A 4-year time series (2001–2004) of the partial pressure of CO 2 (pCO 2 ) and air-sea CO 2 fluxes is reported in the Scheldt estuarine plume. This system is oversaturated in CO 2 with respect to the atmosphere, except during the spring phytoplanktonic bloom, and acts as a net source of CO 2 to the atmosphere of 0.7 mol C m -2 yr -1 that represents 7 to 27% of the inner Scheldt estuary CO 2 emission. Results also highlight that a high spatial and temporal coverage of the surface pCO 2 in coastal ecosystems is crucial for reliable estimations of air-sea CO 2 fluxes. The seasonal variations of pCO 2 seem to be more dominated by biological activities (photosynthesis/respiration) than by temperature change. A stochiometrically linked C/P mass balance budget failed to provide net ecosystem production estimates consistent with the pCO 2 dynamics in this area. It is hypothesized that this discrepancy is related to physiological characteristics of the dominant phytoplanktonic species ( Phaeocystis sp. ) within the studied area. On the basis of a preliminary dissolved inorganic carbon input/output budget, the annual emission of CO 2 toward the atmosphere seems to be largely due to the outgassing of the inputs of CO 2 from the inner Scheldt estuary, rather than due to organic carbon degradation.