Review Reports - Characterization of the Meiobenthic Community Inhabiting the Zwin Coastal Lagoon (Belgium, the Netherlands) and the Role of the Sedimentary Environment

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review of the paper “Characterization of meiobenthic community inhabiting the Zwin coastal lagoon (Belgium, the Netherlands) and the role of sedimentary environment” written by Elisa Baldrighi and others.

This research establishes a foundation for future studies, suggesting areas of focus for further investigations into the relationships among environmental factors, community structure, and anthropogenic influences in such ecosystems.

Lines 31-36. This information is not directly related to the authors' findings. Instead, they should provide a more detailed summary of their results, including qualitative data.

Line 58. Here, the authors should be more rigorous and provide examples that illustrate the role of sediment characteristics in shaping environmental conditions in estuarine systems.

Line 63. The text should present a size range for meiobenthic fauna.

Lines 81-82. The authors should include a brief overview of significant previous findings on the structure and function of benthic communities in this area.

Line 206. The authors should provide information on the taxonomic guides used in this study.

Figure 3 requires significant improvement, as the small font size makes it difficult to understand.

Line 234. The authors should explain what they mean by "community composition."

Line 356. The authors reported in the "Materials and Methods" section that their PERMANOVA was based on the Bray-Curtis matrix. Is this metric appropriate for comparing the number of taxa between stations?

Line 453. Which burrowing polychaetes were found in this location?

Lines 449-481. The authors should discuss the temperature and salinity characteristics of the study area. Why were these parameters not included in the analysis?

Lines 501-203. The authors should also provide a comparative table or figure to illustrate their statement that "the meiofaunal abundance values reported in this study are consistent with those observed in other coastal and lagoonal environments in Belgium and worldwide"

Author Response

Rew #1 Comments and Suggestions for Authors

The authors analyzed meiobenthic communities inhabiting the Zwin tidal lagoon. They showed that nematodes dominated the local community, with copepods and copepod nauplii being the second most important group. They provided evidence that meiofauna rapidly colonized the lagoon after it opened to the sea. They also found that structural differences in communities were related to sedimentary properties and food availability. This research establishes a foundation for future studies, suggesting areas of focus for further investigations into the relationships among environmental factors, community structure, and anthropogenic influences in such ecosystems.

Reply: we would like to thank the Reviewer for this positive feedback on our paper. Please find below our replies to the comments.

Lines 31-36. This information is not directly related to the authors' findings. Instead, they should provide a more detailed summary of their results, including qualitative data.

Reply: Following the Reviewer’s suggestion, we added details related to our study (lines 29-32, 33-37).

Line 58. Here, the authors should be more rigorous and provide examples that illustrate the role of sediment characteristics in shaping environmental conditions in estuarine systems.

Reply: Following the Reviewer’s suggestion, we provided examples of the role of sediment features in shaping the estuarine environments and their communities (lines 58-63).

Line 63. The text should present a size range for meiobenthic fauna.

Reply: We added the information accordingly (line 67).

Lines 81-82. The authors should include a brief overview of significant previous findings on the structure and function of benthic communities in this area.

Reply: Following the Reviewer’s suggestion, we added a brief overview of previous findings (lines 86-90).

Line 206. The authors should provide information on the taxonomic guides used in this study.

Reply: Following the Reviewer’s suggestion we added taxonomic guides (line 217).

Figure 3 requires significant improvement, as the small font size makes it difficult to understand.

Reply: The Reviewer is right here. We improved Figure 3 accordingly and we increased the font size. However, we think that including the figure in the main manuscript and reducing the size of it makes the graph less clear. A separate copy in .tif is provided for each of the figures and resolution is very clear.

Line 234. The authors should explain what they mean by "community composition."

Reply: We replaced community composition with community structure for clarity and following the Reviewer’s suggestion, we clarified what we mean with community structure (line 245-246).

Line 356. The authors reported in the "Materials and Methods" section that their PERMANOVA was based on the Bray-Curtis matrix. Is this metric appropriate for comparing the number of taxa between stations?

Reply: Actually, this is what we specified in the M&M section: “PERMANOVA was used also as a univariate analysis (a robust ANOVA) of meiofaunal total abundance, total number of taxa, and environmental variables, after converting the original data matrix into Euclidean distance”. For the number of taxa, we used the Euclidian distance, which is more appropriate for univariate measures (as probably the Reviewer meant).

Line 453. Which burrowing polychaetes were found in this location?

Reply: It would have been interesting the identification of macrofauna polychaetes, but unfortunately this was beyond the scope of our research.

Lines 449-481. The authors should discuss the temperature and salinity characteristics of the study area. Why were these parameters not included in the analysis?

Reply: We would like to thank the Reviewer for the comment. However, T and Salinity were not measured during the sampling activities. For this reason, they were not included in the environmental parameters. Moreover, both T and salinity did not present substantial variability between sampling stations (see report: Cosyns E., Boumon T., De Smet J., Esteban E., Faveyts W., Geunens O., Jacobs I., Jacobs M., Jansen J., Lambrechts J., Le Gurun M., Mertens A., Moens T., Paredis R., Provoost S., Simons T., Thibau K., Van Braeckel A., Van Colen C., Van Gompel J., Van Oost F., Vanoverbeke J., Vantorre R., Verhelst P. & Zwaenepoel A. 2024. Monitoring van het natuurherstel in het grensoverschrijdende Zwin 2011 - 2023. Eindrapport WVI, INBO, Natuurpunt Studie, Nature-ID & Universiteit Gent (Marbiol) i.o.v. Agentschap voor Natuur en Bos, de Vlaams Nederlandse Schelde commissie en de Provincie Zeeland). Taking in consideration results reported in the previous investigations conducted in the Zwin Nature park, we concluded that T and S do not constitute key parameters that might influence the meiofauna community. The most important differences were and are in the sediment features and in the depositional environment(s) (see also: Van Colen C. et al. (2009). Macrobenthic community structure and distribution in the Zwin nature reserve (Belgium and The Netherlands). Journal of the Marine Biological Association of the United Kingdom, 89(3):431-438. doi:10.1017/S0025315409003257).

Reviewer 2 Report

Comments and Suggestions for Authors

General comments

Baldrighi and colleagues present a comprehensive study on the influence of sedimentary environments on the meiobenthic community of the Zwin coastal lagoon. In 2018/2019, large-scale restoration works were carried out in the study area, including enlargement and deepening of the main gully to enhance the water dynamics throughout the lagoon. Remarkably, two years after these restoration activities, the authors reported a relatively rich, diverse, and well-established meiofaunal community, indicating that the interventions improved environmental conditions within the protected area. The study analysed 15 abiotic factors related to sediment characteristics and assessed their influence on meiofaunal diversity, abundance and spatial distribution using numerical modelling approaches. A wide range of field and laboratory methods was employed (e.g., granulometry, TOC, TN, δ¹³C and δ¹⁵N stable isotopes), complemented by advanced statistical techniques (e.g., nMDS, SIMPER, PERMANOVA, DistLM). Overall, the manuscript is scientifically valuable and clearly written; however, the main weakness lies in the presentation of the results.

Minor comments

Line 176: superscript in δ¹³C and δ¹⁵N stable isotopes

Line 188: there is no reason to use capital letter in the word Chlorophyll-a

Line 206: remove the additional full stop ‘(0.5 g l^-1) [36].’

Line 208: ‘n. taxa’ small letter

Line 249: what you mean by ‘lateral variety’? remove, change, or explain

Line 278-279: ‘Table 1’ (not Tab. 1)

Line 304-305: spaces between units

Line 323: looks like additional space before ‘Although’

Line 545: replace the citation with a number from the reference list

Discussion and Conclusions

The discussion is well-written and accurately analyzes and interprets the results, but it is definitely too long and contains too many details. The text should be shortened to enhance readability.

References

Individual references should be formatted consistently in the same way (e.g., 3. Estuar Coast Shels S; 6. Estuarine, Coastal and Shelf Sciences; 56. Estuar. Coast. Shelf Sci.), spaces and dots between initials, presence/absence of DOI number

Figures and Tables

Figure 1: I cannot find a ‘red star’ in the top left corner of the figure.

Figure 2: Scales and axes descriptions in diagrams (water content, grain size, TOC/TN) should be in larger font. In their current form, they are illegible. Abbreviations: vfs=very fine sand (spaces present/absent before/after the equal sign should be consistent throughout the entire text and all figure/table captions)

Figure 3: This graph is completely unreadable. Enlarge the font of the station/sample names and environmental factors. Spread them out so they don't overlap. Remove the circle (it's unnecessary) and move the legend to the graph area. Add information to the axis description about the percentage of variability explained by each axis. Add explanations of abbreviations in the figure caption or refer to explanations given in another figure/table

Figure 4: Enlarge font, change the Y axis description (Ind. 10 cm^-2 spaces and superscript + remove / symbol). Add information what are box and whiskers (SD, SE, minimum/maximum?)

Figure 5: Change the Y axis description (Ind. 10 cm^-2 spaces and superscript + remove / symbol)

Figure 6 and 7: These graphs are also completely unreadable. Enlarge the font of the station/sample names and environmental factors. Spread them out so they don't overlap. Remove the circle, the diagram title (blue one) and the box with transformation data (they are included in figure caption) and move the legend to the graph area.

Table 1: Column headings should be corrected so as not to leave single letters (reduce the font or use an abbreviation). Use superscript in the isotope symbol

Table 3 and 4: Use the full name of factor in the column ‘source’. Align column headers with their contents

Table 6: The position of text in the column header cell should be consistently the same throughout the table.

Author Response

Rew #2 General comments
Baldrighi and colleagues present a comprehensive study on the influence of sedimentary environments on the meiobenthic community of the Zwin coastal lagoon. In 2018/2019, large-scale restoration works were carried out in the study area, including enlargement and deepening of the main gully to enhance the water dynamics throughout the lagoon. Remarkably, two years after these restoration activities, the authors reported a relatively rich, diverse, and well-established meiofaunal community, indicating that the interventions improved environmental conditions within the protected area. The study analysed 15 abiotic factors related to sediment characteristics and assessed their influence on meiofaunal diversity, abundance and spatial distribution using numerical modelling approaches. A wide range of field and laboratory methods was employed (e.g., granulometry, TOC, TN, δ13C and δ15N stable isotopes), complemented by advanced statistical techniques (e.g., nMDS, SIMPER, PERMANOVA, DistLM). Overall, the manuscript is scientifically valuable and clearly written; however, the main weakness lies in the presentation of the results.

Reply: we would like to thank the Reviewer for this positive feedback on our paper. Please find below our replies to the comments.

Minor comments
Line 176: superscript in δ13C and δ15N stable isotopes

Reply: We changed it accordingly (lines 194, 196)

Line 188: there is no reason to use capital letter in the word Chlorophyll-a

Reply: We changed it accordingly (lines 199-201)

Line 206: remove the additional full stop ‘(0.5 g l-1) [36].’

Reply: We changed it accordingly. (line 216)

Line 208: ‘n. taxa’ small letter

Reply: We changed it accordingly (line 218)

Line 249: what you mean by ‘lateral variety’? remove, change, or explain

Reply: We removed it, sorry for this refuse.

Line 278-279: ‘Table 1’ (not Tab. 1)

Reply: We changed it accordingly (line 289).

Line 304-305: spaces between units

Reply: We added it accordingly (lines 311-312)

Line 323: looks like additional space before ‘Although’

Reply: We changed it accordingly.

Line 545: replace the citation with a number from the reference list

Reply: We changed it accordingly (line 560).

Discussion and Conclusions
The discussion is well-written and accurately analyzes and interprets the results, but it is definitely too long and contains too many details. The text should be shortened to enhance readability.

Reply: Following the reviewer’s suggestion, we shortened the Discussion and Conclusions section. In detail, the Discussion and Conclusion paragraph has been shortened from 2751 to 2350 words.

References
Individual references should be formatted consistently in the same way (e.g., 3. Estuar Coast Shels S; 6. Estuarine, Coastal and Shelf Sciences; 56. Estuar. Coast. Shelf Sci.), spaces and dots between initials, presence/absence of DOI number

Reply: We would like to thank the reviewer for pointing this out. We changed the reference style accordingly. Where the DOI is absent, it is because the information is not available.

Figures and Tables
Figure 1: I cannot find a ‘red star’ in the top left corner of the figure.

Reply: We changed it accordingly. We apologize for the refuse in the caption; we added a red square.

Figure 2: Scales and axes descriptions in diagrams (water content, grain size, TOC/TN) should be in larger font. In their current form, they are illegible. Abbreviations: vfs=very fine sand (spaces present/absent before/after the equal sign should be consistent throughout the entire text and all figure/table captions)

Reply: Following the reviewer’s suggestion, we improved Figure 2.

Figure 3: This graph is completely unreadable. Enlarge the font of the station/sample names and environmental factors. Spread them out so they don't overlap. Remove the circle (it's unnecessary) and move the legend to the graph area. Add information to the axis description about the percentage of variability explained by each axis. Add explanations of abbreviations in the figure caption or refer to explanations given in another figure/table

Reply: Following the reviewer’s suggestion, we improved Figure 3. The graph is a PRIMER output, unfortunately we cannot spread out the names and the position of the legend cannot be modified. We hope the reviewer will find the figure readable.

Figure 4: Enlarge font, change the Y axis description (Ind. 10 cm-2 spaces and superscript + remove / symbol). Add information what are box and whiskers (SD, SE, minimum/maximum?)

Reply: Following the reviewer’s suggestion, we improved Figure 4.

Figure 5: Change the Y axis description (Ind. 10 cm-2 spaces and superscript + remove / symbol)

Reply: Following the reviewer’s suggestion, we improved Figure 5.

Figure 6 and 7: These graphs are also completely unreadable. Enlarge the font of the station/sample names and environmental factors. Spread them out so they don't overlap. Remove the circle, the diagram title (blue one) and the box with transformation data (they are included in figure caption) and move the legend to the graph area.

Reply: Following the reviewer’s suggestion, we improved Figures 6 and 7.

Table 1: Column headings should be corrected so as not to leave single letters (reduce the font or use an abbreviation). Use superscript in the isotope symbol

Reply: Following the reviewer’s suggestion, we improved Table 1.

Table 3 and 4: Use the full name of factor in the column ‘source’. Align column headers with their contents

Reply: Following the reviewer’s suggestion, we improved Table 3 and 4. We specified in the captions the abbreviations used in the column ‘source’.

Table 6: The position of text in the column header cell should be consistently the same throughout the table.

Reply: Following the reviewer’s suggestion, we improved Table 6 accordingly.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript aims to characterize the meiofaunal communities in the Zwin lagoon following its re-opening to the sea in 2019, and to explore how sedimentary features and the quantity and quality of organic matter influence community structure and distribution. The study highlights the potential impacts of human-induced changes in sediment dynamics on benthic biodiversity and ecosystem functioning. Overall, the paper is well written and scientifically sound, but I have a few minor suggestions for improvement:

1. Please improve the readability of Figures 3, 6, and 7. For Figure 1, adding coordinates and a scale bar would be helpful.

2. Lines 207–208: Provide a reference for meiofauna identification methodology.

3. Consider moving Table 2 to the Supplementary Materials.

4. Discussion 4.2: The description of rare taxa is rather lengthy and not strongly linked to the main hypothesis. Instead of a general listing, it would be more useful to clarify why these taxa occur only at specific stations or sediment depths. Could they serve as sensitive indicators of sedimentary differences? Do their occurrences support the environmental drivers identified by the DistLM analysis?

Author Response

Rew #3 Comments and Suggestions for Authors

Reply: we would like to thank the Reviewer for this positive feedback on our paper. Please find below our replies to the comments.

Please improve the readability of Figures 3, 6, and 7. For Figure 1, adding coordinates and a scale bar would be helpful.

Reply: Following the Reviewer’s suggestion, we improved figures 1, 3, 6 and 7. We hope figures are now readable.

2. Lines 207–208: Provide a reference for meiofauna identification methodology.

Reply: Following the Reviewer’s suggestion, we added a reference for the meiofauna identification methodology (line 217).

3. Consider moving Table 2 to Supplementary Materials.

Reply: The Reviewer is right; we moved Table 2 to Supplementary Materials (Table S3).

4. Discussion 4.2: The description of rare taxa is rather lengthy and not strongly linked to the main hypothesis. Instead of a general listing, it would be more useful to clarify why these taxa occur only at specific stations or sediment depths. Could they serve as sensitive indicators of sedimentary differences? Do their occurrences support the environmental drivers identified by the DistLM analysis?

Reply: We can understand the Reviewer’s comment here. Clarifying the reason why these taxa occur only at specific stations/depths is challenging since the information available are still scant and the meiofauna organisms have been identified at a higher taxonomic level (genera and species might give more detailed information). We tried to present and discuss our results based on general information available on these taxa and supported by the literature. Certainly, and this is what we stated in the discussion (lines 552-559; 565-576), the presence/absence of rare taxa is a result of specific sediment conditions (e.g., grain size, food availability). We investigated the presence of a correlation between environmental variables and meiofauna community composition (DistLM analysis), but we did not investigate the correlation between each taxon with our environmental features. Indeed, we can speculate that the presence of rare taxa might support the drivers identified by DistLM (it would make sense) - but further analysis should be done before stating this conclusion.

Reviewer 4 Report

Comments and Suggestions for Authors

The article is very relevant, as it records the early stage of the restoration succession in a previously drained area, which is being returned to a natural marine state. All the information on the stratification of bottom sediments is valuable as a good starting point for future monitoring. It is especially important that the article is devoted to the meiobenthos, which is rarely the object of monitoring. It would be even more interesting to follow the very initial stages of this succession immediately after the enlargement and deepening of the Zwin gully. However, the results obtained two years later also point to the amazing ability of the marine biota to form a rich community very quickly.

I have a comment on this article, which requires significant revision of the text.

When analyzing organic matter in the bottom sediments, the authors do not take into account that most of it is made up of interstitial organisms, mainly meiobentos. Meiofauna was not extracted from the soil before analysis. Therefore, it is pointless to assess the dependence of the distribution and taxonomic composition of meiobenthos on the quantity and quality of organic matter, as this is a correlation of meiobenthos with itself. This also makes meaningless the main hypothesis "…that the quantity and quality of organic matter are reflected in the structure and spatial patterns of meiofauna", as well as the conclusion "the hypothesis that depositional and sedimentological features, including availability and quality of OM, are reflected in the structure and distribution of meiofaunal community is once again confirmed". Although the wording of the hypothesis does not simply say "meiobenthos", but "meiofaunal biodiversity", and the conclusions say "meiofaunal community", but this only complicates the matter, since it would require an analysis of the species composition, which was not the objective of this study.

The authors equate "quantity and quality of organic matter" with "food source and quality", using them as synonyms, which is also wrong. The concept of "food source and quality" with this set of methods used could be applied only to two indicators: chlorophyll a and pheophytin. Total organic matter in sediment, which includes dissolved organic matter, detritus, protists, bacteria, diatoms, largely consists of meiobenthos biomass. This is convincingly confirmed by the results of the study: correlation of the meiobenthos abundance with the amount of organic matter both when comparing stations or different layers from each column; by the fact that 94% of the variance in the total meiofauna abundance was due to the quality of organic matter by DistLM; by the mentioned "The distribution pattern in surface and sub-surface sediments of TOC and TN concentrations were closely correlated (r2 = 0.993), while δ13C values remained relatively stable (avg. -22.5‰ +/- 0.3)…". This is exactly what would occur if organic matter contained in living organisms. The composition of organic matter would be of marine origin and uniform within each column.

An indicator of the food supply could be chlorophyll a (for phytophages) or pheophytin (for detritophages) in the sediment. According to supplementary table 3, the amount of phytopigments (both Chl a and PHEO) is greater at station 2 than at station 1, while the number of meiobenthos, on the contrary, is greater at station 1. Of course, the number is not the best indicator: it would be more correct to compare the biomass with the food supply. But this is outside this study.

By the way, it is interesting that in the Principal component analysis, chlorophyll and pheophytin were fall to different Principal components, i.e. they do not correlate. The amount of pheophytin is approximately the same at station 1 and station 2, while that of chlorophyll varies. Chlorophyll concentration is a variable parameter related to which algae and in what quantity have developed at the moment, while pheophytin has a cumulative effect.

To improve this article, I recommend removing from the text all mentions of organic as a food for meiobenthos, (lines 26, 31, 89, 198, 432, 437, 599, 613, 621), as well as the fact that the quantity and quality of organic matter affect the distribution of meiobenthos. Better emphasize that the data obtained reflect the food supply for macrofauna and fish. Accordingly, in the conclusion, it is not worth writing that "the hypothesis that sedimentation and sedimentation features, including the presence and quality of OM, are reflected in the structure and distribution of the meiofaunal community is once again confirmed." Sedimentological features - yes, availability and quality of OM - no.

Annotation

Lines 23-26: “This study aims to characterize the meiobenthic communities inhabiting the Zwin tidal lagoon, located on the border between Belgium and The Netherlands, and to evaluate to what extent sedimentological characteristics and food quantity and quality influence meiofauna composition and distribution” – food quantity and quality was not estimated, but organic matter quantity and composition.

In the annotation, the following fragment seems superfluous:

Lines 31-35: «Meiobenthos represents a multifunctional group with a tight network of biotic interactions. The loss of species or functional groups may provoke cascading effects that have implications for interacting species and ecosystem functioning. Therefore, anthropogenic activities linked to coastal exploitation and changes in sedimentation regimes can affect the complex connections between marine biodiversity, their environment, and related ecological functions». – Everything written is correct, but in the abstract I would like to see a brief listing of the results of THIS study. It would be more important to mention that the organic matter, even in those sediment layers that formed in the period preceding the restoration of sea tides, is of marine origin (due to the burrowing activity of polychaetes)...

The sentence about food availability needs to be removed from the annotation:

Line 30 - «Major structural differences in communities were related to … food availability».

Materials and Methods

2.1. Study area

Why did not indicate the time during which the site is flooded by high tide, how long it is open at low tide, and water salinity near these stations...

Figure 1. «Top left: geographical location of the studied area (i.e., red star)…» - And where is the red star? It is not visible.

2.3. Sedimentological analysis

Lines 158-160: «The second half was sampled 158 according to sedimentological characteristics with the top 4 cm subdivided into SUP samples (i.e., more oxidized and hydrated) and SUB samples (i.e., more compact and reduced)» – abbreviations SUP and SUB are not used further in the text or in the supplements.

2.8 Statistical analyses

Line 231: misprint: the PCA is to createS new multivariate variables – «…to create…».

Discussion and Conclusions

The discussion did not address what follows from the obtained data on the concentration of 15N and 13C isotopes and their distribution in the sediment cores.

4.2. The meiofaunal communities inhabiting the Zwin Natural Park: an anthropogenic created lagoon

Lines 592-593: “The meiofaunal community showed the characteristics of a well-established population dominated by nematodes, and secondary by copepods with their nauplii” – “Well-established” can only be stated if there are observations of dynamics over time. It cannot be ruled out that this is a community of temporarily settled explerent species, which cannot be said to be well-established at all.

Lines 596-598: “Major structural differences in the meiofaunal communities were related to the depositional processes and sedimentary environments characterizing the investigated stations and, among them, the availability of food” - remove references to food availability.

Lines 599-600: «At the Zwin lagoon, the meiofaunal colonization was quick and abundant and even very weak tidal currents were sufficient to suspend and transport these animals to the new environment» - this is a very important result, which it is desirable to reflect in the abstract.

4.3. The influence of the depositional environment on the meiobenthos

Lines 612-613: “In this study, sedimentological variables (e.g., lithology, quantity and quality of food sources) again emerged as key factors influencing meiofaunal abundance and diversity” - remove references to food quality and its influence on meiofaunal abundance and diversity.

Lines 620-624: “These findings confirm that sedimentary dynamics and depositional processes, through their influence on sediment properties (e.g., grain size) and food quality (e.g., OM composition), shape meiofaunal communities and their vertical and horizontal distribution, as reported in other ecosystems e.g., [84; 90-91]” – remove references to food quality and that it shapes and drives meiofauna.

Lines 624-626: “Since food availability drives the vertical sediment of meiofauna, the high subsurface densities observed at St.1 and St.2 are likely attributable to the extensive availability of relatively fresh marine phytodetritus [92-93]” – subsurface meiobenthos densities cause high TOC .

Lines 638-639: “In conclusion, the hypothesis that depositional and sedimentological features, including availability and quality of OM, are reflected in the structure and distribution of meiofaunal community is once again confirmed” – see above.

I really hope that the article can be published in a corrected form.

Comments for author File: Comments.pdf

Author Response

Rew #4 Comments and Suggestions for Authors

Reply: we would like to thank the Reviewer for this positive feedback on our paper. Please find below our replies to the comments.

I have a comment on this article, which requires significant revision of the text.

Reply: The Reviewer’s comment is understandable, but the methods that we used to extract and quantify the organic matter content are for extracting lipids, carbohydrates, proteins and chl-a/phaeopigments from sediments. Decomposing and dead meiofaunal organisms are absolutely part of the OM pool into the sediment, but they represent only a small percentage of the total OM. We do not agree with the statement “this is a correlation of meiobenthos with itself”. The OM pool is mainly constituted by organic matter of terrestrial and marine origine, microbial component, phytoplankton, marine snow, fecal pellets and dead organisms of any kind. We could agree with the referee if from the literature it would have been reported the meiofauna as the most abundant fraction constituting the organic matter, but this is not the case. Please refer to the following papers (which constitute just few of the large body of literature published in the topic) on which we based our analysis on quantifying the sedimentary OM and its quality:

Important References in support of our approach and methods to quantify the quantity and quality of organic matter and to estimate the amount of food for the meiofauna:

Dell'Anno, Antonio, et al. "Assessing the trophic state and eutrophication of coastal marine systems: a new approach based on the biochemical composition of sediment organic matter." Marine Pollution Bulletin 44.7 (2002): 611-622.’
Paradis, Sarah, et al. "Organic matter contents and degradation in a highly trawled area during fresh particle inputs (Gulf of Castellammare, southwestern Mediterranean)." Biogeosciences 16.21 (2019): 4307-4320.
Pusceddu, Antonio, et al. "Quantity and bioavailability of sediment organic matter as signatures of benthic trophic status." Marine Ecology Progress Series 375 (2009): 41-52.
Pusceddu, Antonio, et al. "Organic matter composition, metazoan meiofauna and nematode biodiversity in Mediterranean deep-sea sediments." Deep Sea Research Part II: Topical Studies in Oceanography 56.11-12 (2009): 755-762.
Pusceddu, Antonio, et al. "Seasonal and spatial changes in the sediment organic matter of a semi-enclosed marine system (W-Mediterranean Sea)." Hydrobiologia 397.0 (1999): 59-70.
Pusceddu, Antonio, et al. "Seasonal fluctuations in the nutritional value of particulate organic matter in a lagoon." Chemistry and Ecology 13.1 (1996): 21-37.
Pasquini, Viviana, et al. "Outcomes of feeding activity of the sea cucumber Holothuria tubulosa on quantity, biochemical composition, and nutritional quality of sedimentary organic matter." Frontiers in Marine Science 9 (2023): 1010014.
Pusceddu, A., Carugati, L., Gambi, C., Mienert, J., Petani, B., Sanchez-Vidal, A., ... & Danovaro, R. (2016). Organic matter pools, C turnover and meiofaunal biodiversity in the sediments of the western Spitsbergen deep continental margin, Svalbard Archipelago. Deep Sea Research Part I: Oceanographic Research Papers, 107, 48-58.
Bianchelli, S., Nizzoli, D., Bartoli, M., Viaroli, P., Rastelli, E., & Pusceddu, A. (2020). Sedimentary organic matter, prokaryotes, and meiofauna across a river-lagoon-sea gradient. Diversity, 12(5).
Bianchelli, Silvia, et al. "Trophic status and meiofauna biodiversity in the Northern Adriatic Sea: insights for the assessment of good environmental status." Marine Environmental Research 113 (2016): 18-30.

Although the wording of the hypothesis does not simply say "meiobenthos", but "meiofaunal biodiversity", and the conclusions say "meiofaunal community", but this only complicates the matter, since it would require an analysis of the species composition, which was not the objective of this study.

Reply: At line 216 we specified the level of identification of the meiofaunal organisms to clarify any misunderstanding. The word meiobenthos is used here with the meaning of abundance and diversity of the meiobenthic component (e.g., Gheskiere et al., BIOLOGIE, 72-SUPPL.: 43-49. 2002; Mevenkamp et al., 2018 https://doi.org/10.1016/j.marenvres.2017.11.002). Meiofaunal diversity is widely used to indicate the number of taxa characterizing the community (e.g., Maria et al., 2018 http://dx.doi.org/10.1016/j.jembe.2017.08.005). The word meiofaunal community is commonly used to identify the organisms (not necessarily identified to the species level) which constitute the meiobenthos (e.g., Neira et al., 2018 doi: 10.3389/fmars.2018.00448;

The terminology we used is and it has been commonly used in many papers on meiobenthos/ meiofauna community composition (e.g., Norkko et al., 2019 https://doi.org/10.1016/j.seares.2019.101781; Bonaglia et al., 2020

https://doi.org/10.1016/j.marenvres.2020.104968). In summary, meiobenthos – meiofaunal biodiversity – meiofaunal community are largely used for referring to all the organisms between 32µm and 1 mm in size living int the sediment and to the composition of their community at any taxonomic level of identification (please, refer also to Giere 2009 - Giere, Olav. Meiobenthology: the microscopic motile fauna of aquatic sediments. Berlin, Heidelberg: Springer Berlin Heidelberg, 2009; Giere, Olav. "Physiology, Biochemistry and Meiofauna—A Rarely Touched Terrain." Perspectives in Meiobenthology: Reviews, Reflections and Conclusions. Cham: Springer International Publishing, 2019. 51-61.).

The authors equate "quantity and quality of organic matter" with "food source and quality", using them as synonyms, which is also wrong. The concept of "food source and quality" with this set of methods used could be applied only to two indicators: chlorophyll-a and pheophytin. Total organic matter in sediment, which includes dissolved organic matter, detritus, protists, bacteria, diatoms, largely consists of meiobenthos biomass. This is convincingly confirmed by the results of the study: correlation of the meiobenthos abundance with the amount of organic matter both when comparing stations or different layers from each column; by the fact that 94% of the variance in the total meiofauna abundance was due to the quality of organic matter by DistLM; by the mentioned "The distribution pattern in surface and sub-surface sediments of TOC and TN concentrations were closely correlated (r2 = 0.993), while δ13C values remained relatively stable (avg. -22.5‰ +/- 0.3)…". This is exactly what would occur if organic matter contained in living organisms. The composition of organic matter would be of marine origin and uniform within each column.

Reply: we can understand the Reviewer’s thoughts but again we do not completely agree with the comments. Equate the "quantity and quality of organic matter" with "food source and quality” can be “hazardous” since not all the quantity of organic matter is actually accessible to the organisms, so quantity does not mean necessarily food source eatable and accessible to the meiofauna (but please consider this paper about meiofauna incorporation of organic matter Urban-Malinga, Barbara, and Tom Moens. "Fate of organic matter in Arctic intertidal sediments: is utilisation by meiofauna important?" Journal of Sea Research 56.3 (2006): 239-248). What we do is an estimation of food quantity = food source and of its quality. This approach has been used and validated for many years, and many papers have been published (Please, see above all the References cited and literature therein). However, we rephrased it along the manuscript. Again, and as the Reviewer wrote just above, organic matter into the sediment comes from many sources and meiofauna is just a small percentage of it. Usually, bigger benthic size organisms along with microbial component dominated the benthic stock (please see Rex et al. 2006 Mar Ecol Prog Ser 317: 1–8 for a metanalysis on this topic). Actually, with the protocols that we applied to quantify the sedimentary OM are not reliable for quantifying the contribution of meiofaunal organisms to the poll of OM into the sediment (please refer to the papers cited above). I guess other protocols and methods should be applied to assess this. Most of the variability in meiofaunal abundance and diversity was explained by TN, grain size and phaeopigments, however we hardly believe that this relation is due to the fact that the TN we found came from the nitrogen into the body of meiofaunal organisms. For all these reasons and considering the large body of literature in which the same approach we used have been used as well, we cannot completely agree with the Referee’s comments. With this, we don’t want to affirm that the approach we applied cannot be improved, but it can be considered reliable ( e.g., Pusceddu et al., 2016 https://doi.org/10.1016/j.dsr.2015.11.004) for our scope. We should consider that even if the meiofauna is an abundant component of the benthos, prokaryotes and other unicellular organisms constitute the big part of the bulk.

In this regard, the conclusion "Major structural differences in the meiofaunal communities were related to the depositional processes and sedimentary environments characterizing the investigated stations and, among them, the availability of food" is not justified. The authors assessed not the food value of organic matter FOR the meiobenthos, but the food value OF the meiobenthos for the animals that feed on it (detritivorous macrobenthos, bottom fish, such as flounders, if they live there). An indicator of the food supply could be chlorophyll a (for phytophages) or pheophytin (for detritophages) in the sediment. According to supplementary table 3, the amount of phytopigments (both Chl a and PHEO) is greater at station 2 than at station 1, while the number of meiobenthos, on the contrary, is greater at station 1. Of course, the number is not the best indicator: it would be more correct to compare the biomass with the food supply. But this is outside this study. By the way, it is interesting that in the Principal component analysis, chlorophyll and pheophytin were fall to different Principal components, i.e. they do not correlate. The amount of pheophytin is approximately the same at station 1 and station 2, while that of chlorophyll varies. Chlorophyll concentration is a variable parameter related to which algae and in what quantity have developed at the moment, while pheophytin has a cumulative effect.

Reply: The methods we used to quantify the OM, or better to estimate the OM quantity, composition and quality (e.g., chl-a), is not an estimation of the meiofaunal contribution to OM. The methods we used in our paper are largely known as methods to extract the OM from sediments (see other references below), not to quantify the meiofauna as food source:

Gambi, Cristina, Cecilia Totti, and Elena Manini. "Impact of organic loads and environmental gradients on microphytobenthos and meiofaunal distribution in a coastal lagoon." Chemistry and Ecology2-3 (2003): 207-223.
Manini, Elena, et al. "Benthic microbial loop functioning in coastal lagoons: a comparative approach." Oceanologica Acta1 (2003): 27-38.
Fiordelmondo, C., Manini, E., Gambi, C., & Pusceddu, A. (2003). Short-term impact of clam harvesting on sediment chemistry, benthic microbes and meiofauna in the Goro lagoon (Italy). Chemistry and Ecology, 19(2-3), 173-187.

There are other methods that could probably be used for the scope mentioned by the Reviewer, for instance the stable isotope analysis. We rephrased and we used another word instead of availability of food, that could be misinterpreted. However, we cannot agree with the general comments of the Reviewer.

Reply: Following the Reviewer’s suggestion, we reworded some sentences in the main text to avoid any misunderstanding. However, we cannot agree with the Reviewer in stating that the food we quantified is food supply for macrofauna and fish and cannot be related to the meiofauna. Meiofauna is an important food source for higher trophic levels, agreed! But this is not what we quantified with our analysis (please, consider all the references we reported above). We rephrased availability and quality of organic matter, but we still believe that the conclusions of our work can be reliable. Please, refer also to all the papers we cited in our manuscript to support our work and our approach.

Annotation

Lines 23-26: “This study aims to characterize the meiobenthic communities inhabiting the Zwin tidal lagoon, located on the border between Belgium and The Netherlands, and to evaluate to what extent sedimentological characteristics and food quantity and quality influence meiofauna composition and distribution” – food quantity and quality was not estimated, but organic matter quantity and composition.

Reply: Following the Reviewer’s suggestion, we changed it accordingly (line 25).

Lines 31-35: Meiobenthos represents a multifunctional group with a tight network of biotic interactions. The loss of species or functional groups may provoke cascading effects that have implications for interacting species and ecosystem functioning. Therefore, anthropogenic activities linked to coastal exploitation and changes in sedimentation regimes can affect the complex connections between marine biodiversity, their environment, and related ecological functions. – Everything written is correct, but in the abstract I would like to see a brief listing of the results of THIS study. It would be more important to mention that the organic matter, even in those sediment layers that formed in the period preceding the restoration of sea tides, is of marine origin (due to the burrowing activity of polychaetes).

Reply: Following the Reviewer’s suggestion, we changed the abstract accordingly (lines 29-36).

The sentence about food availability needs to be removed from the annotation:

Line 30 - «Major structural differences in communities were related to … food availability».

Reply: We modified the sentence as “estimation of food quantity” (line 33).

Materials and Methods

2.1. Study area

Why did not indicate the time during which the site is flooded by high tide, how long it is open at low tide, and water salinity near these stations...

Reply: Following the Reviewer’s suggestion, we added the information to the main test (lines 108-109). Unfortunately, we did not measure the salinity at the sampling stations.

Figure 1. «Top left: geographical location of the studied area (i.e., red star)…» - And where is the red star? It is not visible.

Reply: the Reviewer is right here; we corrected the refuse in the Figure caption. We added a red square and geographical location of the studied area.

2.3. Sedimentological analysis

Lines 158-160: «The second half was sampled according to sedimentological characteristics with the top 4 cm subdivided into SUP samples (i.e., more oxidized and hydrated) and SUB samples (i.e., more compact and reduced)» – abbreviations SUP and SUB are not used further in the text or in the supplements.

Reply: The Reviewer is right here. To avoid any misunderstanding, we changed the sentence as “distinguished between more oxidized and hydrated samples and more compact and reduced samples” (lines 168-169).

2.8 Statistical analyses

Line 231: misprint: the PCA is to createS new multivariate variables – «…to create…».

Reply: Following the Reviewer’s suggestion, we corrected it accordingly.

Discussion and Conclusions

The discussion did not address what follows from the obtained data on the concentration of 15N and 13C isotopes and their distribution in the sediment cores.

Reply: Following the Reviewer’s suggestion, we addressed what we obtained from the data on 15N and 13C isotopes (lines 468-490).

4.2. The meiofaunal communities inhabiting the Zwin Natural Park: an anthropogenic created lagoon

Lines 592-593: “The meiofaunal community showed the characteristics of a well-established population dominated by nematodes, and secondary by copepods with their nauplii” – “Well-established” can only be stated if there are observations of dynamics over time. It cannot be ruled out that this is a community of temporarily settled explerent species, which cannot be said to be well-established at all.

Reply: Following the Reviewer’s suggestion, we deleted “well-established” (line 603).

Lines 596-598: “Major structural differences in the meiofaunal communities were related to the depositional processes and sedimentary environments characterizing the investigated stations and, among them, the availability of food” - remove references to food availability.

Reply: We replaced it with “estimated food quantity”, since the proxies we used to estimate the quantity of food correlated with the meiofauna (line 606).

Lines 599-600: «At the Zwin lagoon, the meiofaunal colonization was quick and abundant and even very weak tidal currents were sufficient to suspend and transport these animals to the new environment» - this is a very important result, which it is desirable to reflect in the abstract.

Reply: Following the Reviewer’s suggestion, we reported this result into the Abstract (Lines 30-36).

4.3. The influence of the depositional environment on the meiobenthos

Lines 612-613: “In this study, sedimentological variables (e.g., lithology, quantity and quality of food sources) again emerged as key factors influencing meiofaunal abundance and diversity” - remove references to food quality and its influence on meiofaunal abundance and diversity.

Reply: We replaced as “organic matter quantity and composition” as suggested above by the Reviewer (lines 621 and 629).

Lines 620-624: “These findings confirm that sedimentary dynamics and depositional processes, through their influence on sediment properties (e.g., grain size) and food quality (e.g., OM composition), shape meiofaunal communities and their vertical and horizontal distribution, as reported in other ecosystems e.g., [84; 90-91]” – remove references to food quality and that it shapes and drives meiofauna.

Lines 624-626: “Since food availability drives the vertical sediment of meiofauna, the high subsurface densities observed at St.1 and St.2 are likely attributable to the extensive availability of relatively fresh marine phytodetritus [92-93]” – subsurface meiobenthos densities cause high TOC.

Reply: following the Reviewer’s suggestion we reworded as OM quantity and composition. However, we do not agree with the fact that high TOC values are due to meiofauna, which constitute only a small fraction of total organic carbon compared to other components. The fact that organic matter quantity and quality, as we measured it, supports and shapes the meiofauna is reported in so many papers (please see all the references cited in the present paper) that we feel comfortable in supporting this hypothesis as well. And we also feel confident in our results.

Lines 638-639: “In conclusion, the hypothesis that depositional and sedimentological features, including availability and quality of OM, are reflected in the structure and distribution of meiofaunal community is once again confirmed” – see above.

Reply: following the Reviewer’s suggestion we reworded as OM quantity and composition.

I really hope that the article can be published in a corrected form.

Reply: We can understand the comments and doubts raised by the Reviewer, and we tried to reword some sentences. It is true that the meiofauna contributes to the C and N pool, even if for a small percentage (this should be kept in mind and considered in the interpretation of our results and discussion). However, we do not agree with the Reviewer’s critics and the large body of literature published so far in support of our methodological approach and results confirms what we stated in the paper. In our paper we included a set of analysis to quantify different components of organic matter, i.e., PRT, CHO and LIP (not only TOC) along with biopolymeric carbon PRT/CHO ratio and chl-a as proxies for the quality of the OM. This approach has been largely used and scientifically accepted (many papers published!!) as reliable for assessing the QUANTITY and QUALITY of organic matter into the sediments and to relate it to benthic communities. We acknowledge that total OM is not a direct measure of food availability for meiofauna, but we also measured chlorophyll-a and phaeopigments, which are widely recognized as reliable proxies of fresh detritus and algal-derived.

In summary what the Reviewer’s sustains, TOC = meiofauna, is not totally correct and this is not what we have measured in our paper. Even if the meiofauna can contribute to the TOC into the sediments, its fraction is very little compared to the microbial fraction and to the detritus! Indeed, it is not correct to say that the OM is actually the meiofauna. The correlation found by DistLM is a very common pattern reported in benthic systems (e.g., Neira et al., 2001, 2018; Pusceddu et al., 2016; Moccia et al., 2019 Advances in Oceanography and Limnology 10.1: 57-66.).

We hope the Reviewer will be open to understand our point of view supported by many References.

Round 2

Reviewer 4 Report

Comments and Suggestions for Authors

Although the authors did not agree with my main objection, it is enough for me that they took it into account. I hope that the need to separately account for the biomass of the meiobenthos and subtract it from the total organic pool will be taken into account in future studies.