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

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. © © 2013 Author(s).