fCO2sw variability in the Bay of Biscay during ECO cruises

The Bay of Biscay is part of the North Atlantic Ocean, the most important sink of CO2, and a subduction zone of mode waters that favours the entry of carbon to the ocean interior. To investigate the seasonal and interannual variability of CO2 uptake, continuous underway measurements of the partial pressure of CO2 at sea surface were performed along a commercial route between Vigo (Spain) and St. Nazaire (France). An unattended measuring system of CO2 fugacity (fCO2), with meteorological station, and temperature, salinity, oxygen and fluorescence sensors, was installed on board of ships of opportunity (RO-RO L’Audace and RO-RO Surprise). taset collected between December 2002 and December 2004 reported a significant interannual variability; mainly for the winter season. The noticeable increase of fCO2sw during the winter mixing period of 2004 was associated to biogeochemical differences related to nutrient ratios, phytoplankton activity and atmospheric CO2 uptake. Dividing the seasonal cycle into three periods, prebloom (October–February), bloom (March–May) and postbloom (June–September), the fCO2sw for the entire seasonal cycle was correctly predicted by empirical relationships with an error lower than 10 μatm in spite of the high interannual variability. The fCO2sw variability at seasonal scale was mainly controlled by processes of synthesis and remineralization of organic matter during prebloom and bloom periods whereas SST was the key parameter during postbloom period. The surface waters of the Bay of Biscay showed a clear role as atmospheric CO2 sink ranging from −2.7±2.8 (±0.3) to −0.08±0.41 (±0.04) mol C m−2 yr−1 (mean±standard deviation (±error)) throughout each complete period although CO2 release to atmosphere was also observed during short episodes of summer. Using the regular wind speed sources of CO2 fluxes estimation ranged from −1.3±1.7 (±0.1) to −2.4±2.7 (±0.1) mol C m−2 yr−1 at annual scale, exceeding the sink capacity of the nearby regions of the North Atlantic Ocean.