Preprints
https://doi.org/10.5194/egusphere-2024-399
https://doi.org/10.5194/egusphere-2024-399
13 Feb 2024
 | 13 Feb 2024
Status: this preprint is open for discussion.

Quantification and mitigation of bottom trawling impacts on sedimentary organic carbon stocks in the North Sea

Lucas Porz, Wenyan Zhang, Nils Gerrit Christiansen, Jan Kossack, Ute Daewel, and Corinna Schrum

Abstract. The depletion of sedimentary organic carbon stocks by use of bottom-contacting fishing gear and the potential climate impacts resulting from remineralization of the organic carbon to CO2 have recently been heavily debated. In this study, a 3D coupled numerical ocean-sediment-macrobenthos model is used to quantify the impacts of bottom trawling on organic carbon and macrobenthos stocks in North Sea sediments. Using available information on vessel activity, gear components and sediment type, we generate daily time series of trawling impacts and simulate six years of trawling activity in the model, as well as four management scenarios in which trawling effort is redistributed from areas inside to areas outside of trawling closure zones. North Sea sediments contained 552.2±192.4 kt less organic carbon and 13.6±2.6 % less macrobenthos biomass in the trawled simulations than in the untrawled simulations by the end of each year. The organic carbon loss is equivalent to aqueous emission of 2.0±0.7 Mt CO2 each year, half of which is likely to accumulate in the atmosphere on multi-decadal timescales. The impacts were elevated in years with higher levels of trawling pressure and vice versa. Results showed high spatial variability, with a high loss of organic carbon due to trawling in some areas, while organic carbon content increased in nearby untrawled areas following transport and redeposition. The area most strongly impacted was the heavily trawled and carbon-rich Skagerrak. Simulated trawling closures in planned Offshore Wind Farms and Core Fishing Grounds had negligible effects on net sedimentary organic carbon, while closures in Marine Protected Areas had a moderate positive impact. The largest positive impact arose for trawling closures in Carbon Protection Zones, which were defined as areas where organic carbon is most vulnerable to disturbance. In that scenario, the net impacts of trawling on organic carbon and macrobenthos biomass were reduced by 29 % and 54 %, respectively. These results demonstrate that carbon protection and habitat protection can be combined without requiring a reduction in net fishing effort.

Lucas Porz, Wenyan Zhang, Nils Gerrit Christiansen, Jan Kossack, Ute Daewel, and Corinna Schrum

Status: open (until 26 Mar 2024)

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Lucas Porz, Wenyan Zhang, Nils Gerrit Christiansen, Jan Kossack, Ute Daewel, and Corinna Schrum
Lucas Porz, Wenyan Zhang, Nils Gerrit Christiansen, Jan Kossack, Ute Daewel, and Corinna Schrum

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Short summary
Seafloor sediments store a large amount of carbon, helping to naturally regulate Earth's climate. If disturbed, some sediment particles can turn into CO2, but this effect is not well understood. Using computer simulations, we found that bottom-contacting fishing gears release about 1 million tons of CO2 per year in the North Sea, one of the most heavily fished regions globally. We show how protecting certain areas could reduce these emissions while also benefitting seafloor-living animals.