The seasonal variability of the partial pressure of CO₂ (pCO₂) has been investigated in the North Sea, a northwest European shelf sea. Based on a seasonal and high spatial resolution data set the main controlling factors - biological processes and temperature - have been identified and quantified. In the central and northern parts being a CO₂-sink all year round, the biological control dominates the temperature control. In the southern part, the temperature control dominates the biological control at an annual scale, since the shallow water column prevents stronger net-CO₂ removal from the surface layer due to the absence of seasonal stratification. The consequence is a reversal of the CO₂ sea-to-air flux during the spring bloom period, the only time, when CO₂ is taken up from the atmosphere in the southern region. Net community production in the mixed layer has been estimated to 4 mol C m^-2 yr^-1 with higher values (4.3 mol C m^-2 yr^-1) in the northern part and lower values in the southern part (2.6 mol C m^-2 yr^-1).