Spatio-temporal variability and air-sea exchanges of carbon dioxide in three European ecosystems

As a contribution to the understanding of the role of the coastal ocean in the global carbon cycle, we report the annual evolution of the distribution of the partial pressure of CO2 (pCO2) of surface waters in 3 European regions with contrasting ecological and physical characteristic: the plume of the river Scheldt (Belgium), the English Channel (U.K./France) and the Galician upwelling system (NW Spain). The major processes controlling pCO2 spatial and temporal variability, as well as the estimation of air-sea fluxes of CO2 are discussed. The net annual emission of CO2 from the plume of the river Scheldt is estimated between +3 to +5 mmolC m-2 day-1. Provisional air-sea CO2 flux computations suggest that the English Channel is neutral from the point of view of atmospheric coupling. The Galician upwelling system is a net sink of CO2 in the range of -4 to -7 mmolC m-2 day-1. Our data confirm from a direct and observational approach that distal (marginal) continental shelves are net autotrophic while proximal continental shelves (influenced by terrestrial and anthropogenic carbon inputs) are net heterotrophic corresponding to a net sink and net source of atmospheric CO2, respectively. The air-sea fluxes of CO2 in the studied regions exhibit intense seasonal variations but the resulting annually integrated flux is high and probably significant for global carbon budgeting efforts.