The pCO2 distribution in the surface waters of the Elbe estuary and German Bight was investigated during high river discharge in April 1997. pCO2 values of about 1100 μatm found in the upper part of the estuary are, compared to other European rivers, only moderate. In the outer estuary, the enhanced river discharge creates a pronounced river plume reflected in decreased surface salinity. However, pCO2 in this area (250 μatm) is below atmospheric values due to primary production. Comparison with results we obtained during other measuring campaigns show that in the outer estuary supersaturation of CO2 with respect to the atmosphere occurs only in early spring during the time of highest river discharge. In late spring and summer, intensive primary production within the surface layer leads to a pronounced undersaturation (141 μatm), despite a temperature increase of more than 20°C from winter to summer. Undersaturation is enforced by stable stratification of the water column during the productive seasons that diminishes the entrainment of CO2 from below the thermocline into the surface water. In winter, CO2 is close to equilibrium with the atmosphere and controlled by physical processes. Our data suggest that the Elbe estuary represents a sink for atmospheric CO2 over the investigated seasons and that carbon cycling in this area is dominated by biological processes.