The Southern Ocean (SO) contributes significantly to the sequestration of anthropogenic CO2 and is furthermore especially prone to Ocean Acidification (OA). The aim of this thesis was to investigate how key environmental factors influence the manifestation of OA effects on phytoplankton physiology and ecology. Publication I describes systematically occurring inconsistencies between pCO2 values calculated from different pairs of input parameters and their implications for the OA research field. Publication II presents the results of an experiment, in which interactive effects of OA and iron availability on SO primary production and species composition were observed. In publication III, dynamic light was found to strongly alter the effects of OA on the diatom Chaetoceros debilis. The aim of publication IV was to understand how iron, light and other factors control natural SO phytoplankton blooms. In conclusion, there is no universal phytoplankton response to OA. The effects of OA will always be modulated by the respective set of environmental conditions prevailing in the ecosystem of interest, which themselves may be subject to change
