| 12 Mar 2026
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.
Suessle et al. provide a comprehensive study on a very timely and relevant matter, that is, the impact of ocean alkalinity enhancement (OAE) on silica ballasting and subsequent export efficiency. I did enjoy reading this manuscript and complement the authors trying to tackle this (as stated) under constrained, yet, very important aspect of OAE research. Particularly, the combination of both water column and sedimentary measurements presents the key strength of this manuscript and let the authors to draw conclusions made. As this study presents a novel and most foremost significant contribution to the field, I recommend publication after handling some (certainly rather minor) comments. While the manuscript is well-written and well structured, it could benefit from a cleaner presentation and tightening in places (especially statistical section). As this study tackles such an intricate aspect of OAE research with informative value for the field, I suggest the authors highlight further their study in a more broader context. While the limitations of this study are well discussed, I believe the authors should not cut short on highlighting the positive aspects further and discuss these more.
Main comments:
(1) It would be good if the authors include some additional context situating their study in a more global context of OAE applicability/efficiency and the implications of decreased silica ballasting (e.g. see Zhou et al., 2024). Although the authors point out (introduction lines 79-94) the importance of diatoms in coastal regions, what about high-latitude environments, or environments where other functional groups dominate, i.e. coccolithophores? This can then be looped back to in section 5 (Implications and Outlook).
Zhou, M., Tyka, M. D., Ho, D. T., Yankovsky, E., Bachman, S., Nicholas, T., ... & Long, M. C. (2025). Mapping the global variation in the efficiency of ocean alkalinity enhancement for carbon dioxide removal. Nature Climate Change, 15(1), 59-65.
(2) In the methodology section, the authors describe the determination of POPSed using persulfate oxidation (Oxisolv) via pressure cooking. While this is a standard method for total digestion, it converts all phosphorus—both organic and inorganic—into orthophosphate for measurement.
Unlike the carbon analysis, where the authors correctly distinguished between POC and PIC by removing carbonates with HCl, there is no mention of a similar step to account for particulate inorganic phosphorus (PIP). In coastal mesocosm environments, the inorganic fraction (e.g., iron-bound P or mineral apatite) can constitute a significant portion of the total particulate phosphorus pool. Similarly, for nitrogen, the use of untreated samples (TPN) likely includes inorganic fractions such as adsorbed ammonium.
Therefore, labelling these results as 'POPSed' and 'PONSed' is potentially misleading and likely results in an overestimation of the actual organic nutrient fluxes. Although this may be tangential to the primary focus of the study, the authors should revise the terminology (e.g., to TPPSed/TPNSed) or justify the assumption that inorganic fractions were negligible.
(3) For analytical analyses, the authors should give analytical merits of quality (precision/accuracy/limit of detection).
(4) The statistical assumptions, choices, and data handling would benefit from a tightened explanation and clarification. Currently it is somewhat confounding and hard to follow, specifically for an audience not too familiar with these techniques. Specifically the choices and subsequent modes of analyses should be presented clearer.
Minor comments:
Lines 67-69: This statement could benefit from an example as to why/how such export pathways might be modulated.
Lines 90-94: Consider including Hashim et al., 2025, a study constraining mineral precipitation under more natural conditions than laboratory settings.
Hashim, M. S., Marx, L., Klein, F., Dean, C. L., Burdige, E., Hayden, M., ... & Subhas, A. V. (2025). Mineral formation during shipboard ocean alkalinity enhancement experiments in the North Atlantic. Biogeosciences, 22(22), 7149-7165.
Line 104: For completeness, add volume of mesocosms here.
Lines 107-109: Personally, I would refrain from labelling a study ‘first-of-its-kind’ and rather state the novelty by clearly identifying the key gap addressed and why it is important to investigate this. Consider rephrasing.
Line 114 and others: Throughout the manuscript there are inconsistencies in using in situ, in-situ, in-situ. Please revise accordingly.
Lines 133-134: This statement reads a bit odd. The authors treat biological response as an isolated variable, but biological export is sensitive to the initial perturbation, which can have indirect impacts on export. The authors cannot measure ‘governing efficiency’ if the application method itself suppresses the biological pump.
Lines 137-139: Why is the day of filling labelled as the start of the experiment when alkalinity manipulations were applied on day 4? Was is acclimation? Please clarify.
Line 139: How were the sets of six mesocosms chosen? Randomly assigned?
Lines 137-144: Why would ship-based delivery cause immediate and homogeneous delivery across the water column? Ship-based delivery aims to disperse into the mixed layer but arguably mostly at the surface. What is this argumentation based on? Requires further explanation.
Lines 160-163: This sentence is somewhat contradictory. There is growing consensus that low to moderate alkalinity additions are unlikely to have adverse impacts on the biology and functioning. But the authors application of up to ΔTA 1250 μmol kg-1 evidently raises pH substantially with the aim to maximize biological response. Then the use of NaOH with a ‘comparatively low environmental footprint’. Please revise and clarify.
Line 171: Technically correct would be dissolved oxygen. Please revise accordingly throughout.
Line 206: Would be good to introduce (here, or in introduction) the focus on Fucoxanthin as diatom-specific pigment.
Line 209: Include details on extraction time and temperature. What was the analytical precision of pigment determination?
Line 265: Why was only the prokaryotic abundance determined if the study focuses on diatoms? Needs clarification.
Line 275: What was the volume of TA and DIC samples?
Line 520: CCMs not defined.