As one of the largest brackish-water seas, the Baltic Sea has a comparatively low alkalinity and carbonate saturation state, leading to a low carbonate system buffer capacity. As a result of this, Baltic Sea surface waters experience large seasonal changes in pH and pCO₂ and show strong changes in carbonate chemistry in response to biologically mediated carbon turnover, such as primary production, respiration, remineralisation, and calcification. Over the past two decades the Baltic Sea has experienced acidification of its surface waters at a rate more than twice as high as mean ocean acidification. Thus, carbonate system perturbations – diurnally, seasonally, and as a result of CO₂-induced acidification – are much more pronounced than in marine systems. Whether the Baltic Sea biota has adapted to stronger carbonate system variations and is therefore able to better cope with anthropogenic acidification or whether it is less resilient because acidification further amplifies the stress already caused by natural carbonate system perturbations is presently not well understood.

To study the responses of a Baltic Sea plankton community to CO₂-induced acidification, we conducted an in situ mesocosm experiment with the Kiel Off-Shore Mesocosms for Future Ocean Simulations (KOSMOS) in the framework of the BMBF project Surface Ocean Processes in the Anthropocene (SOPRAN). Nine mesocosm units, each enclosing ∼55 m³ of water, were deployed and moored at 59° 51.5' N, 23° 15.5' E in the Tvärminne Storfjärden, an open archipelago at the southern tip of Finland. The carbonate system of the enclosed water was manipulated to cover a pCO₂ gradient from ambient (235 µatm) to 1650 µatm. From June 14 to August 7 2012, a wide range of physical, chemical, and biological parameters were monitored daily in the enclosed water bodies. Along with rate measurements of some key biological processes, these data provide a comprehensive overview of the ecological and biogeochemical responses of a Baltic Sea plankton community to anthropogenic acidification.