Abstract

To elucidate the different oceanographic and biogeochemical conditions controlling N2O cycling and its exchange across the air-sea interface, N2O, O2, nutrients, and other hydrographic variables were measured monthly at a fixed station located on the continental shelf at ∼36°S. Sampling lasted one year and included upwelling (53% of the time, principally in austral summer) and non-upwelling periods (47% of the time, transitional months between austral winter and summer). Vertical N2O distribution showed three patterns: nearly homogeneous profiles throughout the water column (18% of the time); a strong N2O maximum at intermediate depths (27% of the year); and a gradual N2O increase with depth (55% of the time). The last two patterns showed significant negative correlations with oxygen and positive correlation with nitrate concentrations, suggesting that nitrification is the process responsible for N2O production. Estimated air-sea fluxes (- 1.4 to 331 μmol m- 2 d- 1) were towards the atmosphere (effluxes) during most of the time with the exception of the transition months, when a N2O flux toward the ocean was observed. The highest fluxes were observed during the upwelling period, confirming the importance of this process as an outgasing mechanism and a trigger for O2 consumption in the water column that favors N2O production by nitrification. The average annual N2O flux (24.6 μmol m- 2 d- 1) could indicate the relevance of seasonal upwelling ecosystems in the global N2O balance. © 2006 Elsevier B.V. All rights reserved.