the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Sediment heterogeneity affects variability of resuspension-induced CO2 production
Abstract. Demersal fishing is a major anthropogenic disturbance to marine sediments, with global implications for benthic carbon cycling and greenhouse gas emissions. Resuspension of sediment organic carbon during bottom trawling enhances oxic mineralisation, converting stored organic matter into aqueous CO2 and reducing the long-term carbon storage potential of the seafloor. Sediment heterogeneity likely plays a role in the vulnerability of sedimentary organic carbon to resuspension, but spatial estimates of trawling-induced CO2 release from resuspended sediment rarely account for this heterogeneity. In this study, we conducted a large-scale survey in the Hauraki Gulf, New Zealand, to assess how sediment characteristics affect resuspension-induced CO2 production (RCO2P). Using a resuspension assay at 57 sites, we quantified RCO2P and it with measurements of sediment grain size, organic matter content and quality, and phytopigments. Boosted regression tree modelling revealed that organic matter content has the strongest influence, with a non-linear relationship to RCO2P and interaction effects with water depth and medium sand content. Vulnerability to CO2 release was highest in sediments with > 3 % organic matter and < 27 % medium sand, particularly at depths between 55 – 95 m. Our results demonstrate that sediment heterogeneity must be accounted for in regional assessments of seafloor carbon storage and disturbance impacts. For this, the resuspension assay offers a practical tool to empirically assess carbon storage vulnerability and can complement model-based approaches to inform spatial management of demersal fisheries.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-3045', Anonymous Referee #1, 04 Aug 2025
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AC1: 'Reply on RC1', Ines Bartl, 18 Sep 2025
We thank the reviewer for their thorough assessment of the manuscript which has drawn us to focus more clearly on the purpose of the work presented in this manuscript. With our study we aim to identify sediment types that are at highest risk of producing CO2 when resuspended and to identify what relationships and interactive effects of highly variable sediment characteristics influence the variability of resuspension-induced CO2 production. This forms the basis for upscaling the risk of resuspension-induced CO2 releases across heterogenous seafloor spaces to inform decision makers. We agree that for our purpose of this manuscript we need to explain more clearly what can be learnt from the Boosted regression tree (BRT) results in the context of identifying vulnerable sediment types and discuss potential underlying mechanisms. We address this by a thorough revision of the BRT analysis and a more detailed presentation of the results and expanding the discussion on potential mechanisms that could explain the spatial pattern of RCO2P in the Hauraki Gulf. Please find attached the point-by point responses and a a document detailing our BRT analysis revisions.
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AC1: 'Reply on RC1', Ines Bartl, 18 Sep 2025
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RC2: 'Comment on egusphere-2025-3045', Anonymous Referee #2, 25 Aug 2025
Bartl and Thrush carried out replicated manipulation experiments of superficial (3 cm) sediments obtained from a total of 57 sites in the Hauraki Gulf (New Zealand) characterized by varying edaphic characteristics to estimate CO2 release due to resuspension. The study poses foundation on an early manuscript (Barts et al. 2025 Meth Ecol Evol) describing the resuspension assay and how this could provide an estimate of the vulnerability of marine sediments ability to store carbon when exposed to bottom disturbance and applies it in the field at a basin scale.
Using machine learning methods, and after the removal of collinear explanatory factors, they identified organic matter (OM) content (but not its freshness), sand contents and water depth as the factors most influential on the CO2 release from the manipulated sediments. Based on these results and using scatter heatmaps, the authors identified values of the explanatory factors representing thresholds of vulnerability to CO2 release by sediment resuspension. According to those thresholds, a large portion of the sediments of the Gulf under scrutiny show from moderate to high attitude to releasing CO2 when disturbed. The authors conclude that the assessment of the vulnerability of marine sediments ability to store carbon must account for sediment heterogeneity. Ultimately, the authors prompt that sediment heterogeneity must be accurately considered when deploying plans of spatial management of demersal fisheries (one of the most disturbing anthropogenic factors on the sea bottoms).
The paper is well written and easy to read. The premises in the introduction and the logical flow that brought the authors to the study design are both clear. The methodology is clear as well (but see below for some missing details) and the data analysis conducted rigorously. The results highlight the most important insights obtained from the experiments. Nonetheless, some issues emerge dealing with the description and reliability (or possible biases) of the resuspension assay and with the discussion, which is limited and provides the analysis only a small portion of the other (not considered factors) possibly explaining (>40%) of the CO2 release variability. The discussion, as it is presented now, opens more questions than the answers (though credible) the study provides. I’m convinced that the effort in doing this study and the potential of the obtained results can be conceivably more properly and deeply discussed.
I’m convinced that this study deserves publication and the information it includes could become a starting point to include sediment heterogeneity among the factors to be considered when planning demersal fisheries, but it needs a larger effort to discuss more affordably the strengths and the weaknesses of the presented results.
Below I provide some suggestions to improve the manuscript.
The putative role of sediment heterogeneity on the effects of impacts (including the natural ones) affecting the sea bottom is somewhat obvious. This, however, has been not considered in deep by science nor policy making. Despite the authors declare it, a deeper and more accurate description of the potential implications of considering it is not anticipated in the introduction, nor extensively addressed in the discussion.
The manipulation method used for resuspending the sediment in this study derives from an early paper published by the same scholars’ team. Despite they prompted in the former article “Depending on the research question, we recommend trials with local sediments to determine optimal incubation times, core sizes, and sediment to water ratios”, no mention of these important data is provided in the manuscript. How much this affects the observed interactions? Moreover, other important details about the resuspension assay are missing, which obliges the reader to jump between the two papers for these details, that could be added in a supporting methodological file. This, obviously, does not prejudice the validity of this manuscript. Nonetheless, either in the former paper or in this manuscript, there is a possibly relevant bias: the energy employed to resuspend the sediment was apparently the same for all samples (“The jar was topped with filtered seawater, sealed airtight and gently inverted for 30 s. The sediment-water mixture was left to settle for 30 s, the jar was re-opened and the initial oxygen concentration was measured”). Considering the ample variety of grain size composition of the manipulated sediments, this could represent a possible uncontrolled simplification. This appears a possible not irrelevant bias: since sediment grain size is tightly related with its compactness, different sediments are differently impacted by the same amount of energy. This means that the simulated resuspension cannot represent the mechanic impacts of trawling on sediments with different characteristics. In this sense, the authors can use their results only to discuss the effects of a severe resuspension on sediments, but without providing a comparison of the energy used to simulate resuspension and that causing the resuspension by the “average” trawl, the results cannot be effectively translated into possible demersal fishery management plans accounting for sediment heterogeneity.
In the methodological paper the authors identify different RQ constants, but here they used only one. Is this correct? Is there any possibility to distribute better this constant?
The lack of a control with OC-free sediments would have helped to clarify whether the CO2 production measured in the resuspended microcosms depends only from OC mineralization. I see this now is unfeasibly corrected, but a possible bias from this should be acknowledged.
The authors used the OM:Phyto ratio as a proxy for OM freshness. They referred to a paper in which the authors, however, used the percentage fraction of phytopigments and biopolymeric C (which does not account for total OM). This, honestly, does not modify the significance of its “new” use, but should be acknowledged.
The lack of information about the composition (and origin) of the OM in the sediments makes difficult making any generalization. The authors acknowledge that different proportions in labile and refractory components could lead to different reactivity to osygen of OM. Nonetheless, this issue needs to be discussed more deeply.
Citation: https://doi.org/10.5194/egusphere-2025-3045-RC2 -
AC2: 'Reply on RC2', Ines Bartl, 18 Sep 2025
We thank the reviewer for their thorough assessment of the manuscript which has drawn us to focus more deeply on purpose of our study. We aim to identify sediment types that are at highest risk of producing CO2 when resuspended and to identify what relationships to highly variable sediment characteristics influence the variability of resuspension-induced CO2 production. This forms the basis for upscaling the risk of resuspension-induced CO2 releases across heterogenous seafloor spaces to inform decision makers. We will address the reviewer's suggestions by adding more details to the description of the resuspension assay and discussing the observed patterns of our results in more depth and a more balanced discussion on the strengths and weaknesses, and highlight the implications for spatial fisheries management more clearly. Please find attached our point-by point responses and a thorough revision of our BRT analysis.
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AC2: 'Reply on RC2', Ines Bartl, 18 Sep 2025
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Bartl and Thrush conduct experiments to gauge the amount of additional CO2 released from sediment disturbance using incubations of natural sediments sampled from Hauraki Gulf, New Zealand. The authors analyse their results using a machine learning method and find non-linear relationships and interaction effects between additional CO2 release and sediment characteristics. It is concluded that assessments of carbon storage vulnerability must account for sediment heterogeneity.
In general, the paper is well written, and the methodology clearly described. However, I found the current presentation and interpretation of results to be lacking. The fact that sediment heterogeneity needs to be accounted for when assessing carbon impacts is already well-established (and somewhat trivial), and the usefulness of the resuspension assay has already been introduced in the earlier work by Bartl et al. (2025). The results of the BRT model are interesting, but they are presented in a quite condensed manner, and it is not laid out clearly what exactly we can learn from them.
I do think the data collected and the experiments done are valuable and useful, but the discussion focuses almost exclusively on the BRT results, which are difficult to interpret, since such ML methods tend to obfuscate possibly straight-forward interactions and relationships. I encourage the authors to dig a bit deeper into their data through additional analyses and/or to present the BRT results in more detail, and to discuss the possible mechanistic explanations for the observed patterns. I list some specific suggestions below, along with other comments.
General comments:
Specific comments
References
Bartl, I., Evans, T., Hillman, J., Thrush, S., 2025. Simple assay quantifying sediment resuspension effects on marine carbon storage. Methods Ecol Evol 16, 309–316. https://doi.org/10.1111/2041-210X.14479.
Epstein, G., Middelburg, J.J., Hawkins, J.P., Norris, C.R., Roberts, C.M., 2022. The impact of mobile demersal fishing on carbon storage in seabed sediments. Glob Change Biol 28, 2875–2894. https://doi.org/10.1111/gcb.16105.
Hiddink, J.G., van de Velde, S.J., McConnaughey, R.A., Borger, E. de, Tiano, J., Kaiser, M.J., Sweetman, A.K., Sciberras, M., 2023. Quantifying the carbon benefits of ending bottom trawling. Nature 617, E1-E2. https://doi.org/10.1038/s41586-023-06014-7.
Kalapurakkal, H.T., Dale, A.W., Schmidt, M., Taubner, H., Scholz, F., Spiegel, T., Fuhr, M., Wallmann, K., 2025. Sediment resuspension in muddy sediments enhances pyrite oxidation and carbon dioxide emissions in Kiel Bight. Communications Earth & Environment 6, 156. https://doi.org/10.1038/s43247-025-02132-4.
Zhang, W., Porz, L., Yilmaz, R., Wallmann, K., Spiegel, T., Neumann, A., Holtappels, M., Kasten, S., Kuhlmann, J., Ziebarth, N., Taylor, B., Ho-Hagemann, H.T.M., Bockelmann, F.-D., Daewel, U., Bernhardt, L., Schrum, C., 2024. Long-term carbon storage in shelf sea sediments reduced by intensive bottom trawling. Nature Geoscience. https://doi.org/10.1038/s41561-024-01581-4.