Impact of bottom trawling on sediment biogeochemistry: a modelling approach

De Borger, Emil; Tiano, Justin; Braeckman, Ulrike; Rijnsdorp, Adriaan D.; Soetaert, Karline

doi:https://doi.org/10.5194/bg-18-2539-2021

Articles | Volume 18, issue 8

https://doi.org/10.5194/bg-18-2539-2021

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

https://doi.org/10.5194/bg-18-2539-2021

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

Articles | Volume 18, issue 8

Research article

|

22 Apr 2021

Research article |

| 22 Apr 2021

Impact of bottom trawling on sediment biogeochemistry: a modelling approach

Emil De Borger, Justin Tiano, Ulrike Braeckman, Adriaan D. Rijnsdorp, and Karline Soetaert

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Final revised paper (published on 22 Apr 2021)
Supplement to the final revised paper
Preprint (discussion started on 21 Sep 2020)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of "Impact of bottom trawling on sediment biogeochemistry: a modelling approach"', Sarah Paradis, 27 Oct 2020
- AC1: 'Reply to reviewer 1', Emil De Borger, 09 Dec 2020
RC2: 'Review of "Impact of bottom trawling on sediment biogeochemistry: a modelling approach"', Antonio Pusceddu, 12 Nov 2020
- AC2: 'Reply to reviewer 2', Emil De Borger, 09 Dec 2020

Peer-review completion

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

ED: Reconsider after major revisions (11 Dec 2020) by Aninda Mazumdar

AR by Emil De Borger on behalf of the Authors (16 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (12 Jan 2021) by Aninda Mazumdar

RR by Sarah Paradis (20 Jan 2021)

Suggestions for revision or reasons for rejection

This paper presents novel results on the effects of the disturbance of bottom trawling on the seafloor, and sheds light on the effects in the mineralization pathways of organic matter on trawling grounds. The authors have addressed most of the questions and comments, and the quality of the revised manuscript has notably improved in this second version. Nevertheless, there are a few key aspects that should still be addressed.

Main comments:
1. Abstract: Since penetration depth of tickler chain and electric pulse trawl are not significantly different (Fig. 2C), please refrain from using “deeper” and “shallower” descriptions of both gear types in the abstract. Instead, I would suggest simply stating “two gear types with contrasting degrees of disturbance of the seafloor”.
2. Line 118-119: Using the measured DIC flux as the upper boundary organic carbon input flux. How can the measured DIC flux be used to estimate OC input? When you refer to OC flux, do you refer to particulate OC or dissolved OC? This sinusoidal carbon flux was estimated using what data? Is it a real representation of carbon input to the seafloor? Why use an amplitude of 1?
3. The varying OC flux in the different stations is not addressed in the methods. In lines 137 – 139. “A sinusoidally varying carbon deposition flux with the model derived carbon flux (Cflux, Table 3) as the annual average, and imposing an amplitude of 1 was used as the upper boundary organic carbon flux (Figure 2 A). This resulted in differing organic carbon fluxes for each location.” However, this refers to the temporal variations in OC flux, and not the spatial variations. Could you please specify how you obtained the OC flux stated in Table 3?
4. At the beginning of the discussion (4.1 Organic carbon depletion) you mention that trawling removes OC through erosion, and that less OC is redistributed due to the absence of bioturbation. However, bottom trawling also mixes up sediment, which you consider in section 2.1.3 Disturbance modelling. Please include all of these aspects in this section of the discussion: trawling-derived erosion, mixing, and reduced bioturbation. In your disturbance simulation, the effect of the removal of bioturbators is far greater than the sediment mixing effect of bottom trawlers, which I consider is an important aspect to include in this section. To further add to this discussion, would the sediment mixing effect caused by bioturbators and bottom trawling be similar with an increasing trawling frequency? In certain fishing grounds, bottom trawling often occurs on a daily basis (i.e. Catalan margin).
5. The effect of grain size and permeability in mineralization pathways are mainly described in section 4.2 Changes to mineralization pathways, lines 345-358. However, these effects are, in my opinion, unnecessarily repeated in section 4.3 Reducing gear penetration depth, lines 389-400. I highly suggest to discuss the effect of grain size and permeability only in section 4.2, and avoid going into detail in section 4.3, since this complicates the comprehension of the latter section. Moreover, I would suggest to change the section title to “Reducing gear disturbance on the seafloor”, since it more appropriately describes the content of the section.
6. Lines 421-422. A better citation to “Site-specific conditions such as rates of biogeochemical recovery and sedimentation rates need to be known to determine the resilience of ecosystems to trawling, and fine-tune management plans” would be Paradis et al. (2021), Geophysical Research Letters, that addresses the effect of seasonal trawling closures in organic matter content of trawling grounds combined with the sedimentological characteristics.
7. Code availability. Please upload your code and data to Github or a similar repository.

Technical corrections
Line 53-54: “Observed biogeochemical changes caused by sediment resuspension can lead to the instantaneous release of nutrients from the sediment into the water column”. Please add a reference to this release of nutrients from the sediment into the water column. For instance, one of the findings of the INTERPOL project published by Durrieu de Madron et al., 2005, Continental Shelf Research, studies this.
In line 225, “In FineL, MudL and MudH the ratio of labile organic carbon (FDET) to semi-labile organic carbon (SDET) increased between 25 and 34 % (Figure S3, supplement)”. However, Figure S3 actually refers to FDET / (FDET + SDET). Moreover, in lines 83-84, you define SDET as slow decaying fraction, while here it is defined as semi-labile organic carbon (as well as in line 236). Please maintain consistency in the nomenclature.
Lines 207-210. Please add the mineralization pathways of the Coarse sediment station, as you do for the remaining stations.
Line 262. “[…] at 4 out of 5 stations”. Instead of phrasing it this way, say “at all stations except MudH”, or something along those lines. It is easier for the reader to follow.
Line 306. Add that these impacts could be far greater in deep-seafloor environments. Similar studies as those explained in this study should be conducted.
Line 313. Mengual et al., 2016 addresses trawling in shallow environments, and not in the deep-sea.
Line 332. “(, Figure 7)”. Erase the “, “ when citing the figure.

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ED: Publish subject to minor revisions (review by editor) (23 Jan 2021) by Aninda Mazumdar

AR by Emil De Borger on behalf of the Authors (05 Feb 2021) Manuscript

ED: Publish as is (02 Mar 2021) by Aninda Mazumdar

AR by Emil De Borger on behalf of the Authors (12 Mar 2021)

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Short summary

Bottom trawling alters benthic mineralization: the recycling of organic material (OM) to free nutrients. To better understand how this occurs, trawling events were added to a model of seafloor OM recycling. Results show that bottom trawling reduces OM and free nutrients in sediments through direct removal thereof and of fauna which transport OM to deeper sediment layers protected from fishing. Our results support temporospatial trawl restrictions to allow key sediment functions to recover.