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Ocean alkalinity enhancement reduces silica ballasting during export due to amplified dissolution
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DOI：10.5194/bg-23-4691-2026.png)
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Abstract
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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.
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