Distribution and biogeochemical control of total CO 2 and total alkalinity in the Baltic Sea

Abstract

The distributions of total CO 2 ( C T) and total alkalinity ( A T) were determined in the Baltic Sea in summer 2008 and winter 2009, respectively. The cruises covered all major basins between the Kattegat, the north of the Gulf of Bothnia and the east of the Gulf of Finland. A C T-salinity diagram indicated that the surface water C T was mainly controlled by A T. Except for the Gulf of Riga, A T decreased with decreasing salinity and resulted in a strong C T gradient in the surface water between the Kattegat to the northern regions of the Baltic Sea. Whereas the biologically induced modulations of the A T-controlled C T distribution were relatively small in the surface layer, the effect of the organic matter decomposition on the C T levels in the below-halocline water was large. The C T fraction generated by mineralization ( min C T) in the deeper water layers was determined from the difference between the measured C T and the preformed C T. The preformed C T was estimated on the basis of the CO 2 partial pressure and the A T which the considered deep water mass had during its last contact with the atmosphere. The min C T accumulation was largest in the deep basins of the central Baltic Sea where maximum values of up to 450 µmol kg − 1 were found in the bottom water of the Gotland Sea. This was attributed to long-lasting stagnation periods and to an intense organic matter input. Considerably lower min C T (factors 4–5) were obtained for the Gulf of Bothnia due to both a more frequent deep water renewal and a reduced organic matter flux.