Abstract. Anthropogenic carbon dioxide (CO₂) emissions are causing severe changes in the global inorganic carbon balance of the oceans. Associated ocean acidification is expected to pose a major threat to marine ecosystems worldwide, and it is also expected to be amplified in the Baltic Sea where the system is already exposed to relatively large natural seasonal and diel pH fluctuations. We studied the responses of larvae of the benthic key species Macoma balthica to a range of future CO₂ scenarios using six  ∼  55 m³ mesocosms encompassing the entire pelagic community. The mesocosms were deployed in the northern Baltic Sea in June 2012. We focused on the survival, growth and subsequent settlement process of Macoma balthica when exposed to different levels of future CO₂. The size and time to settlement of M. balthica increased along the CO₂ gradient, suggesting a developmental delay. With ongoing climate change, both the frequency and extent of regularly occurring high CO₂ conditions are likely to increase, and a permanent pH decrease will likely occur. The strong impact of increasing CO₂ levels on early-stage bivalves is alarming as these stages are crucial for sustaining viable populations, and a failure in their recruitment would ultimately lead to negative effects on the population.