the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jun 2025
Mapping organic carbon vulnerable to mobile bottom fishing in currently unfished areas of the Norwegian continental margin
Abstract. Organic carbon stored in continental margin sediments might be at risk by widespread mobile bottom fishing, potentially leading to reductions of organic carbon stocks, increased ocean acidification, additional atmospheric carbon dioxide emissions and a reduction of the buffering capacity of the ocean. Spatially explicit studies that have been conducted to inform marine management have so far looked at organic carbon stocks that have already been affected by mobile bottom fishing. Here, we focus instead on areas on the Norwegian continental margin that are currently not fished, based on fishing data covering the years 2009–2020. We estimate that the surface sediment layer (0–2 cm) in unfished areas covering 765,600 km2 contains 139.2 Tg of organic carbon. Based on data from a meta-analysis of demersal fishing impacts on organic carbon density, we estimate that 16.4 Tg (1.8–29.6 Tg) of organic carbon might be vulnerable to mobile bottom fishing in a scenario where each grid cell is fished evenly over the entire area and down to the full depth of the surface layer. Approximately one third of the vulnerable organic carbon is currently located in existing area-based protection measures. Additional protection could be guided by hotspots of vulnerable organic carbon, which are exclusively found in the Barents Sea. We argue that the protection of vulnerable organic carbon that is at high risk of being lost e.g. in areas becoming accessible to fishing due to sea ice retreat such as in the northern Barents Sea should be given a high priority.
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Status: open (until 15 Aug 2025)
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RC1: 'Comment on egusphere-2025-2159', Anonymous Referee #1, 03 Jul 2025
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I have reviewed an earlier version of this manuscript once before for another journal. I can see that the authors have addressed a few of my original concerns in this new submission, but not others. Therefore, I have repeated my remaining comments below, along with a few new ones.
In general, I find this to be an interesting and relevant study that adds to the ongoing discussion on seabed carbon management, and I believe it will be useful for many researchers and policymakers interested in this topic. I am not an expert on the machine learning method used in the mapping, but I tend to believe it is trustworthy since it is based on previously published studies. Nevertheless, I do have one general criticism related to the impact calculation (see general comment #1) and a few issues with the interpretation of the results in relation to management recommendations (see general comments #2+#3). I hope that the authors can alleviate my concerns by expanding/adapting the manuscript.
General comments:
- The authors estimate bottom fishing impacts on carbon based on (1) assumptions about resuspension and settling and (2) measured changes in carbon density, and they treat both as separate, independent effects. In reality, however, the two are intrinsically linked: Sediment disturbance partly oxygenates organic carbon during resuspension and mixing, leading to enhanced remineralization, and partly winnows fine particles following resuspension. Both can lead to a decreased carbon content. In other words, the effects of sediment resuspension may already be contained in the measured carbon losses (i.e., Eq. (5) is not quite correct, because δρOC is a function of δd). I suspect that treating carbon loss and resuspension separately may cause an overestimation of the net effect. Though the effects are difficult to separate without dedicated process-based modelling, I recommend the authors discuss this and associated uncertainties by showing both effects separately, i.e., what the effects of only resuspension vs. only carbon loss are.
- The authors base much of their discussion and recommendations around the estimated “recovery time”, i.e. the time until the prior sediment carbon stock has been reached following one bottom fishing event (SVR=1), but I question whether this is really a meaningful metric for spatial management. In my view, when it comes to seabed carbon management, recovery of accumulation rate is more relevant, that is, how quickly (if at all) the seafloor can regain its function as a carbon sink following protection. Perhaps the authors can clarify this issue in their discussion by distinguishing these separate aspects of (1) standing stock depletion and (2) sequestration rate reduction, in relation to potential climate and ecosystem impacts (see also my detailed comment #5).
- I would like to see a discussion of “loss” due to (1) resuspension and transport, which does not necessarily affect the seabed’s overall capacity for carbon storage/sequestration and is therefore not as relevant for climate or ocean acidification, vs. (2) loss due to remineralization, which is more relevant and is what most previous studies on the topic were concerned with. Right now, this is only mentioned in the introduction, and while I understand that estimating remineralization following disturbance might be beyond the scope of this study, I do recommend that the authors at least address the issue in their discussion in relation to their own results.
Detailed comments:
- 191: consider making this a numbered equation.
- 289-290: As I understood from 2.7, carbon densities were calculated from OC content and DBD according to Eq. (3). Doesn’t the impact on OC densities then simply follow from the decrease in carbon content in this case, with no significant influence from bulk density? The way it is presented now reads as though the impact on OC density has been confirmed independently. The same is repeated in 338-339. While it makes sense to focus on OC density for gauging overall impacts, I find the way the meta-analysis is presented now to be misleading and I recommend removing the statistical analysis for impacts on OC density.
- 340-349: The distinction between density and concentration is important and I agree with the authors’ call for consistency in terminology. I also agree that bulk density should be reported. However, I disagree with the framing in l.340-343, which implies that previous assessments of bottom fishing impacts may be erroneous for not distinguishing density and concentration, and that this study is the first to show depletion of carbon stocks by bottom fishing. As far as I am aware, previous studies have accounted for porosity/bulk density when calculating stocks (e.g. Zhang et al., 2024, Nat. Geosci.) Though changes to bulk density by bottom fishing have not been accounted for in previous studies, bottom fishing does not seem to change the bulk density of the sediment according to the authors’ own results (Table 1), meaning that change to carbon concentration is indeed the driving factor behind carbon stock depletion by bottom fishing. Therefore, I don’t think the authors have really shown that impacts on bulk density need to be accounted for when gauging bottom fishing impacts on carbon stocks. Their results rather imply the opposite (which would also be a useful result). In any case, I feel this section should be toned down or clarified.
- 362-366: It may be useful to consider the total area considered for UK and Norway here and calculate OC disturbance/area to make a more meaningful comparison.
- 397-405: As per my general comment, I question the utility of the “recovery”-concept when it comes to seabed carbon management. In my understanding, the difference between “recoverable” and “irrecoverable” seabed carbon is that accumulation rates are much lower for irrecoverable stocks. However, carbon lost/remineralized from a “recoverable” site will have the same potential climate impact as carbon lost from an “irrecoverable” site. The net flux will not change just because one site has active accumulation and the other does not; net loss over time is the same in both cases. The distinction would only be important if future carbon accumulation rates also changed as a consequence of past bottom fishing, e.g. due to changes to benthic community structures and associated ecosystem functions, or if future carbon accumulation was somehow otherwise limited, e.g. by accommodation space (unlikely considering sea level rise).
- 4: the choice of different color mappings for the three scenarios makes it difficult to compare them among each other. The authors may consider choosing intervals that can be used for all three panels (perhaps using non-linear scaling/intervals).
Citation: https://doi.org/10.5194/egusphere-2025-2159-RC1
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