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

Abstract. Excess nitrogen gas (N₂^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, N₂^xs 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 N₂^xs 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., N₂^xs 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 N₂^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.