N_2O 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 N_2O air-sea fluxes in the central part of the SPG (132° to 114°W) were slightly negative (-0.48±0.44 μmol m~(-2) day~(-1)), whereas in its eastern part (114°W to 81°W), they were slightly positive (0.41±0.34 μmol m~(-2) day~(-1)), reaching up to 18 μmol m~(-2) day~(-1) in the coastal area (130 km from the coast). The transition between oceanic and coastal conditions was characterized by an abrupt increase in N_2O emissions from 80W 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 N_2O released into the atmosphere from the eastern part of the SPG came largely (70-80%) from the N_2O produced in the SL (2.0 × 10~(-3) μmol m~(-3) day~(-1)). In the coastal area, N_2O production in the SL reached up to 1.16 μmol m~(-3) day~(-1), and 28% of the N_2O released into the atmosphere was upwelled though the pycnocline by Ekman transport. The annual N_2O emissions estimated for the eastern South Pacific reach 50 Gg of N_2O, confirming the importance of this region for global emissions and reaffirming coastal upwelling centers as areas of strong production and outgassing.
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