Persistent effects of sand extraction on habitats and associated benthic communities in the German Bight

Mielck, Finn; Michaelis, Rune; Hass, H. Christian; Hertel, Sarah; Ganal, Caroline; Armonies, Werner

doi:https://doi.org/10.5194/bg-18-3565-2021

Articles | Volume 18, issue 12

https://doi.org/10.5194/bg-18-3565-2021

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

https://doi.org/10.5194/bg-18-3565-2021

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

Articles | Volume 18, issue 12

Research article

|

17 Jun 2021

Research article |

| 17 Jun 2021

Persistent effects of sand extraction on habitats and associated benthic communities in the German Bight

Finn Mielck, Rune Michaelis, H. Christian Hass, Sarah Hertel, Caroline Ganal, and Werner Armonies

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Final revised paper (published on 17 Jun 2021)
Supplement to the final revised paper
Preprint (discussion started on 20 Feb 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review bg-2020-17', Anonymous Referee #1, 12 May 2020
- AC1: 'BG-2020-17-responses_to_reviewer_#1', Finn Mielck, 11 Jun 2020
RC2: 'Persistent effects of sand extraction on habitats and associated benthic communities in the German Bight', Anonymous Referee #2, 24 May 2020
- AC2: 'BG-2020-17-responses_to_reviewer_#2', Finn Mielck, 11 Jun 2020

Peer-review completion

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

ED: Reconsider after major revisions (13 Jul 2020) by Lennart de Nooijer

AR by Finn Mielck on behalf of the Authors (04 Aug 2020) Author's response Manuscript

ED: Reconsider after major revisions (28 Aug 2020) by Lennart de Nooijer

ED: Referee Nomination & Report Request started (27 Oct 2020) by Lennart de Nooijer

RR by Anonymous Referee #1 (09 Nov 2020)

RR by Anonymous Referee #3 (11 Nov 2020)

Suggestions for revision or reasons for rejection

The authors have gone a good length to improve the manuscript following constructive and detailed comments provided by reviewers 1 and 2. Improvements include a more elaborate description of the context of this study including previous work done in the area, a better definition of research questions for this study, better explanation of the methods, and more elaborate discussion on the longer term expectations regarding recovery to the original state at the sand extraction pits.

However, whilst both reviewers recommended to elaborate on the benthic communities established in relation to the different habitats formed as a result of dredging, which is probably the most interesting aspect of this study, the authors to my opinion could have done much better than the minimalistic 16 lines of text in results section 3.2 dealing with benthos, and statistical analysis merely addressing differences in benthos between more recent and past disturbance versus undisturbed state. In the discussion the authors state that the species composition of the macrozoobenthic infauna changed according to sediment composition in the dredging pits, but this is not substantiated by any direct analysis of fauna versus sediment characteristics.

An underlying problem in the analysis of habitats is that habitat mapping based on hydroacoustics (illustrated in Fig. 2 and 4) seems to distinguish a much greater habitat heterogeneity in the dredging pits, comprising patches of coarse sand, fine sand and mud, than the uniform mud drape which is described in the text. To me, this gives the impression that the hydroacoustics often fail to distinguish the mud drape, but rather record the acoustically more reflective sand present below the mud. This would need to be properly acknowledged.

Regarding the hydroacoustically delineated habitats as shown in Fig. 4, no mention is made of seafloor relief in the study area except the dredging pits, even though a quick glance at the bathymetric and acoustic backscatter maps in Fig. 2 is sufficient to note a distinct coincidence of features of seabed relief and backscatter in the seabed areas surrounding the dredging pits. To me these features seem to represent low dunes composed of fine sand, lying on top of a coarse-grained Pleistocene sand which is outcropping in troughs in between the dunes. And the pattern of alternating dunes and troughs seems to extend also into the older dredging pits, especially in the northern part of the study area. This makes me wonder if the so-called “Wanderfeinsand” is actually moving across the shallower dredging pits, making habitats at these sites more dynamic than suggested by the authors.

Regarding ground truthing of hydroacoustically delineated habitats, it does not help that the miniature pie charts presented in Fig. 4 are concealing the habitat present at the sampling location, and that some of the pies are overlapping.

In the attached document, I have included a large number of smaller mostly textual comments, and also some repeating of more general points of criticism raised above.

Referee Report: PDF

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ED: Reconsider after major revisions (27 Jan 2021) by Lennart de Nooijer

AR by Finn Mielck on behalf of the Authors (24 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (25 Mar 2021) by Lennart de Nooijer

AR by Finn Mielck on behalf of the Authors (26 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (06 Apr 2021) by Lennart de Nooijer

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Short summary

Marine sand mining is becoming more and more important to nourish fragile coastlines that face global change. We investigated the largest sand extraction site in the German Bight. The study reveals that after more than 35 years of mining, the excavation pits are still detectable on the seafloor while the sediment composition has largely changed. The organic communities living in and on the seafloor were strongly decimated, and no recovery is observable towards previous conditions.