Bottom trawling causes highly increased transport of fine sediments in the Western Baltic Sea – with long-term consequences for coastal seas.
Currents stir up sediments from the seabed and transport them further on. Bottom trawling increases this transport by up to a third, as researchers from the Helmholtz-Zentrum Hereon have now shown in a new model study. This can reduce the Baltic Sea’s function as a natural carbon sink and can have significant effects on the nutrient balance of coastal seas.
When particularly dense water flows over the seabed as a so-called bottom current, it leaves behind gutter-like contour lines and thus changes the shape of the seabed. In order to better understand these current paths, scientist of the Institute of Coastal Systems – Analysis and Modeling studied two mud depocenters in the Baltic Sea. They compared various computer simulations with geological and oceanographic data. These confirmed that reliable predictions of the current paths in the Baltic Sea can be made and that past current patterns can also be explained. On this basis, the researchers then investigated the influence of bottom trawling on sediment distribution in the Baltic Sea.
“The novel aspect of our approach is that we include the actual bottom trawl activity of individual vessels in our simulations instead of only estimating the distribution of fishing fleets, as has been common in previous studies. Thus, we also take into account seasonal and weather-related fluctuations in fishing activity,” explains Dr Lucas Porz, lead author of the study. (Source: Hereon Press Release)
Read the complete Hereon Press Release:
==> Current pathways in the Baltic Sea
Porz, L., Zhang, W., & Schrum, C. (2022): Natural and anthropogenic influences on the development of mud depocenters in the southwestern Baltic Sea. Oceanologia, doi:10.1016/j.oceano.2022.03.005
Abstract:
The morphological evolution of two mud depocenters in the southwestern Baltic Sea is investigated by comparison of numerical model results to geological and oceanographic data. The pathways of dense currents during episodic dense-water inflows from the North Sea are shown to correspond to current pathways inferred from contouritic depositional geometries in the flow-confining channels within the study area. A favorable comparison of model results to published current speed observations shows that the mesoscale dynamics of individual inflow events are reproduced by the model, indicating that external forcing and basin geometry rather than internal dynamics control the mesoscale dynamics of inflow events. The bottom current directions during inflows show high stability in the flow-confining channels and explain the contouritic depositional geometries. Asymmetric depositional features in the channels are qualitatively reproduced in the model. Bottom currents are less stable in areas without contouritic features, possibly resulting in an overall diffusive effect on sediment distribution in those areas. In a simulation of resuspension by bottom-contacting fishing gear, inter-basin sediment transport is increased by 4–30%, depending on the area, compared to the case of natural hydrodynamic resuspension. The model predicts an increased winnowing of the finest sediment fraction due to bottom trawling, leading to an overall coarsening-to-fining trend in the direction of net sediment transport. The results show that rather than hemi-pelagic background sedimentation, episodic events with high bottom current velocities as well as bottom-trawling induced resuspension are responsible for the present-day and future morphological configuration of the mud depocenters in the southwestern Baltic Sea.