Ocean Alkalinity Enhancement (OAE) is a potential strategy for gigatonne-scale atmospheric CO₂ removal. OAE uses alkaline substances such as olivine, steel slag, or a strong base (NaOH) to convert seawater CO₂ into (bi)carbonate, thereby enabling uptake of additional CO₂ from the atmosphere. A critical knowledge gap is how the unavoidable physical and chemical perturbations associated with OAE could influence marine plankton communities and how potential side-effects compare to impacts of climate change. We conducted 19 ship-based experiments in the Equatorial Pacific, examining three prevalent OAE substances (NaOH, olivine, and steel slag) and their impacts on natural phytoplankton populations. Our experiments simulated realistic and moderate alkalinity enhancements between 29-16 μmol kg^-1 to demonstrate that NaOH-OAE had a negligible impact on phytoplankton while providing predictable amounts of alkalinity. Conversely, olivine-OAE disrupted plankton, especially cyanobacteria, heterotrophic bacteria, and picoeukaryotes while only providing 0.06 mmol alkalinity g^-1. Steel slag-OAE moderately changed phytoplankton communities but also fertilized growth while delivering 8 mmol alkalinity g^-1. Our study shows that the impacts of different OAE substances range from negligible to disruptive, thereby helping to determine which OAE methods could be suitable for equatorial Pacific applications, which ones require further research, and which ones should be dismissed.

phytoplankton functional types (PFTs),natural photosynthesis,Ocean alkalinity enhancement,trace metal