Biogeochemical responses to ocean alkalinity enhancement in the North China Sea

Xin, Xiaoke; Liu, Yimeng; Chang, Ruize; Lei, Ruobin; Zhu, Minghao; Wang, Xiaotong; Wang, Xue Chao; Liu, Jihua

doi:10.5194/egusphere-2026-5281

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https://doi.org/10.5194/egusphere-2026-5281

Preprints

03 Sep 2026

| 03 Sep 2026

Biogeochemical responses to ocean alkalinity enhancement in the North China Sea

Xiaoke Xin, Yimeng Liu, Ruize Chang, Ruobin Lei, Minghao Zhu, Xiaotong Wang, Xue Chao Wang, and Jihua Liu

Abstract. Ocean Alkalinity Enhancement (OAE) has been proposed as a promising marine carbon dioxide removal strategy. Yet its ecological consequences remain poorly understood, particularly in dynamic coastal systems characterised by high environmental variability. Here, we investigated the effects of OAE on plankton community structure and biogeochemical processes in the eutrophic North China Sea (NCS) using a microcosm experiment with natural seawater collected offshore. Two OAE scenarios were evaluated: CO₂-equilibrated and CO₂-non-equilibrated treatments, both targeting an increase in total alkalinity (ΔTA) of approximately 500 µmol kg⁻¹. Neither OAE treatment significantly affected the abundance of phytoplankton and microzooplankton. Similarly, plankton communities exposed to OAE maintained their natural levels of chlorophyll a, taxonomic composition, biodiversity, and particulate organic carbon throughout the experiment. In contrast, substantial TA loss occurred in the CO₂-non-equilibrated OAE treatment, indicating reduced alkalinity retention under non-equilibrated conditions. This study was conducted within the framework of the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP), which was established to standardise OAE microcosm experiments and systematically assess the environmental impacts of OAE across diverse marine ecosystems. Our findings indicate that NCS phytoplankton and microzooplankton exhibited high tolerance to both CO₂-equilibrated and non-equilibrated OAE scenarios under the experimental conditions tested, while highlighting the need to monitor and mitigate TA loss alkalinity loss during OAE implementation in warm coastal environments.

Received: 03 Sep 2026 – Discussion started: 03 Sep 2026

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Xiaoke Xin, Yimeng Liu, Ruize Chang, Ruobin Lei, Minghao Zhu, Xiaotong Wang, Xue Chao Wang, and Jihua Liu

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-5281', Anonymous Referee #1, 28 Sep 2026

This manuscript addresses a timely and important topic focusing on the ecological impacts of OAE in coastal systems, and contributes valuable data to the growing OAEPIIP intercomparison effort. The manuscript is generally well-written and the experimental design follows standardized protocols. The experimental design is appropriate for the research question, and the inclusion of both equilibrated and non-equilibrated treatments is a strength. However, methodological details are insufficiently described, and the conclusions need to be more carefully qualified. I recommend minor revision before the manuscript can be considered for publication in Biogeosciences.
Comments
L15-20: The "substantial TA loss" in the non-equilibrated treatment is arguably the most important result of the study. However, the abstract provides no mechanistic explanation. Please propose potential mechanisms.
L24: "TA loss alkalinity loss" — duplicated phrase; correct to "TA loss" or "alkalinity loss."
L14: "Neither OAE treatment significantly affected...", please specify the statistical test in the abstract, or at least indicate that this is based on GAMMs.
L50-60: The paragraph on OAE and plankton cites several studies but does not discuss the mechanistic pathways by which OAE might affect plankton (e.g., pH stress, CO₂ limitation, trace metal availability, nutrient speciation). A brief mechanistic overview would strengthen the introduction.
L140-147: Statistical analysis: GAMMs are mentioned but not described in detail. What was the model structure? What were the fixed and random effects? How was temporal autocorrelation handled? Provide more detail.
L165 Fig1: The claim of a TA:DIC loss ratio of ~2:1 (Section 4.3, "Implications for OAE") is the primary evidence for CaCO3 precipitation. This ratio must be shown explicitly.
L172-174: "Initial BSi levels were high, likely due to resuspended particles caused by windy conditions at the time of sample collection." — This is speculative. Move to Discussion or remove.
L185: "Phytoplankton assemblages were dominated by small-sized cells (< 5 µm) until day 4, with no larger plankton observed by microscopy." — This is important for interpreting the lack of response. Discuss whether the community was dominated by pico- and nanoplankton throughout, and whether this affected sensitivity to OAE.
L235: "The lack of detectable responses is likely be attributed..." — grammatical error; correct to "is likely attributable to".
L255-260: It is suggested to discuss seasonal and interannual variability in the NCS and how this might affect generalizability.
L262: Section 4.2 concludes that "there was little evidence that moderate alkalinity enhancement influenced the measured pools of particulate organic carbon and nitrogen." However, the claim that "OAE can alter inorganic carbon cycling without necessarily producing short-term changes in plankton-derived particulate organic matter" is strong. Please rephrase.
L300: "Consequentially" — use "Consequently."
L302: The limitations section does not discuss the closed-system artifact (no gas exchange, no dilution, no vertical mixing) or the bottle effect.
L328: "moderate OAE can be implemented without detectable short-term effects" — this is too strong. Replace with "did not detect short-term effects under the experimental conditions tested."
References: Check for consistency in citation style. Some references are cited as "(Bach, 2024)" while others are "(Bach et al., 2019)."

Citation: https://doi.org/10.5194/egusphere-2026-5281-RC1
- AC1: 'Reply on RC1', Xiaoke Xin, 28 Oct 2026
  
  This manuscript addresses a timely and important topic focusing on the ecological impacts of OAE in coastal systems, and contributes valuable data to the growing OAEPIIP intercomparison effort. The manuscript is generally well-written and the experimental design follows standardized protocols. The experimental design is appropriate for the research question, and the inclusion of both equilibrated and non-equilibrated treatments is a strength. However, methodological details are insufficiently described, and the conclusions need to be more carefully qualified. I recommend minor revision before the manuscript can be considered for publication in Biogeosciences.
  We sincerely thank the reviewers for their time, effort, and constructive comments and suggestions, which have helped improve the quality of our manuscript. We have carefully considered and addressed all the concerns raised by the reviewers and revised the manuscript accordingly. Our point-by-point responses are provided below. All revisions are highlighted in blue in our responses and tracked in the revised manuscript for ease of reference.
  
  Comments
  L15-20: The "substantial TA loss" in the non-equilibrated treatment is arguably the most important result of the study. However, the abstract provides no mechanistic explanation. Please propose potential mechanisms.
  Reply: We thank the reviewer for the suggestion and have provided the potential mechanisms in the abstract.
  L17-19: In contrast, substantial TA loss occurred in the CO2-non-equilibrated OAE treatment due to high saturation state, indicating reduced alkalinity retention under non-equilibrated conditions.
  L24: "TA loss alkalinity loss" — duplicated phrase; correct to "TA loss" or "alkalinity loss."
  Reply: We thank the reviewer for pointing it out. The phrase has been corrected to "TA loss".
  L22-25: Our findings indicate that the YS phytoplankton and microzooplankton exhibited high tolerance to both CO₂-equilibrated and non-equilibrated OAE scenarios under the experimental conditions tested, while highlighting the need to monitor and mitigate TA loss during OAE implementation in warm coastal environments.
  L14: "Neither OAE treatment significantly affected...", please specify the statistical test in the abstract, or at least indicate that this is based on GAMMs.
  Reply: We appreciate the reviewers for the thoughtful suggestion and have revised the sentence accordingly.
  L15-17: Similarly, plankton communities exposed to OAE maintained their natural levels of chlorophyll a, taxonomic composition, biodiversity, and particulate organic carbon throughout the experiment, as indicated by generalised additive mixed models.
  L50-60: The paragraph on OAE and plankton cites several studies but does not discuss the mechanistic pathways by which OAE might affect plankton (e.g., pH stress, CO₂ limitation, trace metal availability, nutrient speciation). A brief mechanistic overview would strengthen the introduction.
  Reply: We appreciate the author for the constructive comment and have provided a mechanistic overview to introduce how OAE might affect plankton.
  L40-44: By changing carbonate chemistry, e.g., carbon availability, elevated pH and calcium carbonate saturation, OAE may affect phytoplankton photosynthesis, calcification, and community composition. These changes may, in turn, influence microzooplankton through shifts in prey availability and physiological responses to pH, potentially triggering cascading effects across marine trophic levels (Oschlies et al., 2023; Renforth and Henderson, 2017).
  L140-147: Statistical analysis: GAMMs are mentioned but not described in detail. What was the model structure? What were the fixed and random effects? How was temporal autocorrelation handled? Provide more detail.
  Reply: We have described the statistical analysis models in the Method.
  L161-168: Briefly, four models were fitted to assess whether alkalinity treatments affected the temporal trends and absolute values of each parameter. The models represented four scenarios: no treatment effects, effects on temporal trends only, effects on absolute values only, or effects on both. Individual microcosms were included as random intercepts to account for unexplained variability. Model assumptions were evaluated by visually inspecting residual heteroscedasticity and temporal autocorrelation. The models were compared using the Akaike information criterion (AIC), with lower values indicating a better balance between model fit and complexity.
  L165 Fig1: The claim of a TA:DIC loss ratio of ~2:1 (Section 4.3, "Implications for OAE") is the primary evidence for CaCO3 precipitation. This ratio must be shown explicitly.
  Reply: We are grateful to the reviewer for the thoughtful suggestion and have provided the TA:DIC loss ratio along with the aragonite saturation state. We have also expanded the discussion on the underlying mechanisms driving secondary precipitation.
  L335-341: Although mineral precipitation was not directly quantified, the observed stoichiometric relationship suggests that CaCO₃ formation may have contribute substantially to the observed alkalinity loss when Ω_aragonite exceeds a critical threshold. No net alkalinity loss was observed in Eq-OAE treatment because the carbonate system remained within conditions that prevented mineral precipitation. In contrast, nEq-OAE caused sustained alkalinity loss due to stronger carbonate chemistry perturbations from direct OH^- addition. Unlike Eq-OAE, which adds alkalinity in proportions of and and consistent with atmospheric equilibrium, nEq-OAE may generate critical oversaturation before sufficient air–sea equilibration occurs. This process may be particularly relevant near the sea surface, where atmospheric equilibration is slow (Hartmann et al., 2022; Suitner et al., 2024).
  L172-174: "Initial BSi levels were high, likely due to resuspended particles caused by windy conditions at the time of sample collection." — This is speculative. Move to Discussion or remove.
  Reply: We agree and have removed it from the manuscript.
  L185: "Phytoplankton assemblages were dominated by small-sized cells (< 5 µm) until day 4, with no larger plankton observed by microscopy." — This is important for interpreting the lack of response. Discuss whether the community was dominated by pico- and nanoplankton throughout, and whether this affected sensitivity to OAE.
  Reply: We thank the reviewer for pointing this out.
  L284-287: The phytoplankton community was dominated by small-sized species, which are less limited by CO₂ diffusion via carbon-concentrating mechanisms, potentially making them more tolerant to OAE-induced changes in carbonate chemistry (Raven 1998; Marañón et al. 2013).
  Marañón, E., P. Cermeño, D. C. López-Sandoval, T. Rodríguez-Ramos, C. Sobrino, M. Huete-Ortega, J. M. Blanco, and J. Rodríguez. 2013. Unimodal size scaling of phytoplankton growth and the size dependence of nutrient uptake and use. Ecology Letters 16: 371–379. doi:10.1111/ele.12052
  Raven, J. 1998. The twelfth Tansley Lecture. Small is beautiful: the picophytoplankton. Functional Ecology 12: 503–513. doi:10.1046/j.1365-2435.1998.00233.x
  
  L235: "The lack of detectable responses is likely be attributed..." — grammatical error; correct to "is likely attributable to".
  Reply: The typo has been corrected to "is likely attributable to".
  L275-276: The lack of detectable responses is likely attributable to the limited physiological sensitivity of the resident plankton community to the carbonate chemistry perturbation imposed in this study.
  L255-260: It is suggested to discuss seasonal and interannual variability in the NCS and how this might affect generalizability.
  Reply: We thank the reviewer for this suggestion. We agree that seasonal and interannual variability are important considerations. In the revised manuscript, we have explicitly acknowledged that our experiment was conducted during a single experimental period and have discussed the generalizability of our findings.
  L371-376: Furthermore, this study was performed during early summer period and therefore captures only a limited window of ecosystem response. Seasonal hydrographic variability, water-column mixing, grazing dynamics, and episodic nutrient inputs may all influence ecosystem responses under natural conditions. Consequently, the magnitude and direction of the observed responses may differ across seasons and years. Future studies spanning multiple seasons and years would help constrain the range of variability in OAE responses in this high-variability coastal system.
  L262: Section 4.2 concludes that "there was little evidence that moderate alkalinity enhancement influenced the measured pools of particulate organic carbon and nitrogen." However, the claim that "OAE can alter inorganic carbon cycling without necessarily producing short-term changes in plankton-derived particulate organic matter" is strong. Please rephrase.
  Reply: We appreciate the reviewer’s valuable suggestion and have revised the wording accordingly.
  L323-326: This apparent decoupling between the carbonate chemistry response and measured particulate organic matter suggests that OAE may sequester CO₂ without necessarily producing short-term changes in plankton-derived particulate organic matter under the experimental conditions tested.
  L300: "Consequentially" — use "Consequently."
  Reply: Done.
  L302: The limitations section does not discuss the closed-system artifact (no gas exchange, no dilution, no vertical mixing) or the bottle effect.
  Reply: we thank the reviewer for the suggestion and have included the closed-system artifact discussion accordingly.
  L379-383: Experimental enclosures may influence microbial community dynamics and succession, and limit extrapolation to natural environments (Bach and Taucher, 2019; Carpenter, 1996). Nevertheless, enclosure-related effects, commonly referred to as “bottle effects,” also inherently affect the control and treatment groups. Thus, comparisons between these groups remain informative, although potential differences in their responses to enclosure conditions should be acknowledged.
  L328: "moderate OAE can be implemented without detectable short-term effects" — this is too strong. Replace with "did not detect short-term effects under the experimental conditions tested."
  Reply: Done.
  L400-403: Overall, this study provides evidence that moderate OAE did not detect short-term effects on productive coastal plankton communities under the experimental conditions tested while emphasising that deployment efficiency depends not only on biological responses but also on the retention of added alkalinity.
  References: Check for consistency in citation style. Some references are cited as "(Bach, 2024)" while others are "(Bach et al., 2019)."
  Reply: We have corrected the citation accordingly and verified the citations throughout the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2026-5281-AC1
RC2:
'Comment on egusphere-2026-5281', Anonymous Referee #2, 10 Oct 2026
Ocean alkalinity enhancement (OAE) holds great potential as a marine carbon dioxide removal (mCDR) technology; however, its ecological impacts and biogeochemical dynamics must be thoroughly evaluated before broad-scale implementation.
In this study, the authors investigated the responses of plankton community structure and associated biogeochemical parameters to OAE using indoor microcosms with natural coastal seawater collected from Aoshan Bay, Yellow Sea. This work was conducted within the standardized framework of the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP), which aims to systematically assess environmental impacts across diverse marine regimes.
Overall, the manuscript is well-written, logically structured, and provides valuable regional data showing high physiological and ecological tolerance of phytoplankton and microzooplankton under both CO2-equilibrated and non-equilibrated OAE scenarios. However, there are several key points regarding terminology, figure presentation, and discussion depth that should be addressed before publication.
Major Comments:
The title currently includes "biogeochemical responses". However, the authors primarily measured particulate organic carbon (POC) and particulate organic nitrogen (PON) as biogeochemical metrics, alongside biological indicators (chlorophyll-a, microzooplankton abundance/biomass, cell counts). Therefore, I suggest changing “biogeochemical responses” in the title to “biological responses” or “plankton community responses” to better reflect the core focus of the measurements.

The term “North China Sea” used in the title and abstract is not commonly recognized in standard international oceanographic literature. Please revise this throughout the manuscript to “Yellow Sea” (or “coastal waters of the Yellow Sea, northern China / Aoshan Bay”), which is globally standard and avoids ambiguity.

A notable finding is the significant total alkalinity (TA) loss under the nEq-OAE treatment, indicating reduced alkalinity retention under non-equilibrated, high-pH/high-TA conditions. This observation is critical for evaluating OAE efficiency. The authors should expand the discussion on the underlying mechanisms driving secondary precipitation. Specifically, calculating and discussing the saturation state of calcium carbonate during the initial addition phase, as well as the potential role of particle-induced heterogeneous nucleation, would greatly strengthen this section.

As this study was conducted within the frameworkof OAEPIIP, the authors should contextualize their findings by citing and discussing other recent OAEPIIP studies conducted in different marine ecosystems (e.g., oligotrophic open ocean, Mediterranean, or subpolar waters). Comparing the buffering capacity and ecological sensitivity of this high-variability coastal system (Aoshan Bay) with other regions will highlight the broader scientific value of this study.

In Figure 2, a noticeable increase in Chl-a concentration was observed in the late stage of the experiment under the Eq-OAE treatment (though statistically non-significant). This subtle trend warrants mention and brief discussion in the text

6. The manuscript should explicitly address the limitations of short-term (14-day) closed microcosm setups. The long-term ecological consequences of OAE should be discussed to provide a balanced perspective for future field trials.
Minor and Technical Comments:
In Figures 1, 2, and 5, the figure legends are incomplete; the definitions for the different colored lines/symbols representing the respective treatments (e.g., Control, Eq-OAE, nEq-OAE) are missing or not clearly indicated in the caption.

In Figure 2, estimates or measurements of microzooplankton biomass (e.g., carbon biomass converted via cell volume) should be added to provide a direct comparison with phytoplankton biomass dynamics.
Citation: https://doi.org/10.5194/egusphere-2026-5281-RC2
- AC2:
  'Reply on RC2', Xiaoke Xin, 28 Oct 2026
  Ocean alkalinity enhancement (OAE) holds great potential as a marine carbon dioxide removal (mCDR) technology; however, its ecological impacts and biogeochemical dynamics must be thoroughly evaluated before broad-scale implementation.
  In this study, the authors investigated the responses of plankton community structure and associated biogeochemical parameters to OAE using indoor microcosms with natural coastal seawater collected from Aoshan Bay, Yellow Sea. This work was conducted within the standardized framework of the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP), which aims to systematically assess environmental impacts across diverse marine regimes.
  Overall, the manuscript is well-written, logically structured, and provides valuable regional data showing high physiological and ecological tolerance of phytoplankton and microzooplankton under both CO2-equilibrated and non-equilibrated OAE scenarios. However, there are several key points regarding terminology, figure presentation, and discussion depth that should be addressed before publication.
  We sincerely thank the reviewers for their time, effort, and constructive comments and suggestions, which have helped improve the quality of our manuscript. We have carefully considered and addressed all the concerns raised by the reviewers and revised the manuscript accordingly. Our point-by-point responses are provided below. All revisions are highlighted in blue in our responses and tracked in the revised manuscript for ease of reference.
  
  Major Comments:
  The title currently includes "biogeochemical responses". However, the authors primarily measured particulate organic carbon (POC) and particulate organic nitrogen (PON) as biogeochemical metrics, alongside biological indicators (chlorophyll-a, microzooplankton abundance/biomass, cell counts). Therefore, I suggest changing “biogeochemical responses” in the title to “biological responses” or “plankton community responses” to better reflect the core focus of the measurements.
  
  Reply: We thank the reviewer for the suggestion and have edited the title accordingly.
  Plankton community responses to ocean alkalinity enhancement in the Yellow Sea.
  The term “North China Sea” used in the title and abstract is not commonly recognized in standard international oceanographic literature. Please revise this throughout the manuscript to “Yellow Sea” (or “coastal waters of the Yellow Sea, northern China / Aoshan Bay”), which is globally standard and avoids ambiguity.
  
  Reply: We thank the reviewer for the correction. We have updated the term “North China Sea” to “the Yellow Sea” in the manuscript.
  A notable finding is the significant total alkalinity (TA) loss under the nEq-OAE treatment, indicating reduced alkalinity retention under non-equilibrated, high-pH/high-TA conditions. This observation is critical for evaluating OAE efficiency. The authors should expand the discussion on the underlying mechanisms driving secondary precipitation. Specifically, calculating and discussing the saturation state of calcium carbonate during the initial addition phase, as well as the potential role of particle-induced heterogeneous nucleation, would greatly strengthen this section.
  
  Reply: We thank the reviewer for the suggestion. We have provided the aragonite saturation state in Fig.1 and expand the discussion on the mechanisms driving secondary precipitation.
  L335-341: Although mineral precipitation was not directly quantified, the observed stoichiometric relationship suggests that CaCO₃ formation may have contribute substantially to the observed alkalinity loss when Ω_aragonite exceeds a critical threshold. No net alkalinity loss was observed in Eq-OAE treatment because the carbonate system remained within conditions that prevented mineral precipitation. In contrast, nEq-OAE caused sustained alkalinity loss due to stronger carbonate chemistry perturbations from direct OH^- addition. Unlike Eq-OAE, which adds alkalinity in proportions of and and consistent with atmospheric equilibrium, nEq-OAE may generate critical oversaturation before sufficient air–sea equilibration occurs. This process may be particularly relevant near the sea surface, where atmospheric equilibration is slow (Hartmann et al., 2022; Suitner et al., 2024).
  As this study was conducted within the framework of OAEPIIP, the authors should contextualize their findings by citing and discussing other recent OAEPIIP studies conducted in different marine ecosystems (e.g., oligotrophic open ocean, Mediterranean, or subpolar waters). Comparing the buffering capacity and ecological sensitivity of this high-variability coastal system (Aoshan Bay) with other regions will highlight the broader scientific value of this study.
  
  Reply: We thank the reviewer for this constructive suggestion. We agree that comparing our findings with OAEPIIP studies from other marine ecosystems would help highlight the broader scientific value of this work. We are currently preparing a synthesis and meta-analysis of the OAEPIIP datasets across different marine ecosystems, which we hope will provide insight into broader applicability and implications of our findings.
  In Figure 2, a noticeable increase in Chl-a concentration was observed in the late stage of the experiment under the Eq-OAE treatment (though statistically non-significant). This subtle trend warrants mention and brief discussion in the text
  
  Reply: We thank the reviewer for this careful observation. Upon re-examining the data, we found that this increase was driven by a single replicate rather than a consistent response across all replicates of that treatment. We therefore consider it most likely to reflect random variability rather than a treatment effect, which is consistent with the lack of statistical significance.
  L270-273: The increase in Chla concentration observed late in the experiment under nEq-OAE treatment was driven by a single replicate rather than a consistent response across replicates. This pattern therefore most likely reflects random variability rather than a treatment effect.
  The manuscript should explicitly address the limitations of short-term (14-day) closed microcosm setups. The long-term ecological consequences of OAE should be discussed to provide a balanced perspective for future field trials.
  
  Reply: We thank the reviewer for this suggestion. We agree that the limitations of the short-term (14-day) closed microcosm setup and have expanded our discussion to specify the constraints of the closed, short-term design. We have also added a discussion of potential long-term ecological consequences and their implications for the design of future field trials.
  L369-376: In addition, our microcosm experiment was conducted under controlled laboratory conditions with limited duration (14 days), and therefore cannot fully reproduce the complex, dynamic environmental conditions of the YS. Furthermore, this study was performed during early summer period and therefore captures only a limited window of ecosystem response. Seasonal hydrographic variability, water-column mixing, grazing dynamics, and episodic nutrient inputs may all influence ecosystem responses under natural conditions. Consequently, the magnitude and direction of the observed responses may differ across seasons and years. Future studies spanning multiple seasons and years would help constrain the range of variability in OAE responses in this high-variability coastal system.
  Minor and Technical Comments:
  In Figures 1, 2, and 5, the figure legends are incomplete; the definitions for the different colored lines/symbols representing the respective treatments (e.g., Control, Eq-OAE, nEq-OAE) are missing or not clearly indicated in the caption.
  
  Reply: We thank the reviewer for bring this to our attention and have added the definitions to the caption in Fig. 1-5.
  L175, 195, 213, 233, and 247: Gray triangles, pale mint-green dots, and light orange squares represent the Control, CO₂-equilibrated OAE (Eq), and CO₂-non-equilibrated OAE (nEq) treatments, respectively.
  In Figure 2, estimates or measurements of microzooplankton biomass (e.g., carbon biomass converted via cell volume) should be added to provide a direct comparison with phytoplankton biomass dynamics.
  
  Reply: We agree with the reviewer that comparing phytoplankton and microzooplankton biomass would provide additional insights into their respective responses to Ocean acidification and help distinguish changes in abundance from changes in biomass. However, cell volume was not measured during the experiment, which prevents us from reliably estimating biomass from the available data. Therefore, we are unable to provide a robust biomass comparison in the present study.
  
  Citation: https://doi.org/10.5194/egusphere-2026-5281-AC2

Xiaoke Xin, Yimeng Liu, Ruize Chang, Ruobin Lei, Minghao Zhu, Xiaotong Wang, Xue Chao Wang, and Jihua Liu

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Short summary

We added alkaline substances to seawater to test carbon dioxide (CO₂) removal and its impact on tiny marine life. In our experiment with coastal water from northern China, microplankton remained healthy and diverse. But without balancing added CO₂, much alkalinity was lost, reducing its effectiveness for CO₂ removal. This suggests that while coastal plankton may tolerate these additions, careful management is needed to prevent alkalinity loss in warm waters like the North China Sea.


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