Distribution of surface fCO2 and air–sea fluxes in the Southwestern subtropical Atlantic and adjacent continental shelf

Measurements of the fugacity of CO2, salinity, in situ temperature, nutrients and chlorophyll-a were made in November 1997, January/February and June 1998, during the “Ocean Circulation in the Southwest Atlantic Region” program (COROAS). The surface water fugacity of CO2, fCO2(sw), in the open ocean was 362.8 μatm, corresponding to a fugacity difference between surface water and atmosphere (ΔfCO2) of +3.2 μatm in November; 425.5 μatm (ΔfCO2 = +75.0 μatm) in January/February, and 350.6 μatm (ΔfCO2 = −4.0 μatm) in June, respectively, for the latitudes south of 25.3°S. These fCO2(sw) values suggest that only during winter does this oceanic area absorb any atmospheric CO2, whereas in the remaining seasons CO2 is released into the atmosphere. North of 25.3°S CO2 was released into the atmosphere during all the seasons, with highest ΔfCO2 at the continental shelf. The contribution of biological and temperature effects to the seasonal fCO2(sw) variations was calculated by dividing the region sampled into three subsections: the continental shelf, the continental slope and the open ocean. These calculations were compared with normalized fCO2(sw) and it was concluded that the main contributions to the fCO2(sw) gradients in the open ocean are the in situ temperature and the exchange of CO2(g) with the atmosphere. In the slope and shelf zones the biological regenerative processes must also be considered. The estimated net sea–air CO2 fluxes ranged between −1.1 and 0.1 mmol m−2 day−1 for the open ocean, 0.4 and 3.7 mmol m−2 day−1 for the slope and 0.3 and 9.8 mmol m−2 day−1 for the shelf zone, using the Wanninkhof 92 formula. It is reasonable to assume that the study area is a source of CO2 for the atmosphere and the sea–air CO2 fluxes over the continental shelf and slope could lead to refinements regarding the calculations of continental shelf pump.