Ocean Alkalinity Enhancement (OAE) is an approach to sequester atmospheric CO₂ in the ocean and may alleviate ocean acidification. A large-scale mesocosm experiment in Norway tested calcium- and silicate-based OAE, increasing total alkalinity (TA) by 0–600 µmol kg^-1 and measuring CO₂ gas exchange. While TA remained stable, we found mineral-type and/or pCO₂/pH effects on coccolithophorid calcification, net community production and zooplankton respiration, providing insights for future OAE trials.