Biogenic nitrogen gas production at the oxic–anoxic interface in the Cariaco Basin, Venezuela
Excess nitrogen gas (N 2xs) 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 2xs concentrations were below detection in surface waters, increasing to ~ 22 μmol N kg −1 at the oxic–anoxic interface ([O 2] < ~ 4 μmol kg −1, ~ 250 m). Below the oxic–anoxic interface (300–400 m), the average concentration of N 2xs 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., N 2xs 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 N 2xs 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.