production at the oxic-anoxic interface in the Cariaco Basin, Venezuela

Excess nitrogen gas (N-2(XS)) 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, N-2(XS) concentrations were below detection in surface waters, increasing to similar to 22 mu mol N kg(-1) at the oxic-anoxic interface ([O-2]<similar to 4 mu mol kg(-1), similar to 250 m). Below the oxic-anoxic interface (300-400 m), the average concentration of N-2(XS) was 24.7 +/- 1.9 mu mol N kg(-1) in September 2008 and 27.5 +/- 2.0 mu mol N kg(-1) in March 2009, i. e., N-2(XS) concentrations within this depth interval were similar to 3 mu mol 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 N-2(XS) 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.