The dramatic impacts of global climate change have driven marine carbon dioxide removal innovation, including ocean alkalinity enhancement (OAE), in an attempt to keep global warming under 2 °C. We conducted a laboratory experiment to assess the impacts of brucite inspired alkalinity addition (BIAA) as an OAE approach on the carbonate chemistry, biogeochemistry, and composition of the natural Santa Barbara Channel phytoplankton community sourced from a spring upwelling event. The BIAA treatment used MgCl2 * 6H2O and NaOH to yield a total alkalinity (TA) concentration of ∼3000 µmol kg−1, in contrast with the untreated seawater controls (TA = ∼ 2300 µmol kg−1). Our results suggest that BIAA altered the phytoplankton community composition, including reduced contribution of diatoms and enhanced numbers of Prymnesiophyceae (coccolithophores and Phaeocystis sp.). These results are in agreement with observations that biogenic silica content was lower under BIAA treatment. While the concentration of particulate inorganic carbon was consistently higher compared to controls, these differences were not statistically significant. Results revealed no differences between control and BIAA treatment in particulate organic carbon and particulate organic nitrogen (PON) concentrations. The proxy for cellular photosynthetic health Fv/Fm revealed that cells were photosynthetically healthy for both control and BIAA treatments, but values were lower in the BIAA treatment at the beginning of the exponential phase. While statistical power limitations of laboratory results might restrict applicability to other systems, our overall results suggest that BIAA has a differential impact on phytoplankton functional groups and their biogeochemical performance.