Coastal Phytoplankton Response to Acidification and Warming Under Differing Levels of Nutrient Availability

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Comments for authors in the file

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Influence of nutrient availability on coastal phytoplankton response to acidification and warming

 

The manuscript presents interesting empirical observations. However, Interpretation is frequently overextended beyond the data, and some statistical support is inconsistent. Thus, I recommend major revision of the manuscript to further address my concern in the manuscript.

 

1. In the Abstract, the authors statements such as “for robust prediction of coastal phytoplankton response” are overstated, given that the study is based on only two mesocosm experiments in a single geographic region. The scope does not justify broad predictive claims.
2. The study is framed as addressing combined stressors, but I think this is not novel per se. The authors fail to clearly articulate what new insight is provided beyond existing mesocosm or multi-driver studies.
3. Although three replicates per treatment are used by the authors, all mesocosms are located within a single pond system, raising concerns about the shared environmental conditions as well as lack of true independence among replicates.
4. Daily removal and replacement of 25 L seawater with ambient water may introduce external variability (nutrients, microbes, grazers). Thus, it can compromise the assumption of a closed experimental system.
5. Sampling frequency differs between ND and NR experiments (daily vs. every second day). I think this will potentially affect the comparability between experiments and detection of short-term dynamics.
6. The author’s mesocosm setup does not adequately describe the grazing pressure, viral lysis and bacterial interactions. I think these factors are critical drivers of phytoplankton dynamics and may confound treatment effects.
7. Nutrient addition strategy in the study lacks ecological realism. Daily nutrient spikes may not reflect natural pulsed or continuous inputs, potentially biasing phytoplankton responses.
8. Results interpreted by the authors as meaningful include values such as p = 0.08 and p = 0.13, which are not statistically significant under conventional thresholds. The relaxed p < 0.1 criterion is not consistently justified and risks inflating false positives.
9. The comparison between ND and NR experiment is weak. I believe that the two experiments differ in Initial biomass, nutrient regime, temperature, and seasonal context. Despite this, they are frequently compared directly by the authors, which is methodologically questionable.
10. The conclusion that warming drives biomass increases is not defensible without a warming-only treatment. Grazing effects are dismissed based on unpublished or “personal communication” data, which is not acceptable as primary evidence.
11. Regarding diatom response, strong emphasis on diatom ecology is not matched by robust species-level analysis. The dominance of a single species (C. closterium) may reflect mesocosm artefacts rather than natural dynamics.
12. The authors claims about trophic transfer efficiency and food web impacts are overextended, given the short experimental duration and lack of higher trophic level response. Statements about eutrophication feedbacks are speculative and not empirically supported.

 

Comments on the Quality of English Language

 The English could be improved to more clearly express the research.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Review of the manuscript: "Response of temperate coastal phytoplankton to future ocean acidification and warming is modulated by nutrient availability"

The issues of climate change and ocean acidification are inherently interconnected, and addressing them requires appropriate methodological approaches. Controlled experiments manipulating temperature and pH represent the most straightforward strategy. The use of mesocosms, which largely circumvent the challenges of top-down control, appears to be the most adequate method for addressing the research problem.

This manuscript presents the results of mesocosm experiments investigating the combined effects of reduced pH and elevated temperature on the structure and dynamics of coastal phytoplankton communities in New Zealand. The study was conducted during both spring and autumn under different nutrient regimes. Factor levels were selected based on a four-year time series of in situ pH and temperature data. The research contributes to our understanding of the role of nutrient availability as a modulator of phytoplankton responses to multiple climate stressors. A significant strength of the work is its analysis of not only the taxonomic composition but also the size structure of the community.

The manuscript aligns well with the scope of the journal, the research topic is timely, the methods generally conform to current scientific standards, and the results are of considerable interest to specialists in biological oceanography, phytoplankton ecology, and climate change research. However, there are opportunities to improve the manuscript's clarity and impact. The following comments should be viewed as recommendations intended to enhance the quality and reader interest in the article.

1. Abstract:The abstract would benefit from being more informative. It is advisable to specify the magnitude of the experimental manipulations (i.e., the range of pH reduction and temperature increase). The response of the size structure should be described more precisely. Additionally, provide the full species name upon first mention (e.g., Cylindrotheca closterium).
2. Introduction:While well-written, the introduction would be strengthened by a more explicit statement of the study's objectives or the specific hypotheses tested.
3. Section 2.3.3 (Phytoplankton):The sources or taxonomic guides used for species identification are not cited in this section.
4. General (Taxonomy):Upon the first mention of a species in the text, the full binomial name (genus and species) must be provided.
5. Section 4.4:The formatting of the in-text citations is incorrect. References are given in an author-date format (e.g., Tortell et al, 2008; Sett et al, 2018), but these entries are not found in the Reference list. Please verify and correct these citations.

The necessary revisions are minor and straightforward. Therefore, I recommend the manuscript for publication after a Minor Revision.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

I have reviewed the revised version of your manuscript and wish to confirm that the amendments introduced adequately address all substantive concerns raised in my initial review. The revisions are both appropriate and sufficient to ensure the work meets the publication standards of [Journal Name].

In my professional assessment, the manuscript is now suitable for publication in its current form.

Author Response

Thank you for comments and your time spent on improving the mansucript 

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you very much for the authors reply to my previous comments. I would like to still raise some important concerns in the revised manuscript.

1) The response to the replication and independence issue is not fully convincing, as it relies on “standard practice” rather than directly addressing concerns about pseudo-replication.

2. The reviewer’s concern about a shared environment implying lack of true independence should be explicitly acknowledged, and the authors should provide a statistical justification rather than only a procedural one.

3. The response regarding missing ecological interactions (e.g., grazing, viruses, and bacteria) appears overly defensive; stating that other studies do not measure these factors is not a sufficient scientific justification. Instead, these should be clearly recognized as key limitations without minimizing their importance.

4. The statistical treatment of p-values is also a major concern, as the justification based on Ronald Fisher is outdated and widely debated in modern ecology; values greater than 0.05 should not be presented as significant but rather described as trends or suggestive evidence.

5. The explanation of the open system due to water replacement is only partially adequate, since applying the same procedure across treatments does not remove confounding variability; this should be acknowledged as a source of uncontrolled variation and framed as a trade-off for achieving greater ecological realism.

The authors have adequately addressed several of the reviewer’s concerns; however, minor revisions are still needed to improve clarity and rigor of the manuscript. 

 

Comments on the Quality of English Language

 The English could be improved to more clearly express the research.

Author Response

Please see the attachment

Author Response File: Author Response.pdf