ƒCO2sw variability in the Bay of Biscay during ECO cruises

Abstract

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 in surface waters were performed along a commercial route between Vigo (Spain) and St. Nazaire (France). An unattended pCO2 measuring system, with meteorological station, and temperature, salinity, oxygen and fluorescence sensors, was installed on board of ships of opportunity RO-RO L’Audace and Surprise. The shipboard measurements show the temporal variations of surface seawater fugacity of CO2 (fCO2 sw), temperature (SST), salinity (SSS), chlorophyll and nutrients (nitrate (NO3 ), phosphate (PO4 ) and silicate (Si(OH)4 ). The gathered dataset throughout the two seasonal cycles reported an important interannual variability; mainly for the winter season. The noticeable increase of fCO2 sw during the winter fertilization of year 2004 was associated to subsequent biogeochemical differences related to nutrient ratios, phytoplankton activity and atmospheric CO2 uptake. The fCO 2 sw distribution throughout the seasonal cycle was correctly predicted from three empirical relationships in spite of the high interannual variability. The highest disagreement were found for fCO2 sw levels lower than 10 μatm The seasonal variability was mainly controlled by nutrients and chlorophyll during pre--bloom and bloom periods whereas SST was the key parameter during post-bloom period. Nevertheless surface waters of Bay of Biscay showed atmospheric CO 2 uptake for all the periods ranging from -2±2 to -0.4±0.6 molC·m-2 ·yr -1 . Using the regular wind speed sources of CO2 fluxes estimation ranged from -1.3 to -2.4 molC·m-2 ·yr-1 at annual scale, exceeding the sink capacity of the nearby regions of the North Atlantic Ocean