Short-term variability of surface carbon dioxide and sea-air CO 2 fluxes in the shelf waters of the Galician coastal upwelling system

Abstract

Using data collected during the DYBAGA and ECO cruises, remote sensing chlorophyll-a estimations and the averaged upwelling index of the previous fortnight (Iw’), we studied the variability of the sea surface CO2 fugacity (fCO2) over the Galician continental shelf during three seasonal cycles. Sea surface salinity (SSS) distribution controlled fCO2 mainly in spring, while sea surface temperature (SST) did during periods of intense cooling in November and warming in June. The uptake of carbon by photosynthetic activity, which was more intense during spring and autumn, masked the surface increase of dissolved inorganic carbon concentration during upwelling events, especially during spring. Significant low correlation between fCO2 and Iw’ was found during spring and summer when upwelling events were observed, whereas no relationship was observed during the downwelling period. High fCO2 exceeding atmospheric values was only found during the summer stratification breakdown. Although sea-air CO2 fluxes showed a marked inter-annual variability, surface waters off the Galician coast were net sinks for atmospheric CO2 in every seasonal cycle, presenting a lower CO2 uptake (~65%) compared to previous published values. Marked inter-annual changes of the sea-air CO2 fluxes seem to be influenced by fresh water inputs on the continental shelf under different meteorological scenarios