Modelling the marine CO2 system in BALTSEM

The main intention of this report is to give a general description of how the marine CO2 system is incorporated into BALTSEM, a coupled physical-biogeochemical Baltic Sea model . The CO2 system involves four parameters; dissolved inorganic carbon (DICor CT), total alkalinity (TA or AT), CO2 partial pressure (pCO2), and pH. If any two of these parameters are known (together with other state variables such as temperature and salinity), the other two can be calculated. DIC and alkalinity are chosen as model state variables as they are conservative with regard to changes in temperature and salinity (as opposed to pCO2 and pH that are strongly dependent especially on temperature). A comprehensive description of the marine CO2 system in general, and the Baltic Sea CO2 system in particular, is given by Schneider (2011). Furthermore, all definitions and calculations employed are described in detail by Dickson et al. (2007). Alkalinity and DIC are mainly transported to the Baltic Sea from rivers, but also from the North Sea. Alkalinity can be produced or consumed by different biogeochemical processes, and is in addition transported to the North Sea through the open boundary in Northern Kattegat. DIC is assimilated by phytoplankton and again released when organic carbon is mineralized. Burial of freshly produced organic material is a permanent sink for carbon, but the major carbon sink is outflow to the North Sea. Differences in CO2 partial pressure between atmosphere and surface water together with wind speed determine the CO2 uptake or outgassing in different sub-basins. The magnitude and direction of air-sea CO2 exchange varies between sub-basins and also seasonally within basins. The Gulf of Bothnia has earlier been described as a source of CO2 to the atmosphere, whereas conflicting results have been reported for the Baltic Proper. Formation and precipitation of calcium carbonate (CaCO3)is a sink term for alkalinity and DIC, whereas dissolution of CaCO3 on the other hand is a source term for both variables. In the Baltic Proper as well as the eastern and northern gulfs, formation of CaCO3 seems to be negligible. The reason is believed to be unsaturation of calcite and aragonite mineral forms of CaCO3 in winter and early spring. In the Kattegat and Belt Sea, calcite seems to be oversaturated throughout the year, and the coccolithophore Emiliania Huxleyicanat times form massive blooms in the Skagerrak-Kattegat area. Nevertheless, precipitation and dissolution of CaCO3 are at this pointnot included as processes in the model, but may be a topic for future studies.A short description of how calcite and aragonite saturation states (ΩCA and ΩAR) are calculated is presented in Appendix B. The main processes that affect the concentrations of DIC and alkalinity are discussed in detail in this report and followed by a description of model forcing and boundary conditions, and an overview of the available observations of CO2 system parameters. Model performance on both long-term average scale, and on short-term scale is analysed in terms of comparisons between model results and observations. Finally, budget calculations reveal the overall main source and sink terms of both DIC and alkalinity in the Baltic Sea