Trawls reduce carbon storage
Intensive fishing on the seabed increases the release of carbon
Flatfish and shrimp are caught in the North Sea by using trawls that are dragged across the seabed. This releases carbon into the water and carbon dioxide (CO2) into the atmosphere, as shown by the latest research at the Helmholtz-Zentrum Hereon. The study is part of the collaborative project APOC. Partners are the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI), the GEOMAR Helmholtz Centre for Ocean Research Kiel and the German Federation for the Environment and Nature Conservation (BUND). The researcher’s effort for reducing the uncertainty in quantitative assessment of the impact of bottom trawling on carbon storage in the North Sea and global shelf seas was recently published in the journal Nature Geoscience.
Normally, the seabed acts as a carbon sink. This means that it stores more carbon than it releases. Researchers from the Hereon Institute of Coastal Systems – Analysis and Modeling have now discovered together with the APOC partners that this function is being impaired by the use of bottom trawls in fisheries. The scientists have analyzed over 2,300 sediment samples from the North Sea.
Geophysicist and lead author Dr. Wenyan Zhang summarizes the findings as follows: “We found that sediment samples collected in intensely trawled areas contained lower amounts of organic carbon than samples collected in areas with less fishing. We were able to attribute this effect to bottom trawling with high confidence. Moreover, our methods greatly reduce the uncertainty in quantitative assessments of the impact at regional to global scales compared to earlier estimates.” Computer simulations have also shown that the carbon concentration in the seabed has decreased continuously throughout the past decades as a result of intensive trawling. Soft, muddy bottoms are particularly susceptible. (Source: Hereon press release)
Read the complete Hereon press release:
==> Trawls reduce carbon storage
Zhang, W., Porz, L., Yilmaz, R., Wallmann, K., Spiegel, T., Neumann, A., Holtappels, M., Kasten, S., Kuhlmann, J., Ziebarth, N., Taylor, B., Ho-Hagemann, H.T.M., Bockelmann, F.-D., Daewel, U., Bernhardt, L., & Schrum, C. (2024): Long-term carbon storage in shelf sea sediments reduced by intensive bottom trawling. Nat. Geosci., doi:10.1038/s41561-024-01581-4
Abstract:
Bottom trawling represents the most widespread anthropogenic physical disturbance to seafloor sediments on continental shelves. While trawling-induced changes to benthic ecology have been widely recognized, the impacts on long-term organic carbon storage in marine sediments remains uncertain. Here we combined datasets of sediment and bottom trawling for a heavily trawled region, the North Sea, to explore their potential mutual dependency. A pattern emerges when comparing the surface sediment organic carbon-to-mud ratio with the trawling intensity represented by the multi-year averaged swept area ratio. The organic carbon-to-mud ratio exhibits a systematic response to trawling where the swept area ratio is larger than 1 yr−1. Three-dimensional physical–biogeochemical simulation results suggest that the observed pattern is attributed to the correlated dynamics of mud and organic carbon during transport and redeposition in response to trawling. Both gain and loss of sedimentary organic carbon may occur in weakly trawled areas, whereas a net reduction of sedimentary organic carbon is found in intensely trawled grounds. Cessation of trawling allows restoration of sedimentary carbon stock and benthic biomass, but their recovery occurs at different timescales. Our results point out a need for management of intensely trawled grounds to enhance the CO2 sequestration capacity in shelf seas.
Zhang, W., & Porz, L. (2024): Chronic intense bottom trawling reduces marine carbon sequestration. Nat. Geosci., doi:10.1038/s41561-024-01589-w
The Problem:
Whereas the consequences of human activities on land are readily observable, measuring underwater effects remains challenging. A notorious instance is bottom trawling, a controversial fishing practice that uses heavy nets to catch seafloor-dwellers such as shrimp and flatfish. Bottom trawls are omnipresent in shelf seas (coastal, usually shallow waters surrounding continents) globally, where they disturb the upper sediment layers and might depreciate seafloor habitats if poorly managed. Research has highlighted the potential of bottom trawls to release a large amount of organic carbon from sediments into the water, where it is partly metabolized by organisms to carbon dioxide. Part of the produced CO2 outgasses to the atmosphere, enhancing anthropogenic climate change. The magnitude of this indirect emission has been heavily debated, with estimates spanning from negligibly small to climate-relevant. Thus, management of bottom trawling for climate protection is hindered by a lack of reliable data and insufficient understanding of the process. A motivation of our study is to provide more robust estimates of this impact, a prerequisite for informed policymaking.
