Ocean alkalinity enhancement (OAE), a proposed method to combat climate change, involves dispersing alkaline materials on the ocean surface to increase the water's ability to store carbon dioxide. In this study, we compared the effects of different alkaline materials (olivine, steel slag, and NaOH) on phytoplankton communities on a similar alkalinity enhancement level which was around 30 umol/kg. Nineteen on-deck 48-hour incubations were conducted on the voyage in the equatorial Pacific Ocean. The results suggested that while NaOH solutions might have minimal impact on phytoplankton, olivine could significantly alter community composition and reduce primary production, particularly affecting Prochlorococcus and benefiting nanoeukaryotes. Steel slag appears to enhance photosynthesis and abundance of microeukaryotes and nanoeukaryotes. This highlights the nuanced interactions between OAE materials and marine life, underscoring the need for careful consideration of environmental impacts when deploying such climate change mitigation strategies.