the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jul 2026
Reviews and syntheses: Experimental evidence of phytoplankton responses to Ocean Alkalinity Enhancement
Abstract. Ocean alkalinity enhancement (OAE) has emerged as a promising carbon dioxide removal strategy aimed at increasing seawater alkalinity and enhancing long-term oceanic carbon uptake. However, concerns remain regarding the potential ecological impacts of large-scale alkalinity manipulation on marine planktonic communities, particularly phytoplankton, which play a central role in marine biogeochemical cycles and primary production. This review synthesizes the current experimental evidence on phytoplankton responses to different OAE approaches, including hydroxide-based, mineral-based, and bicarbonate-based treatments. We compare early conceptual predictions with recent laboratory, microcosm, mesocosm, and modelling studies, highlighting how the field has evolved from theoretical risk assessment toward increasingly mechanistic and experimentally grounded investigations. Current evidence generally indicates that phytoplankton communities exhibit a relatively high tolerance to moderate CO2-equilibrated OAE scenarios, with limited effects on biomass, productivity, and community composition. More pronounced physiological and ecological responses are observed under unequilibrated or mineral dissolution treatments, where rapid pH shifts, trace metal release, and nutrient interactions may alter species-specific performance and community dynamics. Responses vary substantially across taxonomic groups and environmental contexts, reflecting the importance of carbonate chemistry, nutrient availability, and trace metal sensitivity in shaping OAE outcomes. Overall, existing studies suggest that phytoplankton responses to OAE are more nuanced and context-dependent than initially hypothesized, although substantial uncertainties remain regarding long-term ecosystem restructuring and large-scale biogeochemical feedback.
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Status: open (until 26 Oct 2026)
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RC1: 'Comment on egusphere-2026-4055', Anonymous Referee #1, 30 Jul 2026
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AC1: 'Reply on RC1', Sara Groppelli, 16 Sep 2026
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We thank the reviewer for the thorough and constructive comments, which have substantially improved the manuscript. Below we respond to each point in turn. Each response includes the exact revised text as it now appears in the manuscript.
1. General
Reviewer comment: The review aims to synthesize phytoplankton responses to OAE; however, beyond highlighting the ecological importance of marine phytoplankton, the introduction does not adequately explain how or why OAE is expected to influence phytoplankton. As OAE is specifically designed to alter marine carbonate chemistry, its primary effect is to modify the availability of dissolved inorganic carbon species (albeit temporarily). This is fundamental because dissolved CO2 and carbonic acid directly influence phytoplankton carbon acquisition and growth. The introduction would therefore benefit from a concise discussion of why phytoplankton, beyond their ecological importance, are a key group requiring investigation prior to large-scale OAE implementation.
Response: We have added an explanation of the mechanistic link between OAE and phytoplankton physiology, grounded on dissolved inorganic carbon (DIC) speciation and carbon acquisition pathways.
Revised text: "Beyond its ecological relevance, phytoplankton represent a key target group for assessing OAE risks because the technique directly modifies the speciation of dissolved inorganic carbon (DIC) (Renforth and Henderson, 2017). By adding alkalinity, OAE shifts the carbonate equilibrium toward bicarbonate and carbonate ions, reducing dissolved CO₂ and carbonic acid concentrations, at least transiently before air-sea gas exchange restores equilibrium with the atmosphere (Renforth and Henderson, 2017; Bach et al., 2019). Since dissolved CO₂ is the substrate for carbon fixation via RuBisCO and many phytoplankton taxa rely on carbon-concentrating mechanisms sensitive to external CO₂ availability, these changes can directly affect photosynthetic carbon acquisition, growth rates, and, in calcifying taxa, calcification (Reinfelder, 2011). Understanding these physiological responses is therefore essential before large-scale OAE deployment."
Reviewer comment: Within the introduction, the authors use the terms "equilibration state" and "equilibrated" without defining them. Similarly, the explanation of OAE and its effects on the marine carbonate system is overly simplified and does not describe the associated changes in carbon speciation that underpin many phytoplankton responses. I strongly recommend including a more comprehensive explanation of OAE, its impacts on the marine carbonate system, and the distinction between equilibrated and unequilibrated OAE early in the manuscript, ideally before Section 2.1.
Response: A dedicated explanation has been added at the start of Section 2, clarifying the equilibrated/unequilibrated distinction.
Revised text: "A critical distinction throughout OAE research is between equilibrated and unequilibrated (or non-equilibrated) treatments (Bach et al., 2019). In equilibrated treatments, the alkalinity source is brought into equilibrium with CO₂ before being added to seawater, so that dissolved inorganic carbon (DIC) rises together with total alkalinity (TA), approximating the chemical state seawater would eventually reach after complete air-sea gas exchange (Renforth and Henderson, 2017). This produces comparatively small, predictable changes in pCO₂ and pH. In unequilibrated treatments alkalinity is added without prior equilibration, so TA increases while DIC remains momentarily unchanged, producing a more abrupt reduction in pCO₂ and increase in pH and carbonate ion concentration. Unequilibrated treatments are intended to approximate the near-field conditions phytoplankton would experience immediately following alkalinity release in the field, before natural air-sea gas exchange has had time to act, which typically occurs over weeks to years depending on mixed-layer depth and regional gas-transfer conditions (Jones et al., 2014). Because these two deployment strategies expose phytoplankton to different carbonate chemistry perturbations, the distinction between equilibrated and unequilibrated treatments is essential for interpreting and comparing biological responses across studies."
Reviewer comment: Furthermore, throughout the manuscript the authors frequently cite studies without indicating whether they investigated equilibrated or unequilibrated OAE. Given that the manuscript itself highlights that these treatments can produce substantially different biological responses, this distinction should be made consistently throughout to facilitate meaningful comparison among studies.
Response: Equilibration state is now specified at first mention for each cited study were verified against the original source. A full breakdown of equilibration state for all reviewed studies, including those not individually discussed in the main text, is now provided in Supplementary Table 1 (see also response to the Figure 3 comment below).
Reviewer comment: Throughout the manuscript, phrases such as "a liming experiment in the eastern Mediterranean" imply field experiments, while some mesocosm studies are explicitly referred to as field studies. These are fundamentally different experimental approaches and should be described accurately throughout the manuscript (e.g., "a mesocosm study conducted in the eastern Mediterranean").
Response: We have corrected terminology throughout after verifying the original study designs of each cited work.
Reviewer comment: There are also several instances where incorrect references appear to have been cited, often involving different publications by the same first author. In addition, several references cited within the text do not appear in the reference list. I recommend a careful review of all citations before publication.
Response: We identified and corrected.
2. Specific comments (by original line number)
Reviewer comment (L29): "CO2". Here and throughout the manuscript, CO2 should be consistently formatted using subscript.
Response: Corrected throughout the manuscript (CO₂).
Reviewer comment (L50–52): "As of early 2026...". Rather than discussing incomplete annual publication records, it would be more appropriate to state the literature search date.
Response: Replaced with an explicit search date.
Revised text: "The literature search was conducted in March 2026. The apparent decline in publications for 2026 reflects the incomplete nature of the current year's record at the time of search, rather than a true decrease in research activity."
Reviewer comment (L65–66): I am not aware of any OAE field deployments in the North Sea region, nor does the manuscript subsequently discuss any. It is therefore unclear how the description of North Sea characteristics is relevant here. (L68-69): "High buffering capacity and elevated alkalinity". It is unclear what this comparison is relative to. Providing representative values or ranges would improve clarity. Furthermore, as many OAE studies have been conducted in coastal regions with naturally elevated alkalinity, it is not clear how the North Sea differs in this regard. (L70-71): "Air-sea exchange and mixing". This statement is somewhat contradictory. Strong mixing would rapidly dilute chemical perturbations, while rapid air-sea gas exchange would accelerate equilibration and reduce pCO2 perturbations. Together, these processes would shorten the duration of altered carbonate chemistry and potentially make ecological responses more difficult to detect.
Revised text: "In particular, the concentration of OAE research infrastructure in this broader North European region, including the North Sea and adjacent semi-enclosed basins such as the Baltic Sea, can be attributed to a combination of scientific and logistical factors. The region hosts a dense network of marine research institutions and long-term monitoring programs, enabling coordinated experimental efforts and high-frequency observations. These coastal and shelf systems are characterized by dynamic, well-studied plankton communities and strong seasonal bloom cycles, providing a responsive and measurable system in which ecological impacts can be readily observed."
Reviewer comment (Figure 2 caption): The caption refers to different symbols, whereas all points appear to use the same symbol with different colours. Please revise accordingly.
Response: The caption has been changed: Global distribution of studies investigating phytoplankton responses to ocean alkalinity enhancement (OAE). Colours indicate the main experimental approaches used in the reviewed literature, including modelling studies, laboratory cultures, microcosms, and mesocosms.
Reviewer comment (L81): "Ecological realism". This term is conceptually vague. Culture experiments do not lack realism; rather, they isolate physiological and carbonate chemistry responses while excluding broader ecological interactions such as trophic dynamics and community processes.
Response: Reworded exactly along the lines suggested by the reviewer.
Revised text: "...they isolate physiological and carbonate-chemistry responses while excluding broader ecological interactions such as trophic dynamics and community-level processes."
Reviewer comment (L82): "Environmental variability". It is unclear how microcosms inherently limit environmental variability, given that experimental conditions can be manipulated and controlled.
Response: Reworded to attribute the limitation to scale and duration rather than an inherent inability to vary conditions.
Revised text: "although they remain limited in scale and in environmental variability; their small scale and short duration distort real-world marine dynamics (Benton et al., 2007).
Reviewer comment (L108): " In parallel, the carbonate pump involves the formation of biogenic calcium carbonate by marine calcifiers, providing ballast that enhances the sinking of both inorganic carbon and associated organic matter”. While correct, it would be valuable to note that CaCO3 production also releases CO2, and that CaCO3 export has the opposite effect on surface pCO2 to organic carbon export.
Response: Added as requested.
Revised text: "Notably, CaCO₃ production releases CO₂ during calcification, so that export of CaCO₃ has an opposite net effect on surface ocean pCO₂ compared to export of organic carbon via the biological carbon pump."
Reviewer comment (L109): " providing ballast that enhances the sinking of both inorganic carbon and associated organic matter". Ballast refers to a material increasing the sinking velocity of another material. CaCO3 does not ballast itself but instead enhances the sinking of associated organic matter. Please revise accordingly.
Response: Reworded exactly as suggested.
Revised text: "...the carbonate pump involves the formation of biogenic calcium carbonate by marine calcifiers, which acts as ballast enhancing the sinking of associated organic matter."
Reviewer comment (L124): "Mechanism". The term "mechanism" is somewhat misleading in this context, as the authors appear to be referring to methods of alkalinity delivery (e.g., ships, pumps etc.). However, I do not believe this is what is intended. Alternative wording would improve clarity.
Response: Replaced "mechanism" with "chemical release process" to specify the intended meaning and added a new sentence explicitly addressing physical delivery methods (which the reviewer's comment shows could otherwise be assumed as the intended meaning), clarifying that these are a distinct consideration outside the review's primary focus.
Revised text: "OAE approaches can be broadly classified according to the dominant chemical form of alkalinity addition and the associated chemical release process through which alkalinity becomes available in seawater. Beyond this chemical classification, OAE deployment also involves distinct physical delivery methods, such as land-based reactor, ship-based dispersal, or coastal/harbour discharge, which influence the spatial scale, dilution rate, and local intensity of the resulting carbonate chemistry perturbation, but are not the primary focus of this review."
Reviewer comment (L128): "In equilibrated treatments where CO2 and pH are small". Consider revising to: "In equilibrated treatments, changes in CO2 and pH are relatively small."
Response: Changed as suggested.
Revised text: "In equilibrated treatments, changes in CO₂ and pH are relatively small.
Reviewer comment (L-128-130): "Minimal effects on coastal plankton communities". Please clarify which community attributes are being referred to (e.g., abundance, composition, diversity).
Revised text: In equilibrated treatments, changes in CO₂ and pH are relatively small; hydroxide-based OAE therefore had minimal effects on the composition and bloom dynamics of coastal plankton communities
Reviewer comment (L138): "Limestone (calcium carbonate)". Limestone is not synonymous with pure calcium carbonate. Please revise.
Revised text: "... where calcium carbonate from limestone…"
Reviewer comment (L146–147): "Specific chemistry of the alkalinity source". Please clarify which mineral or by-product is being referred to here.
Response: Specified explicitly.
Revised text: "...specific mineralogical composition of the alkalinity source, the release of Mg2+ and associated brucite-derived ions rather than Ca2+ or Na+, can drive species-specific responses independent of alkalinity increase alone."
Reviewer comment (L150–155): "Most chemically controlled forms of OAE". This statement is unclear, as NaOH and Ca(OH)2 additions also directly increase total alkalinity without releasing additional bioactive constituents. Please revise.
Revised text: "Like hydroxide-based approaches, carbonate-based OAE directly increases total alkalinity (TA) without releasing trace metals or nutrients; however, because carbonate salts add dissolved inorganic carbon (DIC) alongside alkalinity, a substantially larger TA increase is required to produce an equivalent shift in carbonate chemistry compared to hydroxide addition (van de Mortel et al., 2025)."
Reviewer comment (Figure 3): In its current form, the heat map provides limited insight into the available literature. Phytoplankton responses depend strongly on the magnitude of alkalinity addition and whether treatments were equilibrated or unequilibrated. Without incorporating these factors, the figure risks being misleading. I recommend replacing or substantially revising the figure and including a summary table detailing study region, alkalinity source, magnitude of alkalinity addition, equilibration state, experimental design, and other relevant variables. Bach and Taucher (2019) provide a useful example. Additionally, the figure indicates olivine has neutral or positive effects, whereas the text states that olivine disrupted plankton communities. These inconsistencies should be resolved.
Response: We traced the apparent inconsistency to its root cause: the original heatmap scored only four functional groups (diatoms, dinoflagellates, coccolithophores, chlorophyll-a), which did not include cyanobacteria, the group reported as negatively affected by olivine in Guo et al. (2025). We have revised Figure 3 to add a fifth "Cyanobacteria" column, which now shows a strongly negative score for olivine, resolving the apparent contradiction and showing that olivine's effects are taxon-specific rather than uniformly positive. The revised figure caption now explicitly states that response scores describe the direction of change in abundance/growth (not a value judgment on ecosystem outcome), and that equilibration state is heterogeneous within each heatmap cell (both NaOH and Ca(OH)2 categories include equilibrated and unequilibrated treatments; the reviewed olivine studies are, to our knowledge, predominantly or exclusively unequilibrated). A new Supplementary Table (Table 1) has been added, providing the region, alkalinity source, magnitude of alkalinity addition, equilibration state, and experimental design for each study underlying Figure 3, following the format suggested by the reviewer (cf. Bach and Taucher, 2019).
Revised text: Figure 3 caption: Semi-quantitative heatmap summarizing changes in phytoplankton abundance/growth across different ocean alkalinity enhancement (OAE) approaches. Response scores were assigned to each study as decrease (−1), no change (0), or increase (+1) in the abundance or growth of each functional group and averaged across studies for each phytoplankton group and OAE methodology. Blue indicates a decrease, green/yellow indicates no change, and red/orange indicates an increase; these scores describe the direction of change in a given taxon's abundance or growth, not a value judgment about ecosystem outcome. Each OAE category aggregates studies using different equilibration states: both the NaOH and Ca(OH)2 categories include equilibrated and unequilibrated treatments, while the olivine studies are predominantly or exclusively unequilibrated. This heterogeneity should be considered when interpreting the averaged scores and is detailed further in Supplementary Table 1."
Revised text: Main text (Section 3.1): "Olivine favors diatom growth and increases chlorophyll-a, likely reflecting the release of dissolved silicate and micronutrients that alleviate nutrient limitation, while simultaneously disfavoring cyanobacteria (Guo et al., 2025), illustrating that olivine's impact is taxon-specific rather than uniformly beneficial or harmful."
Reviewer comment (L161–162): "Nutrient alleviation effects". Please clarify whether this refers to the release of silicate alleviating silicon limitation.
Response: Confirmed and specified.
Revised text: "Olivine-based OAE has been associated with positive responses in diatoms, likely linked to the release of dissolved silicate, which alleviates silicon limitation and supports diatom frustule formation."
Reviewer comment (L182–185): The cited study investigated both equilibrated and unequilibrated OAE, with substantially different responses between treatments. The current summary does not reflect this distinction. Additionally, the reference is absent from the reference list.
Response: The missing reference (Ferderer et al., 2022) has been added, and the text now explicitly reflects that both equilibration states were tested with differing responses.
Revised text: "A Tasmania microcosm study testing both equilibrated and unequilibrated hydroxide-based OAE found that simulated OAE had significant but generally moderate effects on various phytoplankton and heterotrophic bacteria groups, with responses differing between equilibration states and more pronounced effects on diatom silicic acid drawdown and biogenic silica build-up reduced at elevated alkalinity (Ferderer et al., 2022)."
Reviewer comment (L199–200, 209): "Moderate OAE". The manuscript repeatedly uses the term "moderate" without defining the corresponding alkalinity increase. The cited study (Gately et al., 2023) investigated increases of approximately 700–2740 μmol kg-1, which are substantially larger than those expected under realistic deployment scenarios. Recent studies suggest environmental perturbations are more likely to fall within approximately 1–100 μmol kg-1 (e.g., Bach et al., 2026). Similar comments apply to Line 209.
Response: Added explicit magnitude for Gately et al. (2023) and contrasted it with the realistic deployment range, now citing the published version of Bach et al. (2026).
Revised text: "...with growth rates and elemental stoichiometry remaining largely unchanged under limestone- or calcium-based alkalinization at TA increases of ~700–2740 µmol kg⁻¹ (Gately et al., 2023), substantially higher than the ~1–100 µmol kg⁻¹ perturbations expected under realistic deployment scenarios (Bach et al., 2026)."
Reviewer comment (L207–208): The cited study is a mesocosm experiment rather than a field study.
Response: Corrected.
Revised text: "Recent evidence further supports the overall tolerance of diatom communities to OAE. In a mesocosm-based alkalinity enhancement experiment, Groppelli et al. (2026) reported no significant changes in diatom abundances or community composition under low to medium alkalinity additions (~250–550 µmol kg⁻¹)..."
Reviewer comment (L214–215): "Silicic acid drawdown and biogenic silica accumulation". This statement does not accurately reflect the cited study, which reported delayed silicate uptake under equilibrated OAE, delayed and reduced uptake under unequilibrated OAE, and reduced biogenic silica production in both instances.
Revised text: "...Ferderer et al. (2022) reported delayed silicate uptake under equilibrated OAE, delayed and reduced silicate uptake under unequilibrated OAE, and reduced biogenic silica production under both treatments, indicating moderate but ecologically relevant equilibration-dependent alterations of diatom biogeochemistry."
Reviewer comment (L220): "Variables" appears to be a typographical error and should read "variable".
Response: Corrected.
Reviewer comment (L238): This species has since been renamed. Please use the updated taxonomy or provide both names for consistency.
Response: Added the updated taxonomic name at first mention; retained the historical name (E. huxleyi) subsequently for consistency with the cited literature.
Revised text: "The most widespread species, Emiliania huxleyi (recently reclassified as Gephyrocapsa huxleyi based on phylogenomic evidence; Bendif et al., 2023), exhibits threshold-based growth responses to alkalinity enhancement."
Reviewer comment (L240): CO2 should not be expressed in μatm. Please specify whether pCO2 or fCO2 is intended.
Response: Corrected.
Revised text: "...suggesting a threshold pCO₂ of around 100 µatm necessary for optimal growth (Faucher et al., 2025)."
Reviewer comment (L238–240): "Gradual responses". There is insufficient evidence to support this broad statement for non-calcifying phytoplankton. In contrast, several studies have shown rapid reductions in growth below approximately 100 μatm pCO2 (e.g., Paul and Bach, 2020).
Response: Qualified the claim and added the suggested citation.
Revised text: "...the broader claim that non-calcifying phytoplankton show uniformly gradual responses to carbonate chemistry changes is not well supported. Several studies report comparably steep growth reductions below ~100 µatm pCO₂ across a range of taxa, including non-calcifiers (Paul and Bach, 2020)."
Reviewer comment (L246–248): Additional supporting references would strengthen this statement.
Response: Added the classic laboratory sources establishing the optimum-curve response of coccolithophore calcification to pCO2.
Revised text: "...suggesting that field conditions do not fundamentally alter the physiological constraints on calcification (Schneider et al., 2026)" is now preceded by: "This response pattern is consistent with findings from monospecific laboratory cultures (Bach et al., 2011; Sett et al., 2014; Bach et al., 2015)..."
Reviewer comment (L248–249): "Coccolithophore calcification". The cited study measured community calcification rather than species-specific coccolithophore calcification. Increased community calcification could therefore reflect greater coccolithophore abundance rather than enhanced cellular calcification.
Revised text: "...community-level coccolithophorid calcification followed an optimum curve response along the pCO₂ gradient... Because calcification was measured at the community rather than single-species level, this pattern may partly reflect shifts in coccolithophore abundance rather than enhanced per-cell calcification."
Reviewer comment (L253–254): The statement describing a "slightly positive correlation" with slurry concentration requires clarification. Was the relationship statistically significant, and was the response linear, unimodal, or threshold-like? Considering negative impacts at high concentrations.
Response: We have made explicit that statistical significance and response shape were not reported in the source, rather than implying a more rigorously characterized relationship.
Revised text: "In mesocosm experiments using Ca-hydroxide slurry under two contrasting trophic conditions, coccolithophore abundance showed a slightly positive response at low slurry concentrations but was negatively affected at high concentrations, a pattern also observed in diatoms within the same experiments (although the statistical significance and shape of this relationship were not reported) (Bazzicalupo et al., 2024)."
Reviewer comment (L256–258): It is important to specify that this study examined equilibrated OAE. Comparable alkalinity increases under unequilibrated conditions would reduce seawater pCO2 to concentrations well below those that support phytoplankton growth.
Revised text: "Laboratory culture experiments with CO₂-equilibrated, limestone-inspired alkalinity additions... demonstrated that G. huxleyi exhibited neutral growth responses... It is important to note that comparable alkalinity increases under unequilibrated conditions would reduce seawater pCO₂ to concentrations well below those required to sustain phytoplankton growth, meaning this neutral response cannot be generalized beyond equilibrated scenarios."
Reviewer comment (L262): The cited reference does not investigate nickel concentrations and appears to be incorrect.
Response: Confirmed: two distinct "Xin et al., 2024" papers exist; the nickel data belong to Xin, Faucher & Riebesell (2024b), not Xin, Goldenberg et al. (2024a). Corrected throughout.
Reviewer comment (L262–265): The statement that coccolithophores are more tolerant of elevated nickel concentrations than diatoms is unsupported. Please provide appropriate evidence or references.
Response: With the corrected citation (Xin et al., 2024b), the same study directly compares coccolithophore and diatom nickel tolerance under identical conditions, providing supporting evidence.
Revised text: "...considerably lower inhibition than the 60% observed in the diatom Thalassiosira weissflogii under the same experimental conditions (Xin et al., 2024b). This direct comparison supports the observation that coccolithophores are more tolerant to elevated nickel than at least this diatom species, though data remain too limited across taxa to generalize this pattern across all coccolithophores and diatoms, or to establish their relative tolerance compared to other phytoplankton groups."
Reviewer comment (L273): Please identify this as a microcosm study rather than a fjord study.
Response: Corrected.
Reviewer comment (L295): "Carbonate speciation". Consider replacing this with "carbon speciation", as carbonate is only one component (carbon species) of the dissolved inorganic carbon system.
Response: Replaced.
Reviewer comment (L296): "Enhanced alkalinity increases bicarbonate ion concentrations and carbonate saturation states." This statement is not universally correct and depends on whether equilibrated or unequilibrated conditions are being discussed. Under current DIC and TA concentrations (2000 and 2300 respectively) an increase in alkalinity decreases bicarbonate concentrations and increases carbonate concentrations. Please clarify accordingly.
Response: Reworded exactly per the reviewer's own correction.
Revised text: "The primary mechanism through which OAE affects phytoplankton communities is through changes in seawater carbon speciation. Under typical present-day ocean conditions (DIC ≈ 2000 µmol kg⁻¹, TA ≈ 2300 µmol kg⁻¹), enhanced alkalinity increases carbonate ion concentrations and carbonate saturation states while decreasing bicarbonate ion concentrations..."
Reviewer comment (L297): "CO2 should be formatted using subscript."
Response: Corrected.
Reviewer comment (L310): The discussion of nickel toxicity would benefit from comparison with expected nickel concentrations resulting from realistic OAE deployments (e.g., Hutchins et al., 2023; Guo et al., 2022). Without this context, the ecological relevance of the cited experiments is difficult to assess.
Response: Added quantitative context and the suggested references.
Revised text: "For context, dissolution experiments estimate a realistic Ni release of approximately 3 µmol kg⁻¹ per 100 µmol kg⁻¹ increase in alkalinity (Montserrat et al., 2017), several orders of magnitude below the concentrations (up to 100 µmol kg⁻¹) tested in laboratory studies (Hutchins et al., 2023; Guo et al., 2022)."
Reviewer comment (L334–337): The cited manuscript does not support the conclusion presented, as it was conducted under oligotrophic conditions without nutrient additions. Please revise.
Response: Confirmed: two distinct "de Castro et al., 2025" papers exist. The claim matches de Castro et al. (2025b, Oceans journal). Citation corrected accordingly.
Reviewer comment (L362–363): This statement is unclear and appears internally contradictory stating that nutrient limited systems are more resilient to OAE than potentially nutrient limited? Please clarify.
Revised text: "Oligotrophic systems show greater resilience to OAE perturbations than nutrient-rich systems, where combined nutrient and alkalinity additions have elicited negative responses in some taxa, such as coccolithophores (Xin et al., 2024a)."
Reviewer comment (363–364): "Species-specific responses". This statement is vague and unsupported. It is also unclear whether this reflects biological differences or publication bias, given that relatively few phytoplankton functional groups have been examined.
Revised text: "The species-specific responses observed within diatoms, dinoflagellates, and coccolithophores may reflect physiological differences in carbon acquisition and metal handling but could also partly stem from the fact that these three functional groups have received more experimental attention than other phytoplankton groups, making it difficult to distinguish a true biological signal from a sampling or publication bias in the current literature."
Reviewer comment (L366–371): The discussion of real-world deployment scenarios would benefit from explicitly considering dilution. Recent field deployments have demonstrated that carbonate chemistry perturbations diminish rapidly following release because of physical mixing, with important implications for ecological exposure.
Response: Added a citation to the first Australian OAE field trial, demonstrating rapid dilution of the alkalinity plume.
Revised text: "Real-world deployment scenarios must also account for dilution. The first Australian OAE field trial demonstrated that a plume of NaOH-modified seawater dispersed rapidly within meters of the release point: pCO₂ decreased by up to 370 µatm at the point of release, but downstream decreases ranged only from 22 to 77 µatm (Wynn-Edwards et al., 2025). This rapid dilution has important implications for the ecological exposure phytoplankton would realistically experience under field conditions, which is likely to be far more transient and spatially limited than in most laboratory, microcosm, or mesocosm experiments."
3. New references
Bach, L. T., Gill, S. J., Rickaby, R. E. M., Gore, S., & Renforth, P. CO2 removal with enhanced weathering and ocean alkalinity enhancement: potential risks and co-benefits for marine pelagic ecosystems. Front. Clim., 1. https://doi.org/10.3389/fclim.2019.00007, 2019.
Bach, L. T., Riebesell, U., & Schulz, K. G. Distinguishing between the effects of ocean acidification and ocean carbonation in the coccolithophore Emiliania huxleyi. Limnol. Oceanogr., 56(6), 2011.
Bach, L. T., Riebesell, U., Gutowska, M. A., Federwisch, L., & Schulz, K. G. A unifying concept of coccolithophore sensitivity to changing carbonate chemistry embedded in an ecological framework. Prog. Oceanogr., 135, 2015.
Bach, L. T., Tyka, M. D., Wang, B., & Fennel, K. Lethal by design? Resolving differences between experimental and real-world alkalinity perturbations in ocean alkalinity enhancement. J. Geophys. Res. Oceans, 131, e2025JC023598. https://doi.org/10.1029/2025JC023598, 2026.
Bendif, E. M., Probert, I., Archontikis, O. A., Young, J. R., Beaufort, L., Rickaby, R. E. M., & Filatov, D. Rapid diversification underlying the global dominance of a cosmopolitan phytoplankton. ISME J., 17. https://doi.org/10.1038/s41396-023-01377-2, 2023.
Benton, T. G., Solan, M., Travis, J. M. J., and Sait, S. M.: Microcosm experiments can inform global ecological problems, Trends Ecol. Evol., 22, 516–521, https://doi.org/10.1016/j.tree.2007.08.003, 2007.
de Castro, I., Ribeiro, S. C., Louvado, A., Gomes, N. C. M., Cachão, M., Silva Borges, P. F., Brito de Azevedo, E., & Barcelos e Ramos, J. Influence of inorganic nutrients on a North Atlantic microbial community's response to ocean alkalinity enhancement. Oceans, 6(4), 65. https://doi.org/10.3390/oceans6040065, 2025b.
Ferderer, A., Chase, Z., Kennedy, F., Schulz, K. G., & Bach, L. T. Assessing the influence of ocean alkalinity enhancement on a coastal phytoplankton community. Biogeosciences, 19(23). https://doi.org/10.5194/bg-19-5375-2022, 2022.
Ferderer, A., Schulz, K. G., Riebesell, U., Baker, K. G., Chase, Z., & Bach, L. T. Investigating the effect of silicate- and calcium-based ocean alkalinity enhancement on diatom silicification. Biogeosciences, 21(11). https://doi.org/10.5194/bg-21-2777-2024, 2024.
Guo, J. A., Strzepek, R., Willis, A., Ferderer, A., & Bach, L. T. Investigating the effect of nickel concentration on phytoplankton growth to assess potential side-effects of ocean alkalinity enhancement. Biogeosciences, 19(15). https://doi.org/10.5194/bg-19-3683-2022, 2022.
Jones, D. C., Ito, T., Takano, Y., & Hsu, W.-C. Spatial and seasonal variability of the air-sea equilibration timescale of carbon dioxide. Global Biogeochem. Cy., 28(11). https://doi.org/10.1002/2014GB004813, 2014.
Montserrat, F., Renforth, P., Hartmann, J., Leermakers, M., Knops, P., & Meysman, F. J. R. Olivine dissolution in seawater: implications for CO2 sequestration through enhanced weathering in coastal environments. Environ. Sci. Technol., 51(7), 2017.
Paul, A. J., & Bach, L. T. Universal response pattern of phytoplankton growth rates to increasing CO2. New Phytol., 228(6). https://doi.org/10.1111/nph.16806, 2020.
Reinfelder, J. R. Carbon concentrating mechanisms in eukaryotic marine phytoplankton. Annu. Rev. Mar. Sci., 3. https://doi.org/10.1146/annurev-marine-120709-142720, 2011.
Renforth, P., & Henderson, G. Assessing ocean alkalinity for carbon sequestration. Rev. Geophys., 55(3). https://doi.org/10.1002/2016RG000533, 2017.
Sett, S., Bach, L. T., Schulz, K. G., Koch-Klavsen, S., Lebrato, M., & Riebesell, U. Temperature modulates coccolithophorid sensitivity of growth, photosynthesis and calcification to increasing seawater pCO2. PLoS ONE, 9(2), e88308, 2014.
van de Mortel, H., Bednaršek, N., Pelletier, G., Feely, R. A., Müller, J. D., & Gruber, N. Substantial limitations of ocean alkalinity enhancement in mitigating the negative impacts of ocean acidification on marine calcifiers. Environ. Sci. Technol., 59(51). https://doi.org/10.1021/acs.est.5c09298, 2025.
Wynn-Edwards, C. A., Dillon, W. D. N., Akl, J., Neill, C., Anderson, H. J., Lim, H. S., Mongin, M., & Shadwick, E. H. Alkalinity enhancement with sodium hydroxide in coastal ocean waters. Sci. Rep., 15. https://doi.org/10.1038/s41598-025-31606-w, 2026.
Xin, X., Faucher, G., & Riebesell, U. Phytoplankton response to increased nickel in the context of ocean alkalinity enhancement. Biogeosciences, 21(3). https://doi.org/10.5194/bg-21-761-2024, 2024b.
4. New supplementary material
A new Supplementary Table 1 has been added, providing region, alkalinity source, magnitude of alkalinity addition, equilibration state, and experimental design for each study underlying Figure 3, as suggested by the reviewer (submitted as a separate supplementary file).
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AC1: 'Reply on RC1', Sara Groppelli, 16 Sep 2026
reply
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- 1
Review of "Experimental Evidence of Phytoplankton Responses to Ocean Alkalinity Enhancement"
General Assessment
This review addresses a timely and important topic and provides a valuable synthesis of the rapidly expanding literature examining phytoplankton responses to Ocean Alkalinity Enhancement (OAE). The manuscript is generally well organised and brings together studies spanning laboratory cultures, microcosms, mesocosms, and modelling approaches. However, several recurring issues currently weaken the synthesis and should be addressed prior to publication.
The review aims to synthesise phytoplankton responses to OAE; however, beyond highlighting the ecological importance of marine phytoplankton, the introduction does not adequately explain how or why OAE is expected to influence phytoplankton. As OAE is specifically designed to alter marine carbonate chemistry, its primary effect is to modify the availability of dissolved inorganic carbon species (albeit temporarily). This is fundamental because dissolved CO2 and carbonic acid directly influence phytoplankton carbon acquisition and growth. The introduction would therefore benefit from a concise discussion of why phytoplankton, beyond their ecological importance, are a key group requiring investigation prior to large-scale OAE implementation.
Within the introduction, the authors use the terms "equilibration state" and "equilibrated" without defining them. Similarly, the explanation of OAE and its effects on the marine carbonate system is overly simplified and does not describe the associated changes in carbon speciation that underpin many phytoplankton responses. I strongly recommend including a more comprehensive explanation of OAE, its impacts on the marine carbonate system, and the distinction between equilibrated and unequilibrated OAE early in the manuscript, ideally before Section 2.1. Furthermore, throughout the manuscript the authors frequently cite studies without indicating whether they investigated equilibrated or unequilibrated OAE. Given that the manuscript itself highlights that these treatments can produce substantially different biological responses, this distinction should be made consistently throughout to facilitate meaningful comparison among studies.
Throughout the manuscript, phrases such as "a liming experiment in the eastern Mediterranean" imply field experiments, while some mesocosm studies are explicitly referred to as field studies. These are fundamentally different experimental approaches and should be described accurately throughout the manuscript (e.g., "a mesocosm study conducted in the eastern Mediterranean").
There are also several instances where incorrect references appear to have been cited, often involving different publications by the same first author. In addition, several references cited within the text do not appear in the reference list. I recommend a careful review of all citations before publication.
Specific Comments
Line 29: "CO2". Here and throughout the manuscript, CO2 should be consistently formatted using subscript.
Lines 50–52: "As of early 2026...". Rather than discussing incomplete annual publication records, it would be more appropriate to state the literature search date.
Lines 65–66: I am not aware of any OAE field deployments in the North Sea region, nor does the manuscript subsequently discuss any. It is therefore unclear how the description of North Sea characteristics is relevant here.
Lines 68–69: "High buffering capacity and elevated alkalinity". It is unclear what this comparison is relative to. Providing representative values or ranges would improve clarity. Furthermore, as many OAE studies have been conducted in coastal regions with naturally elevated alkalinity, it is not clear how the North Sea differs in this regard.
Lines 70–71: "Air-sea exchange and mixing". This statement is somewhat contradictory. Strong mixing would rapidly dilute chemical perturbations, while rapid air-sea gas exchange would accelerate equilibration and reduce pCO2 perturbations. Together, these processes would shorten the duration of altered carbonate chemistry and potentially make ecological responses more difficult to detect.
Figure 2 caption: The caption refers to different symbols, whereas all points appear to use the same symbol with different colours. Please revise accordingly.
Line 81: "Ecological realism". This term is conceptually vague. Culture experiments do not lack realism; rather, they isolate physiological and carbonate chemistry responses while excluding broader ecological interactions such as trophic dynamics and community processes.
Line 82: "Environmental variability". It is unclear how microcosms inherently limit environmental variability, given that experimental conditions can be manipulated and controlled.
Line 108: " In parallel, the carbonate pump involves the formation of biogenic calcium carbonate by marine calcifiers, providing ballast that enhances the sinking of both inorganic carbon and associated organic matter”. While correct, it would be valuable to note that CaCO3 production also releases CO2, and that CaCO3 export has the opposite effect on surface pCO2 to organic carbon export.
Line 109: " providing ballast that enhances the sinking of both inorganic carbon and associated organic matter". Ballast refers to a material increasing the sinking velocity of another material. CaCO3 does not ballast itself but instead enhances the sinking of associated organic matter. Please revise accordingly.
Line 124: "Mechanism". The term "mechanism" is somewhat misleading in this context, as the authors appear to be referring to methods of alkalinity delivery (e.g., ships, pumps etc.). However, I do not believe this is what is intended. Alternative wording would improve clarity.
Line 128: "In equilibrated treatments where CO2 and pH are small". Consider revising to: "In equilibrated treatments, changes in CO2 and pH are relatively small."
Lines 128–130: "Minimal effects on coastal plankton communities". Please clarify which community attributes are being referred to (e.g., abundance, composition, diversity).
Line 138: "Limestone (calcium carbonate)". Limestone is not synonymous with pure calcium carbonate. Please revise.
Lines 146–147: "Specific chemistry of the alkalinity source". Please clarify which mineral or by-product is being referred to here.
Lines 150–155: "Most chemically controlled forms of OAE". This statement is unclear, as NaOH and Ca(OH)2 additions also directly increase total alkalinity without releasing additional bioactive constituents. Please revise.
Figure 3: In its current form, the heat map provides limited insight into the available literature. Phytoplankton responses depend strongly on the magnitude of alkalinity addition and whether treatments were equilibrated or unequilibrated. Without incorporating these factors, the figure risks being misleading. I recommend replacing or substantially revising the figure and including a summary table detailing study region, alkalinity source, magnitude of alkalinity addition, equilibration state, experimental design, and other relevant variables. Bach and Taucher (2019) provide a useful example. Additionally, the figure indicates olivine has neutral or positive effects, whereas the text states that olivine disrupted plankton communities. These inconsistencies should be resolved.
Lines 161–162: "Nutrient alleviation effects". Please clarify whether this refers to the release of silicate alleviating silicon limitation.
Lines 182–185: The cited study investigated both equilibrated and unequilibrated OAE, with substantially different responses between treatments. The current summary does not reflect this distinction. Additionally, the reference is absent from the reference list.
Lines 199–200: "Moderate OAE". The manuscript repeatedly uses the term "moderate" without defining the corresponding alkalinity increase. The cited study (Gately et al., 2023) investigated increases of approximately 700–2740 μmol kg-1, which are substantially larger than those expected under realistic deployment scenarios. Recent studies suggest environmental perturbations are more likely to fall within approximately 1–100 μmol kg-1 (e.g., Bach et al., 2026). Similar comments apply to Line 209.
Lines 207–208: The cited study is a mesocosm experiment rather than a field study.
Lines 214–215: "Silicic acid drawdown and biogenic silica accumulation". This statement does not accurately reflect the cited study, which reported delayed silicate uptake under equilibrated OAE, delayed and reduced uptake under unequilibrated OAE, and reduced biogenic silica production in both instances.
Line 220: "Variables" appears to be a typographical error and should read "variable".
Line 238: This species has since been renamed. Please use the updated taxonomy or provide both names for consistency.
Line 240: CO2 should not be expressed in μatm. Please specify whether pCO2 or fCO2 is intended.
Lines 238–240: "Gradual responses". There is insufficient evidence to support this broad statement for non-calcifying phytoplankton. In contrast, several studies have shown rapid reductions in growth below approximately 100 μatm pCO2 (e.g., Paul and Bach, 2020).
Lines 246–248: Additional supporting references would strengthen this statement.
Lines 248–249: "Coccolithophore calcification". The cited study measured community calcification rather than species-specific coccolithophore calcification. Increased community calcification could therefore reflect greater coccolithophore abundance rather than enhanced cellular calcification.
Lines 253–254: The statement describing a "slightly positive correlation" with slurry concentration requires clarification. Was the relationship statistically significant, and was the response linear, unimodal, or threshold-like? Considering negative impacts at high concentrations.
Lines 256–258: It is important to specify that this study examined equilibrated OAE. Comparable alkalinity increases under unequilibrated conditions would reduce seawater pCO2 to concentrations well below those that support phytoplankton growth.
Line 262: The cited reference does not investigate nickel concentrations and appears to be incorrect.
Lines 262–265: The statement that coccolithophores are more tolerant of elevated nickel concentrations than diatoms is unsupported. Please provide appropriate evidence or references.
Line 273: Please identify this as a microcosm study rather than a fjord study.
Line 295: "Carbonate speciation". Consider replacing this with "carbon speciation", as carbonate is only one component (carbon species) of the dissolved inorganic carbon system.
Line 296: "Enhanced alkalinity increases bicarbonate ion concentrations and carbonate saturation states." This statement is not universally correct and depends on whether equilibrated or unequilibrated conditions are being discussed. Under current DIC and TA concentrations (2000 and 2300 respectively) an increase in alkalinity decreases bicarbonate concentrations and increases carbonate concentrations. Please clarify accordingly.
Line 297: CO2 should be formatted using subscript.
Line 310: The discussion of nickel toxicity would benefit from comparison with expected nickel concentrations resulting from realistic OAE deployments (e.g., Hutchins et al., 2023; Guo et al., 2022). Without this context, the ecological relevance of the cited experiments is difficult to assess.
Lines 334–337: The cited manuscript does not support the conclusion presented, as it was conducted under oligotrophic conditions without nutrient additions. Please revise.
Lines 362–363: This statement is unclear and appears internally contradictory stating that nutrient limited systems are more resilient to OAE than potentially nutrient limited? Please clarify.
Lines 363–364: "Species-specific responses". This statement is vague and unsupported. It is also unclear whether this reflects biological differences or publication bias, given that relatively few phytoplankton functional groups have been examined.
Lines 366–371: The discussion of real-world deployment scenarios would benefit from explicitly considering dilution. Recent field deployments have demonstrated that carbonate chemistry perturbations diminish rapidly following release because of physical mixing, with important implications for ecological exposure.