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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-328
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-328
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  21 Sep 2020

21 Sep 2020

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This preprint is currently under review for the journal BG.

Impact of bottom trawling on sediment biogeochemistry: a modelling approach

Emil De Borger1,2, Justin Tiano2,1, Ulrike Braeckman1, Adriaan D. Rijnsdorp3, and Karline Soetaert2,1 Emil De Borger et al.
  • 1Ghent University, Department of Biology, Marine Biology Research Group, Krijgslaan 281/S8, 9000 Ghent, Belgium
  • 2Royal Netherlands Institute of Sea Research (NIOZ), Department of Estuarine and Delta Systems, and Utrecht University, Korringaweg 7, P.O. Box 140, 4401 NT Yerseke, the Netherlands
  • 3Wageningen Marine Research, Wageningen University & Research, IJmuiden, the Netherlands

Abstract. Bottom trawling in shelf seas can occur more than 10 times per year for a given location. This affects the benthic metabolism, through a mortality of the macrofauna, resuspension of organic matter from the sediment, and alterations of the physical sediment structure. However, the trawling impacts on organic carbon mineralization and associated processes are not well known. Using a modelling approach, the effects of increasing trawling frequencies on early diagenesis were studied in five different sedimentary environments, simulating the effects of a deep penetrating gear (e.g. a tickler chain beam trawl) and a shallower, more variable penetrating gear (e.g. an electric pulse trawl). Trawling events strongly increased oxygen and nitrate concentrations in surface sediment layers, and led to significantly lower amounts of ammonium (43–99 % reduction) and organic carbon in the top 10 cm of the sediment (62–96 % reduction). As a result, total mineralization rates in the sediment were decreased by up to 28 %. The effect on different mineralization processes differed both between sediment types, and between trawling frequencies. The shallow penetrating gear had a slightly smaller effect on benthic denitrification than the deep penetrating gear, but there were no statistically different results between gear types for all other parameters. Denitrification was reduced by 69 % in a fine sandy sediment, whereas nitrogen removal nearly doubled in a highly eutrophic mud. This suggests that even relatively low penetration depths from bottom fishing gears generates significant biogeochemical alterations. Physical organic carbon removal through trawl-induced resuspension of sediments, exacerbated by a removal of bioturbating macrofauna, was identified as the main cause of the changes in the mineralization process.

Emil De Borger et al.

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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.
Bottom trawling alters benthic mineralization: the recycling of organic material (OM) to free...
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