The marine acid-base system is relatively well understood for oceanic waters. Its structure and functioning is less obvious for the coastal and shelf seas due to the number of regionally specific anomalies. In this review article we collect and integrate existing knowledge on the acid-base system in the Baltic Sea. Hydrographical and biogeochemical characteristics of the Baltic Sea, as manifested in horizontal and vertical salinity gradients, permanent stratification of the water column,

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... ication, high organic matter concentrations and high anthropogenic pressure, makes the acid-base system complex. We summarize in this study the general knowledge on the marine acid-base system as well as we describe the peculiarities identified and reported for the Baltic Sea specifically. In this context we discuss issues such as: dissociation constants in the brackish water, different chemical alkalinity models including contributions by organic acid-base systems, long term changes of total alkalinity, anomalies of borate alkalinity and thee acid-base effects of biomass production and mineralization. Finally, we identify research gaps and specify bottlenecks concerning Baltic Sea acid-base system.