The carbon dioxide system in the Elbe estuary

Abstract

The pCO₂ distribution in thesurface waters of the Elbe estuary and German Bight was investigated during high river discharge in April 1997. pCO₂ values of about 1100 μatm foundin the upper part of the estuary are, comparedto other European rivers, only moderate. In theouter estuary, the enhanced river dischargecreates a pronounced river plume reflected in decreased surface salinity.However, pCO₂ in this area(250 μatm) is below atmospheric values dueto primary production. Comparison with resultswe obtained during other measuringcampaigns shows that in the outer estuarysupersaturation of CO₂ with respect to theatmosphere occurs only in early spring duringthe time of highest river discharge. In latespring and summer, intensive primary productionwithin the surface layer leads to a pronouncedundersaturation (141 μatm), despite atemperature increase of more than 20 °Cfrom winter to summer. Undersaturation isenforced by stable stratification of the watercolumn during the productive seasons thatdiminishes the entrainment of CO₂from below the thermocline into the surfacewater. In winter, CO₂ is close toequilibrium with the atmosphere and controlledby physical processes. Our data suggest thatthe Elbe estuary represents a sink foratmospheric CO₂ over the investigatedseasons and that carbon cycling in this areais dominated by biological processes.