Different coastal marsh sites reflect similar  topographic conditions under which bare  patches and vegetation recovery occur

Wang, Chen; Schepers, Lennert; Kirwan, Matthew L.; Belluco, Enrica; D'Alpaos, Andrea; Wang, Qiao; Yin, Shoujing; Temmerman, Stijn

doi:https://doi.org/10.5194/esurf-9-71-2021

Articles | Volume 9, issue 1

https://doi.org/10.5194/esurf-9-71-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esurf-9-71-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 9, issue 1

Research article

|

11 Feb 2021

Research article |

| 11 Feb 2021

Different coastal marsh sites reflect similar topographic conditions under which bare patches and vegetation recovery occur

Chen Wang, Lennert Schepers, Matthew L. Kirwan, Enrica Belluco, Andrea D'Alpaos, Qiao Wang, Shoujing Yin, and Stijn Temmerman

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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Comments to the manuscript "Different coastal marsh sites reflect similar topographic conditions for bare patches and vegetation recovery"', Anonymous Referee #1, 23 Jul 2020
RC2: 'Review of "Different coastal marsh sites reflect similar topographic conditions for bare patches and vegetation recovery"', Maarten Kleinhans, 26 Jul 2020
AC1: 'Response to reviewers', Chen Wang, 29 Oct 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Chen Wang on behalf of the Authors (05 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Nov 2020) by Orencio Duran Vinent

RR by Anonymous Referee #3 (11 Dec 2020)

Suggestions for revision or reasons for rejection

The manuscript presents an interesting study of the occurrence of bare patches in three contrasting saltmarshes and aims to elucidate how topographic settings determine marsh destruction or revegetation. The introduction and methodology are very clear and the manuscript is generally well-written. However, I have some concerns about the mechanisms proposed in the discussion and the generalizations illustrated in figure 8. I moreover suggest to go through the text to correct some minor unclear formulations and corrections.

1) Main comments:

The manuscript claims that revegetation was studied for three sites but actually only one site showed revegetation. The other two sites were not investigated since there didn’t occur revegetation as partly shown by previous studies. I suggest to rephrase more carefully in the abstract line 25 and the discussion line 345 that this is not a finding particular to this work but only a confirmation of previous studies.

The authors conclude that larger patches cause larger connecting channels but this is not clear from the findings. Alternatively, larger channels could also promote larger bare patches. I suggest to either rephrase the statement in the abstract line 21 and discussion line 358 or carry out an additional analysis tracking connected bare patch size and channel width through time to investigate what occurs first and determine which determines the other.

The discussion about the wave-induced resuspension is only valid for large microtidal marshes, such as Blackwater. I suggest to explore more on alternative hypotheses on tidal channel hydrodynamics (suggested references are in the detailed comments).

Figure 8 generalizes the two findings from Blackwater and Saeftinghe in one figure, which should be separated: wave resuspension within bare patches and revegetation do not co-occur in the presented systems and therefore should be illustrated separately. I suggest to simplify the schematic model showing bed elevation and likelyhood that a certain feature occurs and possibly link to tidal range. Since it is not clear what is the main driver of bare patch formation (waves, SLR, SSC) I would refrain from generalizing the findings in a figure such as presented.

2) Detailed comments:
3.1: I am missing a definition of how you define the difference between channel and connected bare patch/the boundary at which you define it as patch or channel?

Line 165- 167: ‘Field surveys only include selected 165 locations, but with greater vertical accuracy, especially for vegetated areas where LIDAR partially reflects on the vegetation canopy, and open water where LIDAR reflects on the water surface.’
It is not clear what this sentences means – do you mean accuracy is enhanced for the classes vegetated and open water?

Line 227: Please add a reference for the Mann-Whitney U test

Line 233: Please add a reference after ‘growth’

5.1.1
You refer in this paragraph to the peak of the distribution but I am not certain how to interpret this value. Please explain in one sentence on what the distribution and the peak show.

Line 277-278: Please rephrase the sentence, to me it is not clear what this means - between all comparisons of two of the 3 variables?

5.1.3
I do not understand this paragraph from the figure. Where do I see the difference between connected and unconnected bare patches in Fig. 6? Where are the values (% of area) visible in the figure?

Line 347 key point (3): Please be careful with the phrasing: This last point does not emerge from the presented results but was already described in previous publications (as you mentioned in line 220 Schepers et al, 2017). Only the second sentence was shown in this work.

Line 336: What about the unconnected patches?

Line 350: ‘high connectivity’: Do you refer here to the width of the connecting channel in fig.7c?

6.1
I think that the discussion focuses too much on the resuspension by waves but does not explore other optional hypotheses. For example, the time lag between incoming and outgoing tide can result in ebb-dominance in marshes and therefore lead to net sediment export through the channels (e.g. https://www.jstor.org/stable/25736162 or https://doi.org/10.1029/2019WR025942). I think this is a more elegant explanation for sediment export from the marsh and can explain the difference between Saeftinghe and Blackwater. This can furthermore be related to vegetation species, which are mentioned in the text but neglected in the discussion.

Moreover, I suggest to mention that waves are only relevant in microtidal marshes and negligible in the case of Saeftinghe, hence a possible explanation for the difference between the systems (see e.g. https://doi.org/10.1016/j.geomorph.2018.03.025).

I think that autocompaction and organic accretion by different vegetation species are also important phenomena to be mentioned.

Line 444-450: Please be more specific what you mean here. I am not sure what can be concluded from the above paragraph, what is the most likely explanation for large bare areas: SLR, tidal range or sediment supply? Or all these reasons?

3) Figures:
Fig. 3:
Please add in the caption if the small study area is the only data considered in the paper - if yes, it is not clear to me why the blue patches are excluded but the yellow ones are considered.

Fig.4 :
Please mention in figure caption again why there is no lidar data for bare patches at Blackwater and no field data for Saeftinghe

Fig. 6:
I got confused if both connected and unconnected patches were included. Only later I saw that the first class is unconnected channels. I think it would enhance clarity to add that in the figure by text but especially in the caption. Also, I miss the percentages mentioned in the text, maybe it is possible to highlight them.

Fig. 7:
Panel c is not clear to me: Why is 40% of pixels rapidly revegetated with a channel with of 0? I guess the two left data points are unconnected bare patches? I suggest to make that clear, maybe a scatter plot is more representative than a line plot since it is a limited amount of data and maybe separate connected/unconnected patches by a vertical line.

4) Textual comments:

a) Some of the wording I am not familiar with, such as the terms ‘overwash tide’ and ‘tidal frame’. Overwash is usually referred to as waves and I was not sure what the difference between tidal range and tidal frame was.
b) The use of the word ‘connectivity’ is arbitrary: do you mean connected/not connected or degree of connectivity through channel width? This should be defined in the introduction and possibly adjusted throughout the manuscript.
c) You mix the use of the word ‘feature’ and ‘category’ for the different classes vegetated, bar patches etc. (e.g. fig.4 caption) . Please be consistent.
d) I suggest to revisit the punctuation in the manuscript, specifically the use of commas.

4.1) Detailed comments:
Title: occurs = ‘occur’
Line 19: ‘distance from’
Line 151: ‘tidal range’ = ‘tidal amplitude’
Line 212: ‘method as for Saeftignhe’ ‘field elevation survey’: remove ‘elevation’
Line 231: ‘the LIDAR data’
Line 364: ‘on higher’= ‘at higher’
Line 405: ‘feedback’= ‘feed back’
Line 406: ‘remove second ‘may’ before ‘contribute’
Line 478: ‘indicative’ is used twice in a row; ‘recover from’
Caption figure 4: ‘exact numbers’ = ‘total numbers’

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ED: Publish subject to minor revisions (review by editor) (12 Dec 2020) by Orencio Duran Vinent

AR by Chen Wang on behalf of the Authors (22 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Publish as is (05 Jan 2021) by Orencio Duran Vinent

ED: Publish as is (12 Jan 2021) by Heather Viles (Editor)

AR by Chen Wang on behalf of the Authors (17 Jan 2021) Manuscript

Short summary

Coastal marshes are valuable natural habitats with normally dense vegetation. The presence of bare patches is a symptom of habitat degradation. We found that the occurrence of bare patches and regrowth of vegetation is related to spatial variations in soil surface elevation and to the distance and connectivity to tidal creeks. These relations are similar in three marshes at very different geographical locations. Our results may help nature managers to conserve and restore coastal marshes.