Sea level rise in a coastal marsh: linking increasing tidal inundation, decreasing soil strength and increasing pond expansion

Schepers, Lennert; Huyzentruyt, Mona; Kirwan, Matthew L.; Guntenspergen, Glenn R.; Temmerman, Stijn

doi:https://doi.org/10.5194/egusphere-2025-2362

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https://doi.org/10.5194/egusphere-2025-2362

Preprints

26 Jun 2025

| 26 Jun 2025

Sea level rise in a coastal marsh: linking increasing tidal inundation, decreasing soil strength and increasing pond expansion

Lennert Schepers, Mona Huyzentruyt, Matthew L. Kirwan, Glenn R. Guntenspergen, and Stijn Temmerman

Abstract. Coastal marsh conversion into ponds, which may be triggered by sea level rise, is considered an important driver of marsh loss and their valuable ecosystem services. Previous studies have focused on the role of wind waves in driving the expansion of interior marsh ponds, through lateral erosion of marsh edges surrounding the ponds. Here, we propose an additional feedback between sea level rise, increasing marsh inundation, and decreasing marsh soil strength, that further contributes to marsh erosion and pond expansion. Our field measurements in the Blackwater marshes (Maryland, USA), a micro-tidal marsh system with organic-rich soils, indicates that (1) an increase in tidal inundation time of the marsh surface above a certain threshold (around 50 % of the time) is associated with a substantial loss of strength of the surficial soils; and (2) this decrease in soil strength is strongly related to the amount of belowground vegetation biomass, which is also found to decrease with increasing tidal inundation at pond bottoms, the soil has a very low strength. Our finding of decreasing marsh soil strength along a spatial gradient of increasing marsh inundation coincides with a gradient of increasing historical mash loss by pond expansion, suggesting that feedbacks between sea level rise, increasing marsh inundation and decreasing marsh soil strength contribute to amplify marsh erosion and pond expansion.

Received: 20 May 2025 – Discussion started: 26 Jun 2025

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Lennert Schepers, Mona Huyzentruyt, Matthew L. Kirwan, Glenn R. Guntenspergen, and Stijn Temmerman

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-2362', Anonymous Referee #1, 24 Jul 2025
Schepers et al. look to link tidal inundation from sea-level rise to decreasing soil strength in salt marshes, which can be related to the loss of belowground biomass from the increasing tidal inundation. The findings of this paper make sense based on what we know from previous work; however, the methods used here have issues/limitations that need to be addressed. There are also other variables that could impact soil strength in these marshes that are not discussed. Additionally, I believe the authors undervalue previous marsh work in the investigation of tidal inundation on soil strength.
Major Comments:
Much of the paper’s discussion and findings is based on the use of a shear vane device to interpret soil stability. This includes findings that belowground biomass increases soil strength. However, I think the authors need to acknowledge the issues related to using a shear vane as an indicator of soil stability, including that roots within the soils will directly impact the vane measurements. Hence it would make sense that there is a relationship between belowground biomass and soil/root strength if roots impact these measurements. Vane measurements can also be impacted by how fast the vane is rotated, which can vary across users.

The paper also is missing context as to what the differences in soil strength really mean between sites. From a soil strength perspective, is the difference in strengths depicted in Figure 3 and Figure 5 really large enough to generate significant differences in the erodibility of the soil? And how do these differences compare to other studies? Comparing Figure 3 to the penetrologger data in Figure 4, the penetrologger differences are much larger than the vane differences so how does this fit into the narrative? The penetrologger data is not used in this paper as much as the vane data.

For the penetrologger profiles in Figure 4, which have the max strength near the surface, are there other factors other than roots that can also contribute to these differences along depth? For example, how does soil moisture change downcore? You only provide soil moisture data for the topsoil. Where is the BD and OM data? Also, although you may no longer have live roots at deeper depths, you would expect to see dead roots along the profile that should also impact soil strength.

There are other variables not mentioned by the authors that could impact soil strength rather than just belowground biomass, including potential differences in bioturbation and differences in grain size distributions. There are also other factors other than tidal inundation that impact belowground biomass, including nutrient loadings, which can vary even along a marsh site.

Data needs to be made publicly available either through supplemental materials or a data repository before this paper can be published.

Minor Comments:
Sea-level rise should be hyphenated throughout.

Figure 1 can be improved by 1) making the symbol size larger (the symbols in 1b are too small); 2) making some of the text larger, including the labels of a, b, and c in the Figures; 3) adding in a symbol to indicate direction (e.g. a north arrow); 4) correcting the a,b,c labels in the figure caption to be consistent with the labeling in the figures. Also check with the journal requirements to see what type of labeling is required.

Line 107 – yr-1 should be made a superscript and applied throughout the rest of the manuscript.

Also line 107 – Has the rate of SLR changed over time?

Section 2.3. – Later you bring up measurements down to 80 cm but you don’t mention this anywhere in this section.

Section 2.4. heading should be changed to include soil C

Why only collect soils to 15cm depth when you measure soil strength to 30cm?

How much time was soil ashed at 550 to determine organic content?

Organic content is barely mentioned in the manuscript

Table 1 – is the font in the last line of the caption different from the rest of the caption? It looks smaller to me.

Table 2 – font size of this table looks different from the other tables.

Figure 3 – the gray and yellow coefficients are hard to read and all coefficients can be made larger.

Line 223 – unclear what difference the authors are referring to.

Figure 5 – Why are you comparing pond soils and bare patches to just the 30cm marsh depth rather than the 15cm marsh depth?

Line 260 – Do studies that examine soil strength with tidal inundation along a marsh elevation gradient not count?

Line 261 – seems like a word or two is missing here.
Citation: https://doi.org/10.5194/egusphere-2025-2362-RC1
- AC1: 'Reply on RC1', Mona Huyzentruyt, 27 Aug 2025
  
  Dear reviewer,
  We would like to thank you for your very thorough review of the manuscript. We have addressed all comments in the supplement here.
  Kind regrads,
  Mona Huyzentruyt
  
  Citation: https://doi.org/10.5194/egusphere-2025-2362-AC1
RC2:
'Comment on egusphere-2025-2362', Anonymous Referee #2, 25 Jul 2025

In their manuscript Sea level rise in a coastal marsh: linking increasing tidal inundation, decreasing soil strength and increasing pond expansion, Schepers et al. highlight an interesting mechanism – previously demonstrated experimentally – by which sea level rise and associated increases in tidal inundation may promote pond expansion and reduce vegetated marsh area via declines in soil strength, likely driven by reduced belowground biomass. I found the manuscript engaging and generally well written. However, I have several major and minor comments that I strongly encourage the authors to address. In particular, the observational nature of the study – and its inherent limitations – along with the analytical approach, require clearer justification and discussion.
MAJOR COMMENTS
1) The study is observational and conducted in a specific system (microtidal marsh with organic-rich soils), which limits causal inference. This is important to emphasize when comparing with other marsh types and when interpreting findings. Please add a paragraph in the Discussion outlining the study's limitations and potentially suggesting next steps. For instance, in L121-122, while site-specific elevation is acceptable, elevation-driven variation may influence soil strength. Unlike experimental approaches that isolate variables, your study interprets a natural gradient with inherent co-variation. This is valuable, but should be framed as such.
2) Your replication is at the site level, but the 5 locations within sites are treated as independent replicates. Thus, your statistical inference (but see Comments 3 and 4) is confounded with site. While I recognize the logistical constraints of field ecology, please acknowledge this in your limitations section (see Comment 1) and clarify the implications for interpreting your results.
3) There is no dedicated paragraph describing your statistical analysis. Readers need a clear overview of how hypotheses were tested, which variables were used, whether data were averaged, and which models were applied. Please add a paragraph detailing your statistical approach.
4) Related to the above: while correlation may be suitable for Fig. 3, hydroperiod is unlikely to be a response variable influenced by belowground biomass or soil strength. Therefore, regression would be more appropriate to suggest directional relationships in Fig. 2. If you intentionally chose correlation, please explain why. See also Comment 3 regarding the missing statistical analysis section.
RELATIVELY MINOR COMMENTS
5) L14: Replace "method" with "mechanism"; rephrase the sentence accordingly.
6) L14 and L51: Define "soil strength" clearly at first appearance in both the abstract and main text.
7) L69-70: Clarify what is meant by "stable marsh system." Do you mean a system not subject to sea level rise?
8) L70: The phrasing suggests a direct link between soil strength, marsh loss, erosion, and pond expansion. Please revise to reflect the uncertainty of these associations.
9) L72: "Microtidal" should be hyphenated or not consistently throughout. Ensure consistency in terminology across the manuscript.
10) L82-84: Rephrase this section to introduce the experimental design first (number of sites, selection criteria) before referring to site numbers.
11) L88-93 (Figure caption): (1) Capitalization of letters should match figure; (2) use "panel" instead of "figure" for subplots; (3) explain the inset; (4) clarify data source for marsh loss values.
12) L97-98: If you use cardinal directions, mark them on the figure. Otherwise, use terms like "left" or "right."
13) L104: Acknowledge limitations of space-for-time substitution briefly here and expand in Discussion (see Comment 1).
14) L125: Instead of using site numbers, use ecologically meaningful descriptors, e.g., "high-inundation site."
15) L128: Clarify the phrase "5 in each of four categories". This was initially unclear. Introduce categories earlier.
16) L134-135: Remove sentence about north/south pond sampling. It's not in Fig. 1B and feels out of context. This should be explained later when reporting on data collection.
17) L150: Clarify what "marsh point" means. Was there one measurement per location (n=20), or five per location (n=100)? Specify.
18) L158: You sampled soil strength to 30 cm but only harvested biomass to 15 cm. Explain this methodological choice.
19) L162: Again, clarify what "each point" refers to (see Comment 17).
20) L174: Use consistent past tense throughout the Results.
21) L170-171: Briefly describe what red, white rhizomes, etc., are rather than only citing a source. What do they signify ecologically?
22) L177-178 and Table 1: Clarify how hydroperiod (% inundation) was measured and whether values vary within sites. Add standard deviations for variables with within-site variation, as in Table 2. Also, this statement cites Fig. 2, which shows biomass, not hydroperiod, so the reference may be misplaced.
23) L186 and L191: For L191, correlation makes sense when focusing on sites 2-4. For L186, correlation across all sites obscures the non-linear relationship (increase then decrease across hydroperiod). Suggest describing the pattern visually instead and removing the correlation. Please, see my Comment 4 as well.
24) Figures 2 and 3: Why are correlation lines and Pearson coefficients shown only in Fig. 3? Ensure consistency. Also, adjust axes to a 1:1 ratio to avoid overemphasizing y-axis variation. This may mislead interpretations (e.g., Fig. 3b).
25) L206: Remove. Claiming weaker correlations requires a test for slope differences through e.g. a regression-type analysis. Pearson values alone are insufficient, and the differences are not clearly meaningful (please, see my Comment 4 as well).
26) L215: Clarify whether p-value is exactly <0.05 or a general threshold. With such a strong correlation, p should be lower. Inconsistent p-value reporting across the manuscript (sometimes 0.0001 has been used). See Comment 3 on need for a statistical methods section.
27) L232: Figure reference is unclear. In Fig. 5, pond and vegetated marsh soil strength seem similar (e.g., same significance letters). Yet you also refer to Fig. 3a. Clarify which figure supports which comparison.
28) L259: Revisit earlier comments (especially Comment 1) to ensure limitations and uncertainty are clearly reflected in the Discussion.
29) L260-261: Sentence is incomplete or missing a word. Please revise.
30) L264: Remove redundant citation – already referenced at the beginning of the sentence.
31) L300: Depth-related variation affects more than just belowground biomass. Ensure other variables are considered in the limitations paragraph (see Comment 1).
32) L305-308: Define jargon (e.g., "undercutting," "cantilever failures") or replace with simpler language. Ensure accessibility to non-specialist readers.

Citation: https://doi.org/10.5194/egusphere-2025-2362-RC2
- AC2: 'Reply on RC2', Mona Huyzentruyt, 27 Aug 2025
  
  Dear reviewer,
  We would like to thank you for your very thorough review. We have addressed all comments in the suppelement.
  Kind regards,
  The authors
  
  Citation: https://doi.org/10.5194/egusphere-2025-2362-AC2

Lennert Schepers, Mona Huyzentruyt, Matthew L. Kirwan, Glenn R. Guntenspergen, and Stijn Temmerman

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Short summary

In some tidal marshes, vegetation can convert to ponds as a result of sea level rise. We investigated to what extent this is related to decreasing strength of the marsh soil in relation to sea level rise. We found a reduction of marsh soil strength in areas with more inundation by sea water and more ponding, which results in easier erosion of the marsh and thus further expansion of ponds. This decrease in marsh soil strength is highly related to lower content of roots in the soil.


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