Ocean alkalinity enhancement aims to increase atmospheric CO₂ sequestration by adding alkaline materials to the ocean. We assessed the environmental effects of olivine and steel slag powder on coastal plankton. Overall, slag is more efficient than olivine in releasing total alkalinity and, thus, in its ability to sequester CO₂. Slag also had less environmental effect on the enclosed plankton communities when considering its higher CO₂ removal potential based on this 3-week experiment.