Rebuttal plan report

Climatic Controls on Metabolic Constraints in the Ocean

Precious Mongwe¹, Matthew Long², Takamitsu Ito^3, Curtis Deutsch⁴,  and Yeray Santana-Falcón⁵

¹Southern Ocean Carbon Climate Observatory (SOCCO), CSIR, Cape Town, South Africa

²Oceanography Section, Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, United States of America

³School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia  United States of America

⁴Department of Geosciences, Princeton University, Princeton, NJ, United States of America

⁵CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, 31057, France

# Reviewer1

This paper is quite interesting and logically organized. The motivating questions are made clear in the Introduction and the figures are appropriately used to tell the story. In general, the writing is quite clear outside of the Methods section, the last paragraph of the Discussion, and portions of the Conclusions paragraph. The authors will need to correct what seem to be multiple typos throughout the Methods section before this can be published, so I am recommending minor revisions. I also recommend that the authors consider adding a schematic to visually clarify relationships between key metrics of the paper. This would increase the accessibility of the paper significantly and serve as a valuable reference for the Discussion section, particularly when explaining some of the more complex impacts of oxygen and temperature changes on ectotherm habitability of high and low latitudes.

Response: We thank the reviewer for his/her insightful comments and suggestions. Indeed, the methods section had multiple typos in the equations, we apologies for this. We have addressed these typos as pointed out, and we will consider adding a schematic diagram to tie together and summarize the paper.

Line 27: Typo. Signals → signal

Response: Addressed as suggested

Line 44: It may be valuable to incorporate the concept of higher oxygen demand, independent of oxygen supply or circulation changes.

Response: We would like to address this comment; however, it is unclear what the reviewer is suggesting or pointing out with reference to line 44. There may be a mistake in the line reference.

Lines 109-110: Typo? 𝐵 σ is in the equation but you define 𝐵δ

Response: We thank the reviewer for point this out, this was indeed a typo and will be addressed as suggested.

Equation 1: The B term is missing, and this equation should be labeled equation 2. Even though the B term is ultimately dropped, it should be included for clarity, following Deutsch et al., 2020 equation 1.

Response:  The B term was indeed excluded because it drops out, but will be included for completeness as the reviewer suggest.

Line 139: This text is not clear. Please stick to one concept at a time. For example: “Φ' is derived by dividing Φ by Φcrit , so when Φ falls below 1, the organism can no longer sustain its active metabolic demand and will need to make physiological tradeoffs. Account for these active metabolic requirements, we use an adjusted definition of the hypoxic tolerance trait, A c = A o / Φcrit , where A c is termed the “ecological hypoxia tolerance”, consistent with Howard et al., 2020.”

Response: We thank the review for this suggestion, it will be implemented as suggested.

Line 161: It’s not clear how this relationship yields cold tolerance, please elaborate, or reword for accuracy.

Response: This is illustrated in Fig. 1b, where the nearly parabolic curvature of pO₂ at Φcrit indicates an increase in oxygen demand at both low temperatures and high temperatures. Most of the manuscript focuses on the high-temperature oxygen demand based on metabolic demand. Nevertheless, at very low temperatures, gas transfer is limited by the decrease in molecular gas diffusion, and as a consequence, oxygen transfer into the organisms requires energy, leading to cold intolerance. We will add an elaborated explanation to the text.

Figure 1b. It may help to clarify in the figure caption that below the pO2 lines shown, the organism would experience an oxygen deficit relative to its active metabolism requirements, effectively signifying the species-specific hypoxic conditions, based on physiological traits, for this range of temperatures. Figure 2: Center the global map on the Pacific to make the transect location easier to see.

Response: We thank the reviewer for this suggestion, it will be implemented as suggested

Figure 3: Add prime to Φ color bars.

Response: We plan to a add general comment in methods to clarify that Φ text refers to Φ’ throughout the text according to Howard et al., 2020’s definition

Line 347: Can you validate this hypothesis by looking at interannual variations in model density versus temperature or oxygen?

Response: We thank the reviewer for this comment: we plan to either analyses CESM output as suggested or support this point with available literature.

Figure 8 Caption: Note that the same decades for differencing apply to the top row of plots in addition to the bottom row.

Response: We thank the reviewer for this suggested clarification, it will be implemented suggested.

Line 480: Is this a typo? Aren’t high temperature regions mostly suited for organisms with high-temperature tolerance or reduced temperature sensitivity (Figure 2)?

Response: This is not a typo; this phenomenon is better explained by Figure 1.b. Due to high temperatures in the tropics, habitability requires either high oxygen tolerance or high temperature sensitivity (high E_o). High E_o organisms have particularly strong temperature sensitivity at high temperatures. We plan to clarify this text to reflect this point.

Line 494: Should this say epipelagic and mesopelagic? This entire paragraph stands out as being particularly unclear relative to all other text (outside of the methods).

Response: Indeed, this was a typo, it will corrected and we plan to refine the rest of the paragraph.

Line 498: Sentence starting with “At depth” could be reworded for clarity.

Response: This sentence will be removed as suggested

Line 500: By “distinct” do you mean correlations of opposite sign?

Response: Distinct will be replaced by “differences” which will clarify the meaning of the sentence.

Line 509: It’s not clear what is meant by “concomitant pO2 -temperature correlations in the forced trends”. I assume this means trends of the same sign, but it would be ideal if this were clearly stated

Response: We thank the reviewer for this suggestion, it will be implemented as suggested.

Line 521: Suggest changing to: “We find that forced perturbations to pO2 and temperature will strongly exceed those associated with the natural system…”

Response: We thank the reviewer for this suggestion, it will be implemented as suggested.

Rebuttal plan report

Climatic Controls on Metabolic Constraints in the Ocean

Precious Mongwe¹, Matthew Long², Takamitsu Ito^3, Curtis Deutsch⁴,  and Yeray Santana-Falcón⁵

¹Southern Ocean Carbon Climate Observatory (SOCCO), CSIR, Cape Town, South Africa

²Oceanography Section, Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, United States of America

³School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia  United States of America

⁴Department of Geosciences, Princeton University, Princeton, NJ, United States of America

⁵CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, 31057, France

# Reviewer 2

The paper examines the effects of warming and deoxygenation on marine ecosystems by analyzing the temperature sensitivity and oxygen requirements of metabolic rates. Utilizing CESM-LE, the research explores the natural variability and anthropogenic impacts on the support for aerobic metabolisms in marine ecosystems over various timescales. The study emphasizes that future climatic changes will intensify the challenges faced by marine organisms, driving them toward their physiological thresholds and heightening the vulnerability of marine ecosystems to extreme events.

The manuscript is well-written, and the line of thought is clear. I believe this paper is of interest to the general audience of Biogeosciences. I only have very minor technical and clarification questions.

Response: We thank the reviewer for his/her well considered comments and suggestions.

L112: This equation should be labeled as Eq. 1

Response: The Arrhenius metabolic demand equation will be relabelled as equation 1

L113 – 127: The definitions of E_O, E_D, and E_S, are not clear in this section. I suggest bringing the Eq. in L123 earlier.

Repones: We thank the review for this suggestion, it will be implemented as suggest; Line 123 will be introduced earlier.

L241: Could you comment on the negative bias in pO₂ in CESM-LE at 200 and 500 meters? This bias is mainly due to limitations in biogeochemistry or physical circulation. How does this bias project to future scenarios?

Response: This is a well-documented CESM bias. We will provide a description of the sources of this bias and its implications on the text: overexaggeration of habitat loss

L269: OMZ = Oxygen Maximum Zone? This has not been defined in the paper.

Response: Thanks for point this out, corrected

L278: How do you calculate the natural variability? 1σ uncertainty of the period 1920 to 1965?

Repones: Yes, natural variability is calculated as 1σ uncertainty of the period 1920 to 1965, we will state this more explicitly in the text.

L309: Curious if you compared temperature and pO₂ trend between CESM-LE and observation. I am wondering if the CESM-LE shows reasonable trend. Any trend bias in CESM-LE here could project bias in future scenarios.

Response: We did not compare CESM-LE and observations in this study. This comparison, however, has been done in an unpublished multi-ESM study, including CESM. It showed that ESMs generally underestimate oxygen loss as the ocean warms. We will add this material in the supplementary section and include a comment in the main text.

L367: Texts on the left of the bottom row should indicate a trend (difference between 2020–2099 and 1920–1965)

Response. No, these plots show a pO₂-temperature regression at 50 m, 200 m and 500 m, the top row is the natural climate (1920 – 1965) and  bottom row, the forced trend (2020 – 2099). We will refine the text to clarify this.

CESM-LE seems to suggest deoxygenation has started only since ~2000. Observation data, however, support an earlier onset of ocean deoxygenation. Could you comment on this?

Response: Thanks for pointing this out, this reflect CESM’s underestimation of deoxygenation with warming which also came in the above comment. We will add a description that explains this and its effected on the projected impacts.

Rebuttal plan report

Climatic Controls on Metabolic Constraints in the Ocean

Precious Mongwe¹, Matthew Long², Takamitsu Ito^3, Curtis Deutsch⁴,  and Yeray Santana-Falcón⁵

¹Southern Ocean Carbon Climate Observatory (SOCCO), CSIR, Cape Town, South Africa

²Oceanography Section, Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, United States of America

³School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia  United States of America

⁴Department of Geosciences, Princeton University, Princeton, NJ, United States of America

⁵CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, 31057, France

# Reviewer 3

In this manuscript, the authors use a synthesis of empirical data and ESM large ensemble to assess the influence of both oxygen and temperature in determining habitat suitability for a series of ecotypes in the surface and subsurface ocean, and they study how these factors, and their interaction, change distribution of these ecotypes under climate change. The study is compelling, well thought out, and very well written. It was truly an enjoyable read.

The only pointed criticism I have is that it is missing some context on the empirical data used. Although the dataset is referenced in the manuscript, some added text on how it was synthesized and broad description of types of species included, their ecological role, and how the values used were obtained would be helpful. Rough data distribution and possible geographical biases could also be mentioned.

Other than that, any comments I have are very minor (some cosmetic) and I would recommend this manuscript for publication with minor revisions. Specific comments are mentioned below.

The only pointed criticism I have is that it is missing some context on the empirical data used. Although the dataset is referenced in the manuscript, some added text on how it was synthesized and broad description of types of species included, their ecological role, and how the values used were obtained would be helpful. Rough data distribution and possible geographical biases could also be mentioned.

Response: We thank the reviewer for his/her well considered comments and suggestions. We plan to provide a more detailed of the description of the datasets from published literature in the supplementary.

L109: check the exponent on B

Reponses:  This was indeed a typo and it is corrected.

L147: add more information about the dataset used

Response: We plan to use published material to provide a more detailed description of the datasets we used; source, type and distribution in the supplementary material.

Figure 1: the blue star is really hard to see, consider using a different color

Response: Thanks for pointing this out, we will make the appropriate colour correction

Figure 5: Perhaps add a label with the variables to the left to make interpretation easier?

Response: A variable description will be added on the left of figures

L376: Figure 8?

Response: This was as indeed a typo, corrected

Figure 8: could add title with the depth on panels a-c

Response. Thanks for the suggestion, we will add a title with the depth on panels a-c

L479: remove “in the surface ocean”

Response: Removed.