Nitrous oxide fluxes in the central and eastern South Pacific.

N2O air‐sea fluxes were continuously measured on a transect crossing the Subtropical South Pacific Gyre (SPG) from its central part toward its coastal boundary (associated with coastal upwelling off central Chile) during the BIOSOPE cruise (austral spring, 2004). Continuous N2O air ‐sea fluxes in the central part of the SPG (132° to 114°W) were slightly negative (−0.48 ± 0.44 mmol m−2day−1), whereas in its eastern part (114°W to 81°W), they were slightly positive (0.41 ± 0.34 mmol m−2day−1), reaching up to 18 mmol m−2day−1 in the coastal area (130 km from the coast). The transition between oceanic and coastal conditions was characterized by an abrupt increase in N2O emissions from 80°W eastward and was associated with an increase of surface Chl‐a contents. This trend corresponded to the change in trophic status from very oligotrophic to eutrophic. The outward (through the air‐sea interface) and inward (through the pycnocline) N 2O fluxes of the surface layer (SL) were also determined at three representative stations, taking into account turbulent diffusion and vertical advection. The N2O released into the atmosphere from the eastern part of the SPG came largely (70–80%) from the N2O produced in the SL (2.0 × 10−3mmol m−3day−1). In the coastal area, N2O production in the SL reached up to 1.16 mmol m−3day−1, and 28% of the N2O released into the atmosphere was upwelled though the pycnocline by Ekman transport. The annual N2O emissions estimated for the eastern South Pacific reach 50 Gg of N2 O, confirming the importance of this region for global emissions and reaffirming coastal upwelling centers as areas of strong production and outgassing.