Ocean alkalinity enhancement reduces silica ballasting during export due to amplified dissolution

Abstract. Ocean alkalinity enhancement (OAE) is a carbon dioxide removal (CDR) technology proposed to store carbon dioxide (CO2) in the ocean on human-relevant time scales. However, depending on OAE intensity, resulting shifts in seawater carbonate chemistry speciation could alter community-driven biomass build-up, particulate stoichiometry, and transformation during particle export. Using mesocosms in the eutrophic North Sea (Helgoland, Germany), we established six alkalinity levels under two dilution scenarios (localized vs. uniform OAE additions) for 39 d. Total alkalinity (TA) was increased to ΔTAmax= 1250 µmol kg−1 (250 µmol TA kg−1 increments) using NaOH with CaCl2 to simulate cation release during calcium-based mineral dissolution, causing strong carbonate chemistry perturbations (e.g., pHmax > 9.25). To compare community-mediated carbon export across equivalent bloom phases, measurements were assessed within mesocosm-specific bloom and export events rather than on fixed sampling days, thereby accounting for OAE-induced shifts in spring bloom timing. During blooms, average phytoplankton biomass (as the concentrations of chlorophyll a and particulate organic carbon in the water column, POCWC) remained unchanged under unequilibrated OAE. In contrast, silica ballasting ratios declined with increasing pHT: suspended biogenic silica to particulate organic carbon ratios (BSiWC : POCWC, where WC = water column) decreased by up to 50 %, while exported BSiSed : POCSed (where Sed = sediment) decreased by 60 %, indicating intensification during sinking. The stronger decline in sinking compared to suspended BSi : POC is consistent with pH-enhanced BSi dissolution during export. Porosity of sinking particles increased with pHT and co-varied with BSiSed : POCSed, suggesting particle-quality traits can modulate dissolution during transit. Organic matter remineralization metrics showed no response to alkalinity addition, and particle sinking velocities did not scale with suspended or sinking silica ballasting ratios. Across dilution scenarios, unequilibrated OAE may reduce silica ballasting, potentially shoaling carbon remineralization, shortening sequestration timescales, and weakening net CO2 removal, while effects of dissolved silica regeneration on diatom productivity remain unresolved. Quantifying how pH-driven BSi dissolution interacts with bloom and export dynamics will be critical for evaluating OAE efficacy and ecological safety.