Review of “Biogeochemistry of climate driven shifts in Southern Ocean primary producers” by Fisher et al.

This study focuses on climate-driven shifts in Southern Ocean phytoplankton composition and primary production. It provides an analysis of Southern Ocean phytoplankton communities and biogeochemistry simulated in CMIP6 models, making it a contribution to the growing literature on biogeochemistry and future trends in the Southern Ocean.

First of all, I appreciate the massive work done by authors. Although I found the analysis valuable, I have four major concerns about the manuscript that suggest it could benefit from some restructuring. Here are the main areas I believe need attention.

1. Connection Between Text and Figures/Tables: I sometimes had difficulty following the connections between the text and the figures/tables. Therefore, I raised some questions and made some suggestions in the comments below.
2. Results and Discussions: While the discussions on mixed layer depth, light availability, and iron limitation are there, the manuscript would benefit from incorporating additional points. Specifically, more discussion is needed on the roles of grazers and changes in the Antarctic ice sheet and their effects on micro and macronutrients. It is well-known that changes in the Antarctic ice sheet will impact nutrients such as iron and silicate (e.g., Wadham et al., 2019; Death et al., 2014; Tréguer, 2014). However, this point is not mentioned in the paper. Is there a specific reason for that related to ESMs? Additionally, while light and iron limitations are crucial in the Southern Ocean, the impact of grazers on phytoplankton composition is also significant (e.g., Smatecek, 2004). Grazers control phytoplankton bloom phenology in general (e.g., Banse 1994; Banse, 2013), and extensive analyses on zooplankton in CMIP6 simulations exist (e.g., Rohr et al., 2023; Petrik et al., 2022). Including more discussion on this aspect would benefit the manuscript. The summary of climate change pressures and their biogeochemical consequences for ecosystem productivity in Figure 1 is good. However, parts that do not have numbers need better integration into the manuscript's results and discussion sections. Revising this for a more concise format would strengthen the manuscript.
3. Definition of Southern Ocean: The manuscript contains several definitions of the Southern Ocean. In the introduction, readers find that the Southern Ocean is south of the 30S (Gregg et al 2003); in Figure 1, it is south of 65S; in the spatial maps, all are south of 40S. And there are also SOOS regions. Why is this the case? Most messages are for the Antarctic Zone in the manuscript; why not only focus there or SOOS regions?
4. Conclusions: There are some very vague statements, and some of them are not even supported by the analysis that are conducted in this manuscript.

Another critical point is the paper's title, "Biogeochemistry of Climate Driven Shifts in Southern Ocean Primary Producers." The main analysis is based on the results of the CMIP6 models, so it could be helpful to state this in the title. The methods section mentions, "Model and observational data for the Southern Ocean were collected and visualized." What kind of observational data did the authors collect? As seen in the manuscript, the only observational dataset used is the climatology from the World Ocean Atlas 2018. There is no detailed comparison between WOA data and the model results. In my opinion, It should be clearly communicated that this paper mainly aims to identify trends in CMIP6 models.

Furthermore, all citations, including DOIs and formatting, should be checked, since some are missing.

Missing citations in the text:  Nissen et al. (2021), Palter et al. (2010), Moore et al. (2018), Primeau et al. (2013) are listed in the references but need to be cited in the text. I did not check all the references, but there could be more examples of listed but not cited references in the manuscript.

Overall, I think the current version of the manuscript requires significant revisions before it can be considered for publication.

Some additional references that are mentioned above:

Note: Authors do not have to cite them. This is just a suggestion for a start; the list can be expanded.

Banse, K. (1994), Grazing and Zooplankton Production as Key Controls of Phytoplankton Production in the Open Ocean, Oceanography, 7(1), 13–20, doi:10.5670/oceanog.1994.10.

Banse, K. (2013), Reflections about chance in my career, and on the top-down regulated world, Annual Review of Marine Science, 5, 1–19, doi:10.1146/annurev-marine-121211-172359

Death, R., Wadham, J. L., Monteiro, F., Le Brocq, A. M., Tranter, M., Ridgwell, A., Dutkiewicz, S., and Raiswell, R.: Antarctic ice sheet fertilises the Southern Ocean, Biogeosciences, 11, 2635–2643, https://doi.org/10.5194/bg-11-2635-2014, 2014.

Petrik, C. M., J. Y. Luo, R. Heneghan, J. D. Everett, C. S. Harrison, and A. Richardson (2022), Assessment and constraint of mesozooplankton in CMIP6 Earth system models, Earth and Space Science Open Archive, p. 40, doi:10.1002/essoar.10510705.1.

Rohr, T., Richardson, A.J., Lenton, A. et al. Zooplankton grazing is the largest source of uncertainty for marine carbon cycling in CMIP6 models. Commun Earth Environ 4, 212 (2023). https://doi.org/10.1038/s43247-023-00871-w

Smetacek V, Assmy P, Henjes J. The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles. Antarctic Science. 2004;16(4):541-558. doi:10.1017/S0954102004002317

Tréguer, P. J. (2014). The southern ocean silica cycle. Comptes Rendus Geoscience, 346(11-12), 279-286.

Wadham, J.L., Hawkings, J.R., Tarasov, L. et al. Ice sheets matter for the global carbon cycle. Nat Commun 10, 3567 (2019). https://doi.org/10.1038/s41467-019-11394-4

Specific comments

Abstract:

Line 26:

Si* can be known by biogeochemists, but it would be good to specify it here.

Misc is an abbreviation; writing it in a full-form would be better.

Introduction:

Line 59: It is difficult to get this information from Figure S1. Can you calculate and write the regions nearby? Adding a plot of zonal averages could be a good idea.  Why is this SSP2-4.5 scenario, while all the other analyses are SSP5-8.5?

Line 60: I am also curious how much of this change comes from the Coast of Chile and West Africa

Line 79: To be more precise on the point about zooplankton, for example, authors already state the role of zooplankton in the Southern Ocean in the paragraph starting  “In the sea-ice zone, grazing by zooplankton ….” . This indicates the role of zooplankton in the region, but there is not much discussion about that later.

Line 100: Henson et al. 2022 is a good reference for export fluxes, but phytoplankton losses through zooplankton grazing need other references.

Line 115: good reference but can be extended.

Line 120: ‘Here we aim to … “It is difficult to follow whether authors reach these goals at the end of the manuscript. For me, a very clear example is the statement, “Here, we argue that targeting observations to establish phytoplankton-environment response interactions within the regions of the most rapid projected changes is essential to accelerate the improvement of phytoplankton representation in future generations of ESMs.” I do not see this in the rest of the manuscript. If there is, can you make it more clear?

Methods:

Table 1: This table needs to be clarified. Did you use all the 'models selected' for each parameter? If yes, it means that, for example, you use six models for intppdiat but five models for limirrdiat. This example can be expanded. How does it affect your results? I see that in Figure 5, only six models that have intppdiat were used. Is this the case for other analyses? If yes, can you state it in your table?

 2.2 Regional data

I cannot easily connect the analysis of SOOS with the rest of the manuscript. Why is the manuscript suddenly analyzing only Si*, pH, and temperature but not nutrient limitation, iron, NO3, phytoplankton classes, etc. for these subregions? Also, the main focus was always NPP and trying to link it with nutrient limitation, light, and MLD, but suddenly, there is a chlorophyll figure (Figure 3). What is the purpose? If it is about Chl:C ratios, the figure for the carbon fields of phytoplankton can  be useful.

Results and Discussion:

3.1 Physical climate drives biological changes in Southern Ocean water masses

Line 177: “increased buoyancy fluxes (including freshwater inputs) act as opposing drivers of stratification,” can you show buoyancy fluxes as supplementary figures? It is visible that other factors determine the MLD when Figure 2A and Figure 2B are compared. However, it would be better to see them.

Line 196: “CMIP6 models project the greatest increase in productivity to occur across the coastal zone of the Southern Ocean (65-90°S) (Figure 2c), where irradiance limitation is reduced (Figure 2d).” Is this really the case? When I look at Figure 2C, the most significant increase (‘dark red areas’ ) is more in the transitional zones. Ideally, it would be nice to see some numbers to get an idea of the magnitude of the increase.

Line 197: “Conversely, across the Transitional zone (40-50°S), ….. ” It would be good to discuss why IPAR is changing. For example, IPAR is increasing only close to Antarctica, but the limitation reduction occurs in the larger area; why is this the case? Is it related to parametrizations of light limitation in the models?  Is it about clouds or water column structure?

3.2 Changing biogeochemistry of the Southern Ocean

3.2.1 Micronutrient supply and uptake

Line 227: Iron limitation is expected to increase most in the transitional zone ….” I understand that the change in the wind can affect the atmospheric deposition, and if it is shown that the wind changes, it will be affected in the future. Do these models have atmospheric iron deposition, and can you talk about it?  Are there differences among the models?

Line 238: “At the group level, iron limitation…” needs a reference

3.2.2 Macronutrient supply and uptake

Line 256: "Although projections indicate an increase in chlorophyll across such regions (Figure 3), models do not show any increases in nitrate limitation." is it really 'any change'? Figure S3 shows some change until +-20%, which seems higher than the iron limitation.

Line 271: “Changes in Si* correlate with increases in chlorophyll concentration across the same regions (Figure 3), indicative of increased phytoplankton concentrations resulting in a drawdown of silicic acid.” To see this, can you plot the Si* as a subplot and provide the reader with some numbers (e.g. correlation coefficient)? Ideally, comparing models and WOA in maps is a good idea. This could also support the analysis in Table 2.

Figure3: Just a curiosity, Why did you choose to 500 m to integrate chlorophyll?

3.3 Primary production and representation in CMIP6

3.3.1 Phytoplankton classes

To follow the numbers in Figure 4 is difficult. 55%, 74 % etc. Is the second y-label missing in figures A,B,C,D,E,F,? I cannot see these numbers anywhere.

In general, Figure 4 should be better described in the related section.

3.3.2 Ecological dynamics and ecophysiology

This subsection does not really say about ecological dynamics or ecophysiology. If this is the aim, It needs to be extended. If not, it can be combined with another subsection. In this subsection, it is only referred to Figure 4. If the authors want to keep it, I suggest that zooplankton-related discussion be added to this subsection as well.

Line 379:  ‘In a changing ocean,>…. ‘ needs more references

Line 387:  if it is written 'little correlation', please provide numbers.

3.4 Latitudinal productivity projections CMIP6.

Dot ‘.’ In the section title is not need

Line 415: “however ….. “ Why is this the case? Is this related to temperature or nutrient limitation parametrization? I suggest to discuss it. In addition, I guess that the unit in Figure S7's y-axis should be %.

Figure 5: I guess that Figure 5 shows the spatial average of latitudinal bands. I suggest specifying it in the caption. In addition, I suggest showing the spatial maps of change in total, diatom, and non-diatom NPP change in the supporting information (like Figure 2) to support Figure 5.

4.Conclusions

Line 463: AIS was not mentioned before, write full form before using abbreviation

Line 464: “the current iteration …. “ what about Antarctic zone? It contradicts with what is written in Line 276 “Therefore, the large chlorophyll increase and large Si* decline projected in the Weddell Sea is likely driven by an increase in diatoms, whereas the productivity increase with only small changes in both chlorophyll and Si* seen on the west Antarctic Peninsula probably results from an expansion of non-diatom phytoplankton with lower Chl:C.”

Line 465: “However, there is …..” is it really the case all models do not represent grazers. If yes, can you specify and discuss in the discussion part the possible effect of it? You can check Rohr et al. (2023) for this purpose. In addition, where readers can see large uncertainty of up to ±30% ?

Line 477: If “literature contains wide divergence…”, you need to cite them.

4.2 Observational recommendations:

Line 482: Double dot ‘:”  is not needed in the subsection title

Line 483: “We have identified changes in nutrient upwelling …..” Where did you identify the nutrient upwelling strength, eddy strengthening, mutualism and resource competition in the manuscript?

Line 508: “A focus towards defining variability in phytoplankton buoyancy, carbon uptake and release as dissolved organic matter, grazing by zooplankton, interactions with viruses, macronutrient stoichiometry and micronutrient utilisation will enhance our core understanding of Southern Ocean phytoplankton dynamics in a changing climate.” I am having difficulty understanding this statement. There is existing literature (modeling, observational, experimental) that relates to the points made here. Could you clarify the intended message of this sentence?

Review of the manuscript « Biogeochemistry of climate driven shifts in Southern Ocean primary producers».

This is the second time authors submit this paper, and I recognize authors have made a great effort to improve the manuscript from the previous round. I acknowledge authors kept most of the well-written introduction, while improving, and shorten and simplify, the results and discussion sections. My main criticism of the old version was that the original results of the study were diluted in a (otherwise) good review of the state of the art literature, thus obscuring the contributions of the paper. In this version, the analyses are more clearly linked to existing literature, although I still think that most of the information gathered from the study seems mostly a review. Sometimes it is difficult to discern what is an original result from literature.

I also acknowledge the rearrangement of Table 1, though I think it can be presented as supplementary material.

Although the paper is focused on the biogeochemistry, I believe that some discussion on the effect of grazing to primary producers would be beneficial to complement the effects of iron and light limitations.

I found some sentences difficult to follow (the long sentence in lines 367 to 371 is a good example). I would like authors to revisit them for improving readability.

Finally, I think that the last section should be revisited. Some sentences point out to processes that, to my understanding, are not presented in the present study. It seems that recommendations are based to general statements, but not explicitly to the results found in the study.

In summary, I think the paper has benefited from the changes made in the previous iteration, but some additional changes need to be considered before publication.

Specific comments about the text and figures:

Line 45: I think this sentence can be extended up to the global ocean (e.g., Jiang et al., 2019).

Lines 57-58: “...the Southern Ocean phytoplankton…represented xxx% of global marine net primary productivity, which is equivalent to…”.

Line 169: I think a typo here: “silicic acid plus sea surface temperature”.

Line 170: Si* is introduced in line 263. Please move it to here.

Line 198: What about the increase in PP in lower latitudes; and far from the antarctic coast? In fact, these latitudes are still the Southern Ocean? It is not clear what the limits of the Southern Ocean are considered according to the text.

Lines 201 to 205: Are these changes a result of the study, or are referred to other studies?

Lines 207 to 210: Please rephrase this sentence; it can be separated into 3 sentences. Moreover, where are the Ross and Flicher-Ronne ice shelves locations indicated?

Figure 2: I would include an axis of the latitude to get a sense when detailed in the text.

Lines 236 to 247: This seems more of a review.

Line 271 and others: Sometimes sentences are awkwardly written as this one; “...across the same regions, which is indicative of…”.

Line 292: “there is no evidence for a direct effect of acidification on phytoplankton…”. Please review this long sentence.

Line 300: Though interesting, I don’t know if this paragraph is necessary as it does not present or discuss any direct result from the paper.

Line 359: What authors referred by “based on species”?

Lines 379 to 388: May the role of grazing be discussed also here.

Line 448: A citation is lacking here.

Section 4.1. Are these really “Implications”? Results are summarized and suggestions are provided.

Line 484 to 486: I don’t know whether this sentence applies to the manuscript results. Where changes in nutrient upwellling, mutualism, resource competition, etc, have been identified in the text?

Line 491 to 495: Please reformulate this sentence.