Carbon dioxide in European coastal waters

1 We compiled from literature annually integrated air-water fluxes of carbon 2 dioxide (CO2) computed from field measurements, in 20 coastal European 3 environments that were gathered into 3 main ecosystems: inner estuaries, upwelling 4 continental shelves and non-upwelling continental shelves. The comparison of annual 5 cycles of the partial pressure of CO2 (pCO2) in 5 contrasting continental shelves 6 provided insights into the biogeochemical drivers of the CO2 fluxes. The latter were 7 also investigated by comparing CO2 fluxes to net ecosystem (NEP) and net 8 community production (NCP) in 3 contrasted coastal ecosystems. Air-water CO2 9 fluxes were scaled at European regional level and compared to fluxes of atmospheric 10 CO2 in other aquatic and terrestrial compartments. Continental shelves are significant 11 sinks for atmospheric CO2 at an average rate of -1.9 molC m -2 yr -1 that scaled at 12 European level corresponds to an absorption of atmospheric CO2 of -68.1 TgC yr -1 . 13 This sink is equivalent to the one reported for the terrestrial biosphere of -66.1 TgC 14 yr -1 , based on carbon-stock change models. Estuaries are significant sources of CO2 to 15 the atmosphere at an average rate of 49.9 molC m -2 yr -1 that is higher than the CO2 16 emission to the atmosphere from rivers, streams and lakes. The scaled emission of 17 CO2 to the atmosphere from inner estuaries of about 67.0 TgC yr -1 would almost fully 18 balance the sink of atmospheric CO2 computed for continental shelves. However, the 19 scaled emission of CO2 from estuaries to the atmosphere is inconsistent with the 20 potential emission of CO2 based on the fate of river organic carbon during estuarine 21 transit. This discrepancy is most probably due to the poorly constrained surface area 22 estimate of inner estuaries. 23 24