Ocean Alkalinity Enhancement (OAE) is increasingly regarded as a potential strategy to enhance oceanic uptake of atmospheric CO₂ through modification of seawater carbonate chemistry. However, the impacts of different alkaline materials on ecosystem processes and carbon dioxide removal (CDR) potential at the community scale remain insufficiently constrained. Two identical batches of mesocosm experiments were conducted from September to November 2025. Each batch consisted of three 50,000 L tanks and three 120,000 L tanks, representing a control treatment, a brucite treatment (20 mg L^-1), and a steel slag treatment (20 mg L^-1). Brucite addition significantly increased pH and total alkalinity and decreased dissolved inorganic carbon during the initial phase, indicating pronounced alteration of the carbonate system. Despite these geochemical changes, phytoplankton biomass, gross primary production (GPP), respiration (R), and net primary production (NPP) did not differ significantly from the control, although the maximum relative electron transport rate (rETRmax) increased during the later stage. In contrast, steel slag addition induced comparatively minor changes in carbonate chemistry but significantly increased phytoplankton biomass, promoted picoeukaryote dominance, and enhanced photophysiological performance, as indicated by higher Fv/Fm and rETRmax values. Gross primary production and respiration increased concurrently, while net primary production remained unchanged relative to the control. Overall, the two alkaline materials elicited fundamentally different ecosystem responses. Brucite acted primarily as an alkalinity source that modified seawater carbonate chemistry and increased the physicochemical potential for atmospheric CO₂ uptake, whereas steel slag functioned mainly as a biological stimulant with limited effects on seawater alkalinity, enhancing phytoplankton biomass and physiological performance while altering community composition. However, neither treatment resulted in a significant increase in net primary production. These findings suggest that the carbon dioxide removal potential of OAE cannot be inferred solely from changes in seawater alkalinity or biological productivity and highlight the importance of integrating both biogeochemical and ecological responses when evaluating the effectiveness of OAE-based carbon dioxide removal strategies.
 

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