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Summary

  • The initial submission of this article was received on January 5th, 2021 and was peer-reviewed by 2 reviewers and the Academic Editor.
  • The Academic Editor made their initial decision on February 6th, 2021.
  • The first revision was submitted on June 9th, 2021 and was reviewed by 1 reviewer and the Academic Editor.
  • A further revision was submitted on August 11th, 2021 and was reviewed by the Academic Editor.
  • The article was Accepted by the Academic Editor on August 14th, 2021.

Version 0.3 (accepted)

· Aug 14, 2021 · Academic Editor

Accept

I appreciate your careful consideration of the reviewer's comments.

Regards,

Michael

[# PeerJ Staff Note - this decision was reviewed and approved by Robert Toonen, a PeerJ Section Editor covering this Section #]

Version 0.2

· Jul 9, 2021 · Academic Editor

Minor Revisions

I believe this manuscript is acceptable with minor revisions for format and style. I have attached a pdf with my edits. These changes follow recommendations I made previously. Note, I stopped fixing references about half way through.

Regards,

Michael

·

Basic reporting

I have reviewed the new manuscript and changes made by the authors and I have no further comments.

Experimental design

-

Validity of the findings

-

Additional comments

I have reviewed the new manuscript and changes made by the authors and I have no further comments.

Version 0.1 (original submission)

· Feb 6, 2021 · Academic Editor

Major Revisions

We received two detailed reviews. Both reviewers found, and I agree, that the manuscript presents some important data; however, it needs major revisions. In addition to the comments provided by the reviewers please consider the follow revisions. Note, these are just examples, not a comprehensive list.

Line 17. Here and throughout, write more succinctly. Replace “…services, highlighting their role…for long periods.” With “…services, including carbon sequestration.”
Line 20-23. This section is too wordy. Use words once in a sentence.
Line 23-25. Revise to “determine the structure of seagrass meadows in the LPBR and the sources and sinks of carbon in this system.”
Line 27. Provide methods in general terms (spectral analysis and..).
Line 28. Replace with “_____ analysis suggested that the LPBR stores 2.2 ….
Also, use consistent and reasonable significant figures (275 ± 23 not 275 ± 23.49)
Line 30. Be more specific and avoid run-on sentences. Replace “(top 1 m ), .. in Mexico” with “(top 1 m ). Carbon storage decreased/increased with water depth. Seagrass meadows covered 149,613 ha, which represents the largest organic carbon stock (35 Tg C) documented in seagrass meadows in Mexico.”
Line 44. Delete “Due to their structural and functional characteristics”
Line 67. Avoid the passive voice. Replace Among the mechanisms that regulate carbon stocks in seagrasses is the structural complexity of the seagrass meadows. This complexity is a function of the shoot density, leaf area and the specific characteristics of the species (Mazarrasa et al., 2018)” With “Shoot density, leaf area and specific characteristics of the species regulates carbon storage in seagrass meadows (Mazarrasa et al., 2018).”
Line 99. Replace “…such as dredging and the reduction in water quality associated with the increase in sediment supply, which favors the eutrophication processes of the coastal area, reducing the light..” with “such as dredging and eutrophication, which reduce light…”
Line 102. Delete “These damages..organic carbon.”
Line 109. Replace “and few studies relate the habitat health and the structure of the seagrass community in their interpretations (Samper…” with “and few studies relate habitat health and the structure of the seagrass community (Samper..”
Line 127. Considered by whom?
Line 128. Write actively. Replace “The terrestrial surface is dominated by mangrove vegetation and petenes (vegetation islands like hammock ecosystem from Florida USA) that together represent 23% of the total coverage.” With “Mangroves and petenes (vegetation islands like hammock ecosystem from Florida USA) cover 23% of the terrestrial surface.”
Line 146. Replace with “We visited..”
Line 224. Avoid superfluous determiners. Delete “the” whenever possible.
Line 262. Do not start paragraphs by telling the reader to go look somewhere else. Start every paragraph in Results with a summary of the key results. Move Line 274-276 to the start of the paragraph.
Line 267. Replace “The average concentration of NO3- + NO2- in the water from the LPBR was 5.26 ± 0.79 μmol “ with “NO3- + NO2- in the water column of LPBR average 5.3 ± 0.8 μmol…” By the way, that DIN level seems pretty high for an oligotrophic system.
Line 280. As above, start with a topic sentence.
Line 300. Avoid the passive voice. Replace “was found to average” with “averaged.” Follow that example throughout.
Line 320. Delete phrases like “Regarding the storage..” State if C stocks increased or decreased with depth.
Line 347. Delate “according to water quality variables.”
Line 348. Replace “such as” with “from”
Line 358. Revise to “The ____ supports this finding.”
Line 501. Conclusions doesn’t provide any conclusions. Also, don’t cite sources in conclusions. State the highlights of your findings.
Line 521. Stick to a consistent format for all references.

·

Basic reporting

No comment, see below

Experimental design

No comment, see below

Validity of the findings

No comment, see below

Additional comments

This paper assessing the carbon stocks in Los Petenes Biosphere Reserve (LPBR) in Mexico is of interest as it explores the carbon storage using multiple species and different water depths, which are two factors known to affect the carbon storage capacity in seagrass meadows. The manuscript is well written (I noticed, however, some grammatical errors) and I think the main issue is the presentation of the data and I have given some suggestions to that in the detailed comments below. Although, not included in the study but a comparison between the carbon storage within the MPA and outside would have been highly interesting, as well as a comparison with unvegetated sediments, which would have shown the contribution from seagrass meadows to the carbon storage as well as given an indication on the general accumulation of carbon in the area (in relation to for instance hydrodynamics).
Specific comments:

Introduction
Line 44: I suggest replacing “their” with “seagrasses” and place “their” and the second part of the sentence.
Line 52: Write out salt marshes.
Line 55-56: Please revise this statement, 50% of the carbon buried in marine sediments is found in all coastal blue carbon habitats combined (i.e. mangrove, salt marshes and seagrass). See for instance Duarte et al. 2013 - The role of coastal plant communities for climate change mitigation and adaptation.
Line 76: Please change to “water depth”.
Line 85-86: Check the grammar, …how much “that” is of autochthonous “origin”?
Line 90: Corresponds “to” 30%?
Lines 91-93: Please add a reference to this statement.
Lines 93-95: This sounds unreasonable, please consider revising. There are a lot of areas where seagrasses are increasing in area. See e.g. de los Santos et al. 2019 - Recent trend reversal for declining European seagrass meadows.
Line 116: Please change to “water depth”.

Methods
Line 126: The abbreviation LPBR should be moved to the introduction where the name is presented for the first time.
Line 127: Should it be “marine” reserve?
Lines 147-148: …water quality samples and “vegetation samples” were collected?
Lines 151-153: Please rephrase this sentence, “making measurements at 50 cm depth intervals from the surface to the bottom”?
Lines 176-178: Was no ground truthing conducted at the site?
Line 185: Once “cleaned”
Line 199: If the known sediment depth is 8 cm, why extrapolate to 1 m? This gives a higher carbon storage than the actual measured as well as a highly uncertain estimate of the stock, given only 8 cm were measured and 92 cm were extrapolated. I fully understand that 1 m sediment depth was used for comparison and I suggest that these cores should instead be excluded from those comparisons. I also think that these cores add to the natural variability of carbon storage, if only 8 cm has been accumulated, that site might not be the most efficient in carbon trapping.
Line 201: Were the cores correlated to the compaction or was the compaction only measured?
Line 204: the “mid-section” of each interval?
Line 211: LOI was used to determine OM?

Results
Line 281-282: I don’t understand the numbers, %OM (4-61%), %Corg (4-28%). Higher compared to what?
Lines 282-283: But there seems not to be a gradual increase in %Cing?

Discussion
Lines 351-356: This is a long sentence, please consider divide it into two.
Line 359: Should be fig 7?
Line 440: Should be fig 7?
Line 463: This is more of a general conclusion, please consider to move to “conclusion”.
Lines 470-475. The estimated loss of 130.8 Tg should be seen as a maximum, given that all of the carbon is remineralized and released to the atmosphere. This needs to be clear. Also, is this for 1 m of sediment?

Figures and tables.

Fig 6 and fig 8 are not referred to in the text. Or fig 6 is referred as 7.

Fig. 1. Is the black line in fig to the left the reserve boundaries?
Fig. 4. Please change to “species”. There is no mention in the caption what fig a-f stands for, neither does the figure show which depth the circles belong to. I don’t know what this figure adds to the study, please consider removing.
Fig 5. This is a nice figure. Please write what the abbreviations stands for (AB, S etc). I strongly suggest to present the carbon stocks of the different species separately in relation to water depth.
Fig 6. Is this sedimentary carbon or biomass or both? Why is this figure not reported in the results but only referred to in the discussion? Are these stocks including area? I suggest a better way to represent this would be to have a A and B figure where B is a bar graph showing the stocks, that would be easier to interpret.
Fig 7. Numbers represent each water depth? How could you find mangrove sediment at 4 m? This could also be a A and B figure, where B shows the results from the model as mean ± SD contribution of the different sources for each water depth.
Fig. 8 is very illustrative. Instead of showing the isotope signal, it would be more interesting to show the relative contribution from land/mangrove, which seem to decrease from shore. Is the thick blue line representing water discharge?
Table 2. Is MO in fact OM? Why is %Corg and OM almost equal at water depth 5.
Table 4. The first sediment carbon content, to what sediment depth is that calculated?
Table 6. I find it odd that seagrass didn’t contribute at all to the sedimentary carbon pool at 1 m

Reviewer 2 ·

Basic reporting

Language:
The English language is not clear and should be improved. There are very long sentences and grammar errors throughout the manuscript. I recommend a careful review of language. I provide here some examples from the abstract, for full review throughout the manuscript please see specific comments on each section:

L29: Authors use a noun (storage) instead of a verb (store)

L30: “The seagrass community extent for 149,613 ha” should be “The seagrass community extends …”

L33: “source” should be “sources”

L33-34: “If the seagrass extension in the LPBR lost through natural or
anthropogenic impacts could contribute with 130 Tg CO2eq emissions”.
Please rewrite, something like this would be better: “If LPBR seagrass meadows are lost due to natural or anthropogenic impacts, 130 Tg CO2eq emissions could be released.”

Background references:
Overall introduction is well documented and referenced but the structure needs to be improved. Authors should try to have one clear and main idea per paragraph, and restructure the information in a more logical order.

Manuscript structure:
the manuscript follows the appropriate sections.

Figures and Tables:
Figures and tables do not follow acceptable format and content. Specifically:

-Fig 1. Panels in Fig 1 need to be labeled with letters. More information on panel consisting of Sentinel 2a image is needed.
-Fig 2. Authors need to provide much more information in Figure heading and need to follow the format requirements form the journal, like labeling each panel with letters. What are insets on each photo?
-Fig 4. What does a, b, c, d, e ,f ,g , h , i and j mean?
-Fig 5. Please split this data at least in two independent panels: One for biomass stocks and one for sediment stocks.
-Fig 7. Where does the mangrove sediment and mangrove leaves data come from?? Not reported in methods. If these values correspond to data previously reported it needs to be specified in the fig heading.
-Fig 7 and Fig 8 are not mentioned in the text.

-Table 1 summarizes water quality data but is not referred in the text.
L172: This is the first table appearing on the text and it is referred as Table 2. But Table 2 summarizes sediment characteristics, not seagrass coverage as stated. Should it be Supplementary Table 2? If so, supplementary Table S1 is not appearing on the text neither. Please make sure that tables are labeled in the right order, following their appearance on the text.
Supplementary Table 2 needs major revision:
-First, all abbreviations for type of meadow need to be written in full.
-In MxMa (mixed seagrass meadow with macroalgae), no data on which macroalgae sp are present neither their relative contribution.
-In MxSf (mixed seagrass meadow dominated by S. filiforme), shoot density and cover of H. wriigthi and S. filiforme are similar. Why do authors consider it as dominated by S. filiforme? Which parameter do they base this classification on?
-In Sf (monospecific meadow of S. filiforme), why is there info on T. testudinum and H. wriigthi if it is a monospecifc meadow of S. filiforme?
-please consider adding the N of samples per each type of meadow, along the mean SeM, median and Range.

Table 1, 2 and 3. Please provide more information on table header and write variables in full.
Table 2 and 6. "MO" should be "OM". Also consider writing it in full.

Raw data: Supplementary Data set and Supplementary Data cores need clarification on what variables are in each column and units, etc.

Hypothesis: Authors do not state a clear hypothesis. Please provide a clear hypothesis at the end of the introduction section.

Experimental design

This study falls within the aims and scope of this journal.

Authors identify the following knowledge gaps:
At the end of second paragraph, authors identify a first knowledge gap: (L78) “However, insufficient reports exist regarding how the structural complexity of seagrasses is related with carbon sources and their ability to store organic carbon in subtropical regions with the influence of groundwater.”
Then (L81): “Estimates of carbon stocks of seagrass meadows across the world are derived from little data, resulting in the tendency to generalize global carbon stocks in these habitats (Serrano et al., 2014).”
And in third paragraph (L85): “However, research questions remain about these estimates, such as how much of the organic carbon stock in seagrass meadows is allochthonous and how much is autochthonous? (Kennedy et al., 2010; Garcias-Bonet et al., 2019). This question must be addressed at the regional level since the conditions where seagrass meadows are distributed are specific to each region and have implications related to ecosystem connectivity.”

However, Identification of knowledge gaps should be stated more clearly. For instance:
- scarcity of data on the relation between C stocks and seagrass complexity for subtropical regions.
- scarcity of data on C stocks for subtropical seagrasses
- scarcity of data on the contribution of auto- vs allochthonous materials in C stocks

Methodology used here do not follow the highest standards. specific comments:
I think a general rearrangement of Material and methods is needed. Info on stations and surveys should not be included only in the subsection of water quality, it is confusing in the present form.
No clear when sampling events toke place. No clarity in the number of stations and which parameters were assessed in each one.
Authors identify 5 bottom types but data on seagrass coverage is not supporting such classification.
L170-171: “For the classification of the image, four classes were considered based on an analysis of conglomerates with 80% similarity,…” Shouldn’t be 5 classes (i.e. Tt, Sf, MxMa, MxSf, and S)?
Also, provide info on which seagrass spp. are found in mixed meadows.
In MxSf (mixed seagrass meadow dominated by S. filiforme), shoot density and cover of H. wriigthi and S. filiforme are similar. Why do authors consider it as dominated by S. filiforme? Which parameter do they base this classification on?
In Sf (monospecific meadow of S. filiforme), why is there data on T. testudinum and H. wriigthi if it is a monospecifc meadow of S. filiforme?

L 181: “three quadrants of 1 m2 in four 25 m transects (4 quadrants by station)”. Please provide more information here. Are these stations the same as in Fig.1? I do not understand exactly how many 1m2 quadrats were sampled and how they were distributed. Please clarify.

L190-193: “To estimate the average carbon content in the biomass (Cbio) of seagrass by the above and belowground components, the values of the dry weights (g Dw m-2) were converted to carbon equivalents using conversion factors 0.34 and 0.35, respectively (Fourqurean et al., 2012a), after which these values were finally extrapolated to Mg C ha-1.”

The way authors calculate Carbon content in above and belowground biomass is not adequate. Authors used conversion factors to calculate Carbon content from tissue dry weight values using values reported in Fourqurean et al 2012a.
C content ranges widely depending on seagrass species and location. Therefore I strongly suggest that actual concentration of C in above and belowground tissues is analyzed for each seagrass species and type of meadow in order to provide accurate estimates of biomass C stocks, which is one of the main objectives of this study.

Moreover, authors refer to Fourqurean et al 2012a et al for the conversation factors, however, in this work Fourqurean et al. used mean values extracted from Duarte 1990 and other three studies that reported high variability in C content. For example Duarte 1990 reported C content ranging from 24 to 44%.
And here, there is a more recent example of the great variability of C content in seagrass leaves is the extensive work from Duarte et al 2018 with data from more than 200 seagrass tissue samples. Range: 14.98 – 42.58%.

Therefore, I think that data on above and belowground biomass cannot be published in the current form.

L214-215: “The percentage of organic carbon (Corg) was calculated as the difference between inorganic carbon (Cing %) and total carbon (TC %).”
Standard protocols require an acidification step prior to the analysis of C in order to remove carbonates. This step is especially important in carbonate rich sediments like the ones from the study site.
Authors need to support their choose of not acidifying sediments. Sediment C stocks are calculated based on these values, and, therefore, this is a crucial analysis in this work, which in this current form does not follow the standard protocols.

I suggest that authors use more recent models to evaluate the contribution of different sources to the organic carbon in their sediments. Authors could use as well values on delta 15N and not only delta 13C.
Available Bayesian models in R packages are an easy and comprehensive tools that authors could explore. For instance, see Parnell et al 2016.
Parnell, A. (2016). Simmr: a stable isotope mixing model. R package version 0.3.
https://CRAN.R-project.org/package=simmr (accessed Janauary 19, 2016).

Validity of the findings

Raw data has been provided, however, more information needs to be added regarding meaning of acronyms and abbreviations and units in column variables on datasets.
I suggest that authors include an explanatory paragraph accompanying the datasets.

Conclusions are based on data that I believe needs major revision.

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