We conducted a large-scale field experiment using 55 m³ floating containers (mesocosms) to investigate consequences of near-future projected CO₂ elevations (ocean acidification) on a Baltic Sea plankton community in Storfjärden (Finland). The focus of our study was on single- and multicelled small-sized organisms dwelling in the water column. Our results suggest that increasing CO₂ concentrations may change the species composition and promote specific food web interactions.