peer reviewed ; The variations of the partial pressure of CO2 pCO2.and related parameters were determined in surface seawater along the Belgian coast, from January 1995 to June 1996, at both daily and seasonal time scales. The distribution of pCO2 in this area is regulated by river input from the Scheldt, biological activity and hydrodynamics. The contribution of each of these processes varies as a function of the considered time scale: i. the daily variation of pCO2 depends on the tide although modulated by the biological diel cycle; ii. the seasonal variation of pCO2 depends on the input from the Scheldt and the seasonal variations of phytoplanktonic biomass. During winter, the plume of the river Scheldt is oversaturated in pCO2 with respect to the atmosphere. During spring and summer, phytoplankton blooms occur both in the lower Scheldt estuary and in the river plume and may lead to undersaturation of pCO2 in the easternmost area of the river plume. However, the degradation of phytoplankton induces oversaturation of pCO2, in the westernmost area of the plume. Furthermore, the inter-annual variation of pCO2 depends partly on the fluctuations of the discharge of the Scheldt. Our preliminary results strongly suggest that, on an annual basis, the Scheldt plume behaves as a net source of CO2 to the atmosphere