Coral Oasis on Con Dao Islands: A Potential Refuge of Healthy Corals in the Offshore Waters of Vietnam?

Round 1

Reviewer 1 Report

This study investigates the coral cover and community composition over three years at a few sites around Con Dao Islands in Southern Vietnam. The study finds that the coral communities were able to recover following a heat stress event and that there was minimal coral mortality following a bleaching event in 2019. Instances of coral recovery following heat stress are important to understand and this work should be published following revisions.

Generally, there is very little background information for the study and the introduction is mostly specific to Vietnam. There is no mention of El Nino Southern Oscillation (ENSO) and how the 2019 heat stress event is linked to ENSO phases or other events that occurred during this time. Some of the details in the introduction are also not relevant and the second half of the introduction presents as an exhaustive list of previous work undertaken at the CDNP including references to unpublished data.  

The results section was very short and was difficult to follow. Improved clarity of findings is needed. Also suggest including more quantification of results in the discussion, for example, if there was a decline in coral cover – specify what percentage decline occurred.

Only 3 of the 6 sites were surveyed in 2019 and 2020, which is a certain limitation of the study.

Line by line comments:

Line 165: This sentence needs clarification.

175: Remove the word ‘certain’ and perhaps specify that the corals did bleach but seem to have recovered?

Table 1. Please clarify in the table caption where the thermal susceptibility information was sourced from?

Figure 2. This looks like a screenshot straight from the NOAA Coral Reef Watch website. Suggest accessing the raw data (publicly available) and re-plotting for publication. At the very least, it should be referenced properly in the figure caption.

Figure 3: Include/define what the coral bleaching threshold has been based on.

Figure 4: Nice photos. Suggest including a credit to the photographer name e.g. Photo taken by: [name].

Figure 6: These should probably include zero data as well and would be improved if the authors presented the same groups along the x-axis for all plots to make it easier to compare across sites. For example, if there was no sargassum at a site it should show up as a zero on the bar chart? Colour coding for the 9 different groups would help a lot.

Author Response

We are grateful to the Reviewer 1 for valuable comments on our ms. 

The main message of our work is highlighting of 1) the uniqueness of Con Dao archipelago in term of healthy state of coral reefs under regional coral reef decline in surrounding waters offshore Vietnam, and 2) reversal hierarchy of coral bleaching/mortality response on ENSO-related thermal anomalies. Therefore we believe it is reasonable to give a short description what is Con Dao National Park and the history of coral bleaching in this area, especially given the lack of such published data. Background information on El Nino Southern Oscillation (ENSO) has been added in Introduction (lines 70-80).

Word ‘certain’ has been removed.

Table 1. The sources of coral thermal susceptibility are cited.

Figure 2 was properly referenced in the caption

Figure 3. The base for the definition of coral bleaching threshold at Con Dao is presented.

Figure 4. Photographer is mentioned.

Figure 6. We pointed out in Results that Sargassum was sighted only at the Site 6, also we stated in the legend that the figure 6 showed up only dominant taxa at each site. Nevertheless, three categories presented well at each study sites (Stony coral, dead coral and Acropora) can be used for visual comparison across sites.

Author Response File: Author Response.pdf

Reviewer 2 Report

See my comments in the attached pdf.

Comments for author File: Comments.pdf

Author Response

We are very grateful to the Reviewer 2 (who made a lot of comments in pdf version of submitted manuscript) especially for his kind proofreading of the text, which we have taken into account during the revision. Also we corrected text in Results and Figure 2 according to reasonable comment regarding incongruity of considered thermal anomalies and phases of field work in the study area. All correction made in text according to the comments of the three Reviewers are in red. As for the major message of the Reviewer 2, - yes, we agree with him, that our study doesn’t add any new knowledge to the coral reefs but it does highlight a phenomenon of acroporid’ resistance to recurrent thermal stress and reverse thermal susceptibility of corals in the study area that by our opinion are at least worth to be mentioned. Also we absolutely agree that it would have been very useful to determine the type of Zooxanthellae, but unfortunately we didn’t have possibilities for such an analysis. Six times throughout the text from the Abstract to the Discussion of our manuscript the respected Reviewer made comments that our arguments are speculations as they are not supported by data. Of course they are speculations if we don’t have any other measurements or analyses apart from presented in the paper but if other authors in their papers present their assumptions and suggestions in Discussion section without direct supporting data, aren’t they speculate? If we will remove all our ‘speculations’ I believe there will be no matter for discussion and a reason to submit this manuscript for publication.  Anyway, we realize the drawbacks of our work and we will accept and understand if the Editor will follow the recommendation of this Reviewer and will decline our manuscript from further consideration for publication in Diversity.

Author Response File: Author Response.pdf

Reviewer 3 Report

This is a useful contribution providing valuable data on status of benthic cover, and community structure, in Con Dao Archipelago over the previous several years, in respect of prior bleaching episodes from thermal stress. The study was well conceived and conducted. The Tables and Figures are clear and relevant. The manuscript does, however, require moderate revision in respect of English wording and sentence structure. I have not edited the manuscript in these respects. A few specific comments here below:

Title:

I suggest 'Coral Oasis on Con Dao Islands: A potential refuge of healthy corals in offshore waters of Vietnam (rather than 'the refuge' and 'coastal', as the islands are more than 100 km offshore). The authors could consider posing this as a question, rather than statement, given the short time scale of their study, and increasing evidence from elsewhere of the loss of such local refugia as environmental conditions continue to deteriorate via marine heatwaves, coral disease and predator outbreaks, and 'super-typhoons' etc.

In that case, the title would be: 'Coral Oasis on Con Dao Islands: A refuge of healthy corals in offshore waters of Vietnam?'

Abstract

This covers the main findings but needs editing re English sentence structure.

Introduction

Line 60: 'global hotspot' is not correct, when compared with the much more species rich areas of the Coral Triangle (see eg. Veron et al. 2015:

Veron, J.E.N., Stafford-Smith, M, DeVantier, L.M., Turak, E, (2015) Overview of distribution patterns of zooxanthellate Scleractinia. Frontiers in Marine Science 1: 1-19. https://www.frontiersin.org/articles/10.3389/fmars.2014.00081/full .  

Hence, a 'hotspot' (and potential thermal refuge) in Vietnam is more appropriate. The risk of loss of local thermal refugia has recently been assessed by Dixon et al. (2022):

Dixon AM, Forster PM, Heron SF, Stoner AMK, Beger M (2022) Future loss of local-scale thermal refugia in coral reef ecosystems. PLOS Clim 1(2): e0000004. https://doi.org/10.1371/journal.pclm.0000004

It would be worthwhile for the authors to consider their findings in light of the Dixon et al. work.

Line 64: '... severe'

Line 69: SDNP?

Materials and Methods

Line 86: Re typhoon tracks, many pass to the North, moving to the west across the South China Sea, sending large swells from the NE - SE, depending on their respective tracks. See e.g. Wang, Y. (2020). Composite of typhoon induced sea surface temperature and chlorophyll‐a responses in the South China Sea. Journal of Geophysical Research: Oceans, 125, e2020JC016243.
https://doi.org/10.1029/2020JC016243

Line 90: Typhoon strength is typically measured on the Saffir-Simpson scale. The authors could consider including a table and/or figure of recent (eg. from 1990 to present) typhoons with respective tracks relative to Con Dao, showing that Linda passed in very close proximity to the islands.

Line 103: '...being typical'

Line 103: '... dominated by Acroporidae' requires citation / reference or 'see Results'

Results

Line 141: A table showing results of the T test could be included, or as Suppl. Table, and similarly for the ANOVA of changes in cover (Figure 6).

Line 164: '... coral decline ..'

Table 1. the Reference sources for the susceptibility of each genus should be listed next to the relevant text. Also check spellings. Eg.  Lithophyllon, Platygyra, Sandalolitha

Caption for Figure 2 should include more details, rather than 'Southern Vietnam' of the precise location of the NOAA data.

More information should be provided on how the Coral Bleaching Threshold temperature of 30.5 C was determined.

Caption for Figure 4 could be improved by noting which sites the photos are from.

Figure 6. An accompanying table that provides results of the ANOVA should be included here or as Suppl. Table. It is useful for readers to be able to assess the precise levels of significance rather than just those that were < 0.05.

Discussion

Lines 177-191: If there are sufficient data available, the authors could consider inclusion of a figure that shows graphically the changes in bleaching and living coral cover through time (from 1998) for the major taxonomic groups. This would be very informative for readers, and support the case for acclimation. In addition to references 30, 37-40, there are other studies that have assessed acclimation to warming that the authors could cite and discuss. Eg. Schoepf et al. (2019) Stress-resistant corals may not acclimatize to ocean warming ... https://www.nature.com/articles/s41467-019-12065-0 or: https://doi.org/10.1038/s41467-019-12065-0

Lines 195-198: The authors could also include size - growth rate information for the large massive corals (eg. ca 1 cm radial extension per year for Porites), as illustrated in Figure 4D.

Depth, sea state (calm conditions) and levels of illumination (solar radiation) have been found to be important in levels of bleaching and recovery. This may be worth considering given the differences between shallow sites 1, 3 and 6 over time in cover of living and dead corals (Figure 6).

Line 233: '...corallivorous ...'

Author Response

We are grateful to the Reviewer 3 for valuable comments on our ms. All corrections made in text are in red.

The title suggested by the Reviewer is reasonable and we accepted it in our revision. (line1)

Global hotspot: ‘global’ was removed.

We considered the finding of Dixon et al. 2022 together with Schoepf et al. (2019) and other works in Discussion section (lines 240-257).

 

Line 64: '... severe'

Corrected

Line 69: SDNP?

Corrected

 

Line 86: Re typhoon tracks, many pass to the North, moving to the west across the South China Sea, sending large swells from the NE - SE, depending on their respective tracks. See e.g. Wang, Y. (2020). Composite of typhoon induced sea surface temperature and chlorophyll‐a responses in the South China Sea. Journal of Geophysical Research: Oceans, 125, e2020JC016243.
https://doi.org/10.1029/2020JC016243

Corrected. Lines 103-106

 

Line 103: '...being typical'

Line 103: '... dominated by Acroporidae' requires citation / reference or 'see Results'

Corrected. Line 120

 

Results

Line 141: A table showing results of the T test could be included, or as Suppl. Table, and similarly for the ANOVA of changes in cover (Figure 6).

Results of t-test presented in text (line 161)

Table of ANOVA results is provided (Table 2).

 

Line 164: '... coral decline ..'

Table 1. the Reference sources for the susceptibility of each genus should be listed next to the relevant text. Also check spellings. Eg.  Lithophyllon, Platygyra, Sandalolitha

Corrected

 

Caption for Figure 2 should include more details, rather than 'Southern Vietnam' of the precise location of the NOAA data.

Coordinates for the area of Southern Vietnam determined by NOAA is added to the caption

 

More information should be provided on how the Coral Bleaching Threshold temperature of 30.5 C was determined.

The base for determination of coral bleaching threshold is added in caption

 

Caption for Figure 4 could be improved by noting which sites the photos are from.

This information is added to the caption

 

Figure 6. An accompanying table that provides results of the ANOVA should be included here or as Suppl. Table. It is useful for readers to be able to assess the precise levels of significance rather than just those that were < 0.05.

Table with ANOVA results is provided (Table 2)

 

Lines 177-191: If there are sufficient data available, the authors could consider inclusion of a figure that shows graphically the changes in bleaching and living coral cover through time (from 1998) for the major taxonomic groups. This would be very informative for readers, and support the case for acclimation. In addition to references 30, 37-40, there are other studies that have assessed acclimation to warming that the authors could cite and discuss. Eg. Schoepf et al. (2019) Stress-resistant corals may not acclimatize to ocean warming ... https://www.nature.com/articles/s41467-019-12065-0 or: https://doi.org/10.1038/s41467-019-12065-0

Such a figure showing changes in bleaching was created and added as Figure 7.

 

Lines 195-198: The authors could also include size - growth rate information for the large massive corals (eg. ca 1 cm radial extension per year for Porites), as illustrated in Figure 4D.

This information was included in Results and Discussion (lines 194; 223-226)

 

Depth, sea state (calm conditions) and levels of illumination (solar radiation) have been found to be important in levels of bleaching and recovery. This may be worth considering given the differences between shallow sites 1, 3 and 6 over time in cover of living and dead corals (Figure 6).

These factors were considered in Discussion (lines 224-227)

Author Response File: Author Response.pdf