Temporal variability in N 2O water content and its air–sea exchange in an upwelling area off central Chile (36°S)

Abstract

To elucidate the different oceanographic and biogeochemical conditions controlling N 2O cycling and its exchange across the air–sea interface, N 2O, O 2, 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 N 2O distribution showed three patterns: nearly homogeneous profiles throughout the water column (18% of the time); a strong N 2O maximum at intermediate depths (27% of the year); and a gradual N 2O 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 N 2O 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 N 2O 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 O 2 consumption in the water column that favors N 2O production by nitrification. The average annual N 2O flux (24.6 μmol m − 2 d − 1 ) could indicate the relevance of seasonal upwelling ecosystems in the global N 2O balance.