| 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.
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.