Impact of Indian Ocean Dipole Events on Phytoplankton Size Classes Distribution in the Arabian Sea
Round 1
Reviewer 1 Report
This paper investigated the relationship between phytoplankton size classes and IOD in the Arabian Sea. They found that the fraction of micro-phytoplankton increases during negative IOD and the fraction of pico-phytoplankton increases during positive IOD. This study offers important results for the ocean biogeochemistry and physical-biological interaction community. However, further discussions are expected to improve this manuscript by considering some major points as follows:
(1) Detailed information and important references should be added in part 2 (Data and Methods). Which method was used for the quality control of pigment data? How to determine the phytoplankton size classes from field observations? And the datasets information should include the sampling time.
(2) It is well known that the Arabian Sea is featured with a semiannual reversal of the monsoon. The PSC is significantly difference between two monsoons and inter-monsoon periods in the Arabian Sea. I recommend that the authors take seasonal variation into account when reparametrizing the three component model.
(3) Line 179-180: The author needs to specify which set of SSH and MLD products to use in this study.
(4) Figure 4: Why the number of match points between in-situ and reconstructed is less than the number of sampled station (52 vs 156)? (Line 124 and Line 226)
(5) Figure 5: What is the meaning of Figure 5？(Line 229) These data points were calculated from the three-component model, right? I recommended the comparison between satellite derived Fm/n/p with that in situ data to be shown in this figure.
(6) Generally, the significant-test (p-value) should be provided when we calculated correlation between two variables. This is very important step and the authors should add in Figure 7&8. (Line 257-258)
(7) In the whole study, many regressions analysis between DMI and monthly anomalies of PSC, as well as their physical variables like SST, MLD and SSH were used for studying the effect of IOD. However, it’s hard for us to get new results compared with previous studies. Besides, we don’t know whether these correlations are obvious enough. As we know, phytoplankton biomass dynamics in the Arabian Sea are driven by the seasonal reversal of the monsoon wind. How about the seasonal variability of PSC in this area in extreme IOD years compared with normal conditions? Spatial differences could also be discussed in detail. For example, in Figure 9 and Figure 10, we can see obvious features in some typical areas like northeast and southeast part of Arabian Sea. I strongly recommend authors to do further discussion.
(8) The colorbar in Figure 1 is not clear. Also in Figure 8, it’s not easy for readers to get the details. I think these could be modified.
(9) Some new references could be added.
“Shi and Wang, Phytoplankton biomass dynamics in the Arabian Sea from VIIRS observations, Journal of Marine System,2022.227,103670."
Author Response
Please see the attachment
Author Response File: 
Author Response.docx
Reviewer 2 Report
The study by Shunmugapandi et al. on the impacts of the Indian Ocean Dipole on phytoplankton size classes in the Arabian Sea is timely and may prove to be of broad interest. The study has used appropriate methods and datasets and the results are in keeping with previous similar studies. That said I have noted a number of issues of clarification/editing that require attention but the underlying science would appear to be robust.
General comments:
There are multiple grammatical issues and unusual or ambiguous phrases used throughout the ms that require attention to improve clarity and accuracy.
Methods (Section2.1): more information is required on the nature of the in-situ datasets that were collated for use in this study. One presumes that the data consist of size-fractionated chlorophyll measurements but this is not clear. More detail is needed.
Whilst piracy has been a recent problem there are also likely to be older JGOFS era data and other datasets that would be of interest to the authors but I accept that such data largely predate the satellite timeseries used here so ultimately are of limited use. Nevertheless there are older data that can be referred to to address questions such as the typical size class fractions in the Arabian Sea region or how the size classes may vary across the seasons.
Description of the QA process is absent. It is not enough to say that the small dataset went through a QA process. More details are required on how outliers were identified and flagged.
What constitutes a surface sample? This is relevant information as the satellite data represents the first optical depth thus the authors may be excluding sub-surface data that sits within the optical depth of the satellite data if in this case a surface sample is genuinely a surface sample (i.e 0 m).
L124 indicates datasets (i.e. multiple observations). I think the authors may mean data points. Do 156 datapoints as used here represent a sufficiently large enough dataset to have confidence in the results? Some indication of the impact of larger/smaller datasets on the results is relevant particularly when comparing between studies. That the cross-validation dataset consists of only 16 samples seems very small and thus it is important to describe the significance of the sample size.
Figure 2: Can confidence intervals or errors be added to the graphs?
Figure 4 (and section 3.2): In order to gauge the success of the gap-filling exercise and the validity of the reconstructed chl-a dataset Figure 4 should also include a new subpanel with the original satellite-insitu data regression. At present, it is not possible to assess the (L218) “better association” without seeing the initial comparison.
Figure 5: It would instructive to consider including a figure showing the relationship between measured size-fractions and model-derived size-fractions so that an assessment of the accuracy of the model can be better understood.
Table 1: Whilst the comparison between previous studies is important it remains unclear why the various studies differ. Is this due to the differences in the in-situ datasets used in each study or something else? Ultimately, I am unclear whether the work reported here is an improvement on Sahay et al or Brewin et al, or simply a variation on a theme. Some effort is needed in the discussion to better clarify the improvement reported here.
Section 3.2: Please clarify the basis for a random selection of 500 datapoints. This is not well described.
Figure 7: It would be useful to also see the correlation to satellite chl-a here not just the size-fractions. Worryingly, this figure appears to show limited subregional variability for any size fraction. Can the authors comment further on this? Indeed, the statement that (L268: “…the correlation between Fn and the physical variables was vague…” doesn’t seem right given known variabilities for this region.
Figure 7 and 8: The correlations presented here seem to remove considerable sub-regional variability compared to the monthly anomalies shown in Figures 9 and 10 for extreme years. Is this realistic?
L346-357: It is unusual for the nanoplankton to remain so stable against changing environmental conditions. This aspect of the study requires better description and explanation. Is it an artefact of the way nanoplankton chl-a is derived or a genuine reflection of community resilience?
Discussion: A lot of the text of the discussion is repeating statements from the results section and does not adequately resolve the progress made. I would like to have seen greater cross-referencing to the literature, perhaps to explore potential explanations for unusual bloom events, and deeper comparison to similar studies (e.g. Brewin et al, Sahay et al). There definitely should be greater discussion of the significance of the results and why only pico and micro size fractions seem to be responsive to the IOD. From an ecological perspective it seems strange that nanoplankton are less responsive. This should be explored further as ultimately it raises concerns that results from the 3-component model are to simplistic to engage with broader questions on climate impacts on regional productivity, fisheries, and food security. Can anything be gleaned from the in-situ datasets to support the model results?
Minor comments:
P1 L7: opening sentence of abstract is incomplete or unclear
L14: Suggested edit - “applied it to”
L16/17: Unclear sentence , please revise.
L30: Phytoplankton is a plural (i.e. phytoplankton are…)
L34: I would argue that oversimplifying phytoplankton as a single group must definitely does lead to a lack of detailed biological and ecological information. Suggested edit – “…as a single group does lead to a lack of…”
L37: Grammatical errors. Suggested edit – “…community is composed of a specific pigment composition that comprises…”
L52-53: Grammatical issues
L52: Upper surface ocean
L56: “With a long time-series”
L64-65: Sentence unclear, please revise
L83-85 & L91-92: Repetition.
L86-87: Sentence incomplete. Please revise.
L98: Missing word “…role of the iod…”
L100: Replace sensitiveness with sensitivity
L103: “…provide a synoptic scale long term time-series…”
L131-132: Repeated from earlier text
L135: The PSC used here and in most other studies originates from Sieburth et al 1978.
L172: Grammatical error. Suggested edit – 10% of original samples were retained for cross-validation purposes.
L188: Please clarify if monthly PSC anomalies were also obtained from a 17-year climatology timeseries. As written this sentence is unclear.
L198-199: Please rephrase.
L220-223: Some repetition here that can be removed
Figure 6: Please highlight on the figure the years identified as extremes and what criteria are used to denote an extreme year.
L276-287: Some grammatical errors here that require rephrasing.
L281: Not clear what is meant by “While seeing the Fn…”
L296-298 & 321-323: Repeated text. Please simplify and delete repetition.
L309: The comparison to the Brewin et al study would greatly benefit from deeper comparison and explanation for the differences. Currently it seems to indicate that this study has made only marginal improvement over a pre-existing study.
L314: Replace database with dataset
L323-325: Could delete – already made this point
L328-329: Repetition from earlier
L337: Missing word?
L340-345: Unclear. Please edit and rephrase.
L346-357: Can be condensed to remove repetition
L361: Rephase “the more excellent…”
L362: grammar. Please rephrase (“through which challenges”)
L364: Please rephrase “fairly obtained”
L367: Suggested edit – replace highly with strongly
Author Response
Please see the attachment
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
I can see some improvement of the study. However, I still think that deep explanation and discussion could be added in the revised manuscript. Interpreting these correlations coefficients to a reasonable explanation considering the biological or physical forcing effects will be much helpful . Some points are as follows:
1. The first sentence in the abstract in the revised manuscript maybe not necessary.
2. Line 287， “SST and SST” should be “SST and SSH”
3. It’s not precise enough to describe the positive or negative correlations in Figure 8 as it is now, because there are still large spatial differences. More specific description should be added instead of a rough one.
4. What’s the maximum anomaly of F ratio in Figure 9 and Figure 10? Less than 0.1? There are still large spatial departures of these parameters. It’s clear that in pIOD and nIOD years, the mesoscale eddies might have some effect on the variability of phytoplankton size classes. However, these points were not mentioned in this manuscript.
Author Response
I can see some improvement of the study. However, I still think that deep explanation and discussion could be added in the revised manuscript. Interpreting these correlations coefficients to a reasonable explanation considering the biological or physical forcing effects will be much helpful . Some points are as follows:
We would like to thank the reviewer for the careful review of the manuscript and for providing valuable suggestions to improve the quality of the manuscript. Keeping these suggestions in mind, we have prepared the following responses addressing the comments made by the reviewer in a point-by-point manner. The suggestions have been implemented in the revised manuscript.
- The first sentence in the abstract in the revised manuscript maybe not necessary.
Response: As per the suggestion the first sentence in the manuscript has been removed in the revised manuscript.
- Line 287，“SST and SST” should be “SST and SSH”
Response: The typo error has been corrected in the revised manuscript.
- It’s not precise enough to describe the positive or negative correlations in Figure 8 as it is now, because there are still large spatial differences. More specific description should be added instead of a rough one.
Response: As per the suggestion in the comment, the description of Figure 8 has been modified in the revised manuscript.
“To identify the influence of physical drivers on the PSC distribution, we further investigated the connection between the PSC and the physical variables like SST, MLD and SSH in the Arabian Sea region, the mean time-series anomalies of each PSC (Fm, Fn and Fp) corresponding to the SST, MLD and SSH was calculated. The biological and physical factors appeared tightly correlated in the Arabian Sea. Figure 8 indicates that the inter-annual anomalies of Fp are largely positively correlated with the SST and SSH; it shows an inverse relationship with MLD which coincides with the environmental adaptation of picoplankton. In contrast, the Fm shows a negative correlation with the SST and SST, and it has a positive relationship with MLD which also coincides with the favorable condition of microplankton. No correlation was observed between Fn and physical variables (SST, SSH and MLD) as nanoplankton are ubiquitous in na-ture. In Figure 8, throughout the Arabian Sea, the microplankton, picoplankton and physical drivers appear coupled.”
- What’s the maximum anomaly of F ratio in Figure 9 and Figure 10? Less than 0.1? There are still large spatial departures of these parameters. It’s clear that in pIOD and nIOD years, the mesoscale eddies might have some effect on the variability of phytoplankton size classes. However, these points were not mentioned in this manuscript.
Response: The maximum anomaly of F ratio in Figure 9 and 10 is 0.10. Yes, the mesoscale might have some effect on PSC distribution, to observe that we might need depth-wise PSC observation, but we used surface chl-a data for this study. In the revised manuscript we have discussed the effect of the Indian monsoon on PSC distribution and unprecedented changes in PSC due to extreme IOD events.
Reviewer 2 Report
The authors have responded favourably to my initial comments. There remain many grammatical errors (too many to list here) and a few areas where clarification would be welcomed but overall the study is improved.
Section 2.1: The addition of Table 1 is welcomed but could be improved by merging with the text on L125-134.
Quality control: The section on QA processes (starting on L134) should be presented as a new paragraph and expanded. I am not confident that the QA processes could be replicated by future studies given the scant information presented here.
L139: Employ not imply ?
L140: Data points not datasets ?
L153-154: repeated from L40-42
L191: New sentence does not seem to fit into text and is either repeated from L141 or does not quite make sense. please check and clarify
L208: There is no explanation as to why October 2019 and July 2016 have been selected. I suppose this is due to insitu data constraints but this could be clearly stated.
L241: Sentence seems to stop suddenly and perhaps should be joined to following sentence (i.e. remove full stop)
L287: SST is listed twice ?
L299: “observed efficiently” does not make sense to me. Can this be rephrased
Discussion: There is still a lot of repetition from the Methods and Results sections here and not enough discussion of the implications of the study for my liking. For e.g. L339 emphasise the use of DINEOF (essentially repeating elements of the methodology section), and L343-349 repeats basic results. The new text (L350-387) is better but often circular in its arguments e.g. L 353-357. The basic conclusions are sensible in that picoplankton proliferate during pIOD and microplankton during nIOD, but beyond that the discussion fails to articulate the significance of these findings whilst the conclusion section is broadly repetitive of the discussion section and should be shortened or made more succinct. There is no attempt to quantify the relative improvement made by the authors over previous studies not any assessment of the significance of having used a larger dataset. As a result and as stated previously, it remains unclear whether the work reported here is an improvement over previous studies or simply a variation on a theme. Effort is needed to better clarify the improvements made.
Author Response
The authors have responded favourably to my initial comments. There remain many grammatical errors (too many to list here) and a few areas where clarification would be welcomed but overall the study is improved.
We would like to thank the reviewer for the careful review of the manuscript and for providing valuable suggestions to improve the quality of the manuscript. Keeping these suggestions in mind, we have prepared the following responses addressing the comments made by the reviewer in a point-by-point manner. The suggestions have been implemented in the revised manuscript.
Section 2.1: The addition of Table 1 is welcomed but could be improved by merging with the text on L125-134.
Quality control: The section on QA processes (starting on L134) should be presented as a new paragraph and expanded. I am not confident that the QA processes could be replicated by future studies given the scant information presented here.
L139: Employ not imply ?
Response: The word “imply” has been changed to “implement” in the revised manuscript.
L140: Data points not datasets ?
Response: The word “datasets” has been changed to “data points” in the revised manuscript.
L153-154: repeated from L40-42
Response: In response to the comment, the repeated sentence in the L53-154 has been removed in the revised manuscript.
L191: New sentence does not seem to fit into text and is either repeated from L141 or does not quite make sense. please check and clarify
Response: L141 explains the data used for the validation of the reparametrized PSC model and L191 describes the process of DINEOF method which retains 10 % of satellite chl-a values before reconstruction for validation purposes. The L191 has been rephrased in the revised manuscript.
“Followed by 10 % of original chl-a satellite values were retained for cross-validation purposes.”
L208: There is no explanation as to why October 2019 and July 2016 have been selected. I suppose this is due to insitu data constraints but this could be clearly stated.
L241: Sentence seems to stop suddenly and perhaps should be joined to following sentence (i.e. remove full stop)
Response: As per the suggestion in the comment the following correction has been done in the revisd manuscript.
“Using the randomly selected (500 points) gap-filled satellite estimates of chl-a and fractional contribution of PSC (Fm, Fn and Fp) obtained from the reparametrized three-component model the relationship between the chl-a and Fm, Fn and Fp was observed in order to check whether the satellite estimates achieve same relationship as in-situ PSC and chl-a shown in Figure 3.”
L287: SST is listed twice ?
Response: The typo error has been corrected in the revised manuscript.
L299: “observed efficiently” does not make sense to me. Can this be rephrased
Response: In response to the comment, the word “observed efficiently” has been changed to “observed” in the revised manuscript.
Discussion: There is still a lot of repetition from the Methods and Results sections here and not enough discussion of the implications of the study for my liking. For e.g. L339 emphasise the use of DINEOF (essentially repeating elements of the methodology section), and L343-349 repeats basic results. The new text (L350-387) is better but often circular in its arguments e.g. L 353-357. The basic conclusions are sensible in that picoplankton proliferate during pIOD and microplankton during nIOD, but beyond that the discussion fails to articulate the significance of these findings whilst the conclusion section is broadly repetitive of the discussion section and should be shortened or made more succinct. There is no attempt to quantify the relative improvement made by the authors over previous studies not any assessment of the significance of having used a larger dataset. As a result and as stated previously, it remains unclear whether the work reported here is an improvement over previous studies or simply a variation on a theme. Effort is needed to better clarify the improvements made.
Response: As per the suggestion, the major part of the discussion has been modified in the revised manuscript.
Round 3
Reviewer 1 Report
I agree with the acceptation of this manuscript.
Reviewer 2 Report
The authors have addressed my comments. No further suggetions.
