The seasonal cycle of carbon dioxide in Baltic Sea surface waters

A detailed analysis of the seasonal cycle of carbon dioxide (CO2) within the surface waters of the Baltic Proper is presented. Based on the data sets of two cruises during winter and summer, a model has been developed interpolating the annual cycles of dissolved inorganic carbon (DIC) and the partial pressure of CO2 (pCO2). The model balances the CO2 budget of the surface waters referring to the processes determining DIC and pCO2 instead of using non-causal parameterisations. The carbonate system of the Baltic Proper is strongly influenced by the input of low salinity waters during spring and summer originating from the Bothnian Sea, the Gulf of Finland and from the Gulf of Riga as well as by the input of high salinity waters during autumn and winter originating from the North Sea. These hydrographic changes are enforced by the biological processes together causing high annual amplitudes of DIC (up to 250 μmol/kg) and of pCO2 (up to 400 μatm). The mean annual uptake of atmospheric CO2 is calculated to 0.9±0.09 mol CO2/(m2 a) and the mean new production of organic matter amounts to 3±0.3 mol C/(m2 a).