Ocean alkalinity enhancement (OAE) can help to remove CO₂ from the atmosphere. Yet, our study is the first to investigate the link between OAE and phytoplankton in an Earth system model. We show that OAE can indirectly decrease primary production and that biological feedbacks can modify the efficiency of OAE. Our study provides hints for ecological risks of OAE and the quantification of carbon drawdown and can serve as a kick start for other models to consider OAE–ecosystem interactions.