Distribution of the partial pressure of carbon dioxide and air-sea exchanges over the European continental shelf

Human activities have changed the flows of natural biogeochemical cycles since the industrial revolution and global simulations predict important climate and ecological changes for the next century due to the increase of atmospheric CO2. As a contribution to the understanding of the role of the coastal ocean in the global carbon cycle, we report the distribution of pCO2 in surface waters, the major processes controlling the spatial and temporal variability of pCO2 and the estimation of air-sea fluxes of CO2 in various study regions with contrasting ecological and physical characteristics. The study regions cover the proximal continental shelf (nine European estuaries, the plume of river Scheldt, the English Channel) and the distal continental shelf (Bay of Biscay and Galician upwelling system). The nine studied estuaries act as a net source of CO2 and the annual emission extrapolated to the overall European estuaries ranges between +8.2 and +16.4 104 tC day-1 corresponding to 5 to 10% of the anthropogenic CO2 emissions in Europe. The net emission of CO2 from the plume of the river Scheldt is estimated between +50 to +120 tC day-1 which represents 11 to 26% of the one we computed for the Scheldt inner estuary. Provisional air-sea CO2 flux computations suggest that the English Channel acts at annual basis as a net source of CO2 ranging between +560 and +1100 tC day-1. However, the Southern Bight of the North Sea, that has a similar eco-hydrodynamic forcing (permanently well-mixed waters), seems to act at annual basis as a net sink of atmospheric CO2. The Bay of Biscay acts as a net sink of atmospheric CO2 in the range of -5 to -8 mmol m-2 day-1. These fluxes extrapolated to the overall European distal continental shelves give a net annual influx in the range of -2.5 to -4.7 105 tC day-1 that corresponds to an additional sink of atmospheric CO2 of about 45% to the one reported in literature for the North Atlantic Ocean (north of 40°N). The Galician upwelling area acts as a net sink of CO2 in the range of -4 to -7 mmol m-2 day-1. These data contradict the belief that all coastal upwelling areas act as a net source of atmospheric CO2, by analogy with equatorial upwelling systems. Our data also confirm from a direct and observational approach that distal continental shelves are net autotrophic while proximal continental shelves are net heterotrophic corresponding, respectively, to a net sink and net source of atmospheric CO2. The air-sea fluxes of CO2 in the different studied regions exhibit intense seasonal variations but the resulting annually integrated flux is high and significant for global carbon budgeting efforts.