Measurements of the partial pressure of CO2 (pCO2) in surface waters, and other water properties were performed monthly during a 2-year period from February 1998 through February 2000, at a station in the open northwestern Mediterranean Sea (Dyfamed Station). On seasonal timescale, the pCO2 minimum of 300 ?atm in winter was followed by an increase of 120 ?atm (pCO2 reaching 420 ?atm) related to warming of surface waters in summer. Estimates of the underlying processes (mixing, biological activity and air-sea gas exchange) governing the monthly variations of the upper layer pCO2 were obtained from observed variations in total inorganic carbon content (TCO2) in the surface, and from the vertical distribution of physical parameters and TCO2. Monthly variations in TCO2 due to gas exchange were determined from wind speed and from the air-sea pCO2 gradient. The impact of biological activity was estimated from the difference between the observed variations in TCO2 and the evaluations of air-sea exchange and carbon supply by physical processes. Mixing at the base of the mixed layer counteracts the late winter to summer TCO2 drawdown (about 80 mmol/m**3) due to a net organic production of about 100 mmol/m**3. The carbon consumption continues until early summer despite the absence of nutrients in the upper layer from April or May. The net carbon production in the mixed layer during the warming period exceeds by a factor of 1.6 the carbon production deduced from nitrate fluxes and using the usual Redfield C:N ratio of 6.6:1. The TCO2 increase during the autumn is primarily associated with convective vertical mixing induced by upper layer cooling and deepening. On the other hand, the contribution of air-sea gas exchange to TCO2 variations remains relatively small aside from summer months, when the CO2 oversaturation is high and the mixed layer is only 15-20 m depth.