Temporal variability in N2O water content and its air–sea exchange in an upwelling area off central Chile (36°S)

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.