the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Mar 2026
Biological thresholds for marine carbon dioxide removal (mCDR): the effect of changes in carbonate chemistry
Abstract. Marine carbon dioxide removal (mCDR) encompasses a variety of approaches to actively remove CO2 from the atmosphere, which, given current and projected emissions, is necessary to keep global average temperature increases to less than 2 °C. In addition to the removal of CO2 from the atmosphere, mCDR projects would have effects on the marine environment, including changes in ocean carbonate chemistry. These changes in carbonate chemistry can affect marine organisms through multiple physiological pathways. Although research on the effects of ocean acidification over the last fifteen years has advanced understanding of the effects of low pH/high CO2 conditions on marine organisms, much less is known about organismal response to the high pH/low CO2 or high alkalinity conditions that can be generated by some mCDR methods. We created a database of available information on marine species response to the carbonate chemistry conditions that can be generated by mCDR with a focus on identifying carbonate chemistry thresholds at which biological responses occur. The database contains 310 studies, from which we estimated 276 thresholds. In addition to all mCDR studies available to date, we reviewed studies not explicitly designed to address mCDR. These studies used natural and artificially manipulated variations in carbonate chemistry, including increasing pH and alkalinity, to explore basic physiological and biological responses as well as approaches to mitigate ocean acidification. In analyzing the database, we considered a variety of biological responses, both positive and negative, and considered sensitivity by taxa, exposure duration, treatment method and other factors. Using practical definitions of a threshold, including one based on the treatment level leading to the first statistically significant biological response, we describe the distribution of pH and alkalinity threshold values. The data reveal a high tolerance for high pH (>12) and high alkalinity for some species responses under some experimental conditions, but negative response to experimental conditions only slightly above ambient (pH 8.3) for other species responses. A positive effect of pH above ambient levels was observed in six cases, just 3 % of the total. We summarize results by presenting the distributions of threshold estimates collected from individual studies. The uncertainty and variation in organismal response represents a challenge for the development of management guidelines for the developing mCDR industry. Building on our review of existing studies, we suggest several paths forward for improved biological threshold estimates for mCDR-relevant carbonate chemistry conditions.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-1597', Anonymous Referee #1, 22 Apr 2026
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RC2: 'Comment on egusphere-2026-1597', Anonymous Referee #2, 16 Jul 2026
McElhany et al. summarize the currently proposed marine carbon dioxide removal (mCDR) methods and their potential impacts on carbonate chemistry and different marine species. The review analyzes over 300 studies and compiles 276 biological thresholds. This represents a substantial amount of work, and it is challenging to identify safe thresholds for mCDR deployment. Therefore, limiting the scope to pH and alkalinity thresholds is reasonable. I appreciate the authors' time and effort in producing this valuable synthesis, and I hope they find the following comments helpful.
Major comments:
- Page 44: It would be helpful to re-emphasize which groups of organisms are most sensitive to pH change and to summarize the general optimal or safe pH ranges for different marine taxa in the Discussion. Based on the results presented, phytoplankton appear to be more sensitive than invertebrates and fish. This is likely to be one of the key take-home messages that readers will be interested in.
- Line 940: In the Results section, you identify optimal pH ranges for different groups of organisms. Therefore, I am not sure why the Discussion states that "there is no obvious biological tipping point for species sensitivity." Although the dataset has limitations, I think it is equally important to discuss the patterns that were observed, as well as the limitations of the analysis.
- Figure 3: Please define the abbreviations (e.g., Pop. Growth, Pos. Response). I also found the distinction between "taxon" and "study type" somewhat confusing, as field studies necessarily involve particular groups of organisms. I suggest specifying which organisms or communities were investigated in the field studies (are they phytoplankton? Benthic community? etc). If this cannot be done here, it should at least be clarified in Figure 7 where key biological responses were summarized.
Minor comments:
- Line 65: More references are needed here in addition to Fuhrman et al. (2023).
- Box 1: It would be helpful to include an illustration about the different mCDR methods and their underlying mechanisms.
- Box 1: In OAE, there are two distinct approaches: equilibrated OAE and unequilibrated OAE. These are not simply different stages of the same process but represent two different implementation strategies. Specifically, equilibrated OAE involves enclosing a body of seawater, adding alkalinity, allowing the water body to equilibrate with atmospheric CO₂, and then releasing it back into the ocean. These two approaches have different carbonate chemistry and therefore different biological implications. For example, equilibrated OAE results in much smaller changes in pH and DIC upon release.
- Line 81: "...which may contain harmful compounds." Please specify what these harmful compounds may be.
- Line 84: "...leading to a variety of possible coastal ecosystem effects." Please elaborate on what these potential coastal ecosystem effects are.
- Box 2 (Page 5): It is unclear what "system" refers to in this box. Does it refer to the seawater carbonate system, or the broader Earth system including both the atmosphere and the ocean? Please clarify this, as air–sea CO₂ exchange is important for explaining the concepts presented here.
- Box 2 (Page 6): "As seawater pH or carbonate saturation state (Ω) increases, brucite, calcium carbonate, and other compounds may precipitate out of solution." Does Mg(OH)₂ generally precipitate with increasing seawater pH or Ω? This sentence appears to overgeneralize the conditions under which secondary mineral precipitation occurs. While CaCO₃ precipitation is a well-recognized consequence of increased carbonate saturation state, Mg(OH)₂ precipitation is not generally expected under typical seawater OAE conditions and has mainly been reported under specific circumstances (e.g., localized high pH or incomplete dissolution of added brucite). I suggest revising the sentence to distinguish between these processes and avoid implying that increases in seawater pH or Ω generally promote precipitation of both phases. In addition, Suitner et al. (2024a) and Suitner et al. (2024b) appear to refer to the same publication. Please check and correct the duplicate citation.
- Box 2 (Page 6): I suggest citing Eq. (1) after "...which pushes the system to a new equilibrium with a lower pH."
- Box 2 (Page 7): "...followed by a slow reduction in pH if conditions lead to precipitation." Please specify what type of precipitation is being referred to here.
- Box 2 (Table): "As indicated by the asterisk (*) in the table, at equilibrium and to the extent that OAE causes CDR, the pCO₂ in the ocean and atmosphere will be slightly lower than before alkalinity addition." My understanding is that because CO₂ has been permanently transferred from the atmosphere into the ocean, the atmosphere contains slightly less CO₂ than before. Consequently, the final equilibrium pCO₂ of both the atmosphere and ocean should also be slightly lower than the initial value. Is this interpretation correct? If so, please explain this more clearly so readers can better understand.
- Box 2 (Table): "The pre-equilibrium state will differ from that shown in the table if the alkaline material is carbonate-based (e.g., CaCO₃ or Na₂CO₃)." Is CaCO₃ directly used for OAE? It’s hard to dissolve right?
- Box 2 (Table): Why does the HCO₃⁻ concentration decrease in the pre-equilibrium OAE scenario? HCO₃⁻ can shift toward CO₂, but CO₃²⁻ can also convert to HCO₃⁻. Please explain this more clearly.
- Box 2 (Table): Please specify whether the pCO₂ shown refers to seawater or the atmosphere to avoid confusion. In addition, what is the difference between the white and black arrows?
- Table 1: CO₃²⁻ concentration and Ω are also affected by calcification. What do "dissolution" and "fertilization" refer to in this table? Please clarify these terms and double-check that the information is consistent with Box 2.
- Line 149: I suggest replacing "phytoplankton" with "coccolithophores or other calcifying phytoplankton."
- Line 177: Please change "is focused on" to "focuses on."
- Box 3: Consider replacing "discriminate changes in the system" with "detect meaningful changes in the system" or "distinguish meaningful changes from natural variability".
- Line 189: Please explain what is meant by "potential biological impacts of mCDR."
- Table 6: Please remove the extra unit "μmol kg⁻¹" and ensure that units are presented consistently throughout the manuscript. In some places, the unit is written as μmol kg⁻¹, while elsewhere a different format is used.
- Figure 7: Please see comment before. It would be helpful to indicate which organisms were included under the OA experiments and field studies (e.g., fish, phytoplankton communities, benthic invertebrates).
- Figure 8: It may be better to use μmol L⁻¹ instead of mmol L⁻¹ in the first subplot. Please also label each subplot.
- Line 1024–1026: I do not agree with this suggestion, as pH is more commonly reported than H⁺ concentration in both experimental studies and field measurements.
- Line 1034–1037: I appreciate the authors' emphasis on the importance of other carbonate chemistry parameters. I also encourage them to discuss which parameters are most practical for evaluating the biological impacts of mCDR based on the dataset they compiled. For example, DIC may be a particularly important parameter because it directly influences photosynthesis and fish physiology (as mentioned in Lines 121–126).
Citation: https://doi.org/10.5194/egusphere-2026-1597-RC2
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Cited
2 citations as recorded by crossref.
- Stability assessment of calcium carbonate dissolution as a marine carbon dioxide removal mechanism A. Melendez-Perez et al. https://doi.org/10.3389/fmars.2026.1796693
- A sequential gated research framework for addressing potential impacts of marine carbon dioxide removal on fisheries, aquaculture, and Indigenous communities K. Grabb et al. https://doi.org/10.1093/icesjms/fsag076
The manuscript by McElhany et al. examines how mCDR involving changes in carbonate chemistry could impact marine organisms and applies a qualitative systematic review approach to answer this question. They identify thresholds, defined as the level at which the first statistically significant result is found to determine when impacts occur. I consider this is a good approach. However, the manuscript itself has some major issues, including missing papers (and therefore responses). If the manuscript is revised to include all relevant work using appropriate methods, then I would be happy to provide more detailed comments on a revised version of the manuscript.
Major comments:
The authors should follow the procedure of PRISMA: https://www.prisma-statement.org/. Here, the search terms are not exactly noted, search dates etc. in the main methods. This has led to other issues. I can see that they are missing a number of studies that I can think of off the top of my head. For example, what about the work of Steeve Comeau? There are a handful of his papers that explore high pH and/or high TA (through high DIC). None of these are here. The manuscript must be reproducible and must be systematic if it is to be published.
I think it is confusing to use the term “studies” and “articles or reports” as being mutually exclusive. Instead of “studies” I would describe them as “responses”, a term typically used in the field of meta-analyses.
The figure quality needs to be improved for this work to be published. The text is currently grainy (and font sizes inconsistent), and the internal gridlines could be removed, colours improved etc.
Minor comments:
The table legends and tables should be on the same page (except table 5, where are the others?). E.g. the one on Diatoms. This could be better suited in the supplementary materials.
The results section is far too long and very list like at time.
The supplementary data set should provide studies in alphabetical order.
Could line numbers please be added in order to facilitate review?
The introduction presently has too much results. This is an odd approach. Introduce the topic, its importance and relevance and then (maybe) discuss the results at the very end of the introduction.
Page 4: a lot more references required. Here and elsewhere. If it is not a novel finding, please use a reference to introduce the point.
Box 2 the figure should be elementary for many readers. The authors should focus on the table, being more specific about what methods have been proposed to induce OAE and what chemical effects this would have for example. Are all the methods equal? These are the things important to the readers.