Abstract. The functioning and efficiency of ocean alkalinity enhancement (OAE) as a CO2 removal strategy is well investigated in model studies, but risks for the ecosystem are presently not considered in models. Our study examines OAE–phytoplankton feedbacks in an Earth system model by adding carbonate system dependencies to the phytoplankton growth term. OAE is performed between 2040 and 2100 in the exclusive economic zones of Europe, the US, and China, with alkalinity additions reaching 103.2 Tmol yr−1 by the end of the century. Atmospheric pCO2 is reduced by 3–8 µatm. The excess ocean CO2 sink is mainly chemically driven but can additionally be altered by biological feedbacks. Further, net primary production decreases by up to 15 % due to indirect effects of OAE. Our results do not confirm the ecological realization of the direct, physiologically positive effect of OAE on calcifying coccolithophores. Limiting alkalinity addition in locations with high aragonite saturation states is beneficial as it not only reduces the OAE impact on phytoplankton but also increases the reduction in atmospheric pCO2. Our study highlights the need to take ecosystem responses into account when evaluating the effectiveness of OAE.