Charophytes are found in fresh and brackish waters across the globe and play key roles in coastal ecosystems. However, their response to increasing CO₂ is not well understood. The aim of the study was to detect the effects of elevated CO₂ on the physiology of charophyte species growing in the brackish Baltic Sea by measuring net primary production. Mesocosm experiments were conducted in the Kõiguste Bay (N Gulf of Riga) during the field season of 2012. Separate mesocosms were maintained at different p_CO2 levels:~ 2000,~ 1000 and~ 200 µatm. The experiments were carried out with three species of charophytes: Chara aspera, C. tomentosa and C. horrida. The short-term photosynthetic responses of charophytes to different treatments were measured by the oxygen method. The results show that elevated CO₂ levels in brackish water may enhance the photosynthetic activity of charophyte species and suggest that increasing CO₂ in the Baltic Sea could have implications for interspecific competition and community structure in a future high CO₂ world.