Key controls on the seasonal and interannual variations of the carbonate system and air‐sea CO₂ flux in the Northeast Atlantic (Bay of Biscay)

Abstract

Abstract Biogeochemical variations of surface water in the Northeast Atlantic (Bay of Biscay) were examined using high‐frequency underway measurements combined with monthly sampling of carbon‐related variables. The mechanisms controlling seasonal CO 2 variability were investigated by distinguishing the contributions of biological and physical processes to the monthly changes in dissolved inorganic carbon (DIC) and partial pressure of CO 2 ( p CO 2 ). The seasonality of DIC (47–81 µmol kg −1 ) had a single peak with a winter maximum primarily driven by vertical mixing and a summer minimum driven by spring biological removal. Non‐Redfield C:N uptake was observed in the nutrient‐depleted summer but not during the spring bloom. In the North Atlantic, p CO 2 seasonality shows a latitudinal transition: from the temperature‐dominated oligotrophic subtropical gyre to the subpolar region where p CO 2 is dominated by changing concentrations of DIC. In the midlatitude Bay of Biscay, the annual cycle of p CO 2 (61–75 µatm) showed a double‐peak distribution. The summer p CO 2 peak was mainly driven by temperature increase, while the winter peak resulted from the dominant effect of entrainment of subsurface water. Interannual variations of DIC were more pronounced in winter and were driven by the changes in the strength of winter mixing. Higher wintertime concentrations and seasonal amplitudes of DIC were observed in cold years when the mixed‐layer depths were deeper, which appears to be associated with negative phases of the North Atlantic Oscillation. The Bay of Biscay shows a decrease of CO 2 uptake in 2008–2010 (−0.97 and −0.75 mol m −2 yr −1 ) compared to 2002–2004 (−1.47 and −1.68 mol m −2 yr −1 ).