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

Abstract. Ocean alkalinity enhancement (OAE) is a carbon dioxide removal technology (CDR) 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 days. Total alkalinity (TA) was increased stepwise 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., pH > 9.25). Because response patterns were consistent across dilution scenarios, they were treated as replicates and assessed across the common pHT gradient. Average phytoplankton bloom magnitude (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. As OAE delayed spring bloom timing, these effects were only apparent within mesocosm-specific bloom and export events. 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. Remineralization metrics showed no treatment response, and particle sinking velocities did not scale with suspended or sinking silica ballasting ratios. Unequilibrated OAE may reduce silica ballasting, shoal carbon remineralization, and thus shorten sequestration timescales, potentially weakening net CO2 removal, regardless of dilution scenario. Quantifying how pH-driven BSi dissolution interacts with bloom and export dynamics will be critical for evaluating OAE efficacy and ecological safety.