Sediment resuspension : Impacts and extent of human disturbances

Abstract: Resuspension of sediment by anthropogenic disturbances is a concern due to the impacts it has on organisms and ecosystems. Bottom trawling is one major cause of sediment resuspension. A field study showed that a small trawl created a sediment plume 120 - 150 m wide and 15-18 m high (Paper 1). The sediment in the same study was highly contaminated. Blue mussels exposed to the sediment plume showed an increased uptake of contaminants and after 1 month reached levels toxic for human health (Paper 1). In the Baltic Proper, bottom trawling is the main cause of sediment suspension in waters >73 m, where resuspension by wind induced waves is minimal. Compared with dredging, bottom trawling annually resuspends 23-88 times more sediment in this area (Paper 4).Bottom trawling is expanding to deeper waters where sediment resuspension is expected to have larger impacts on organisms compared with shallow water. Deep water sponges create important habitats that are attractive fishing grounds. A laboratory experiment (Paper 3) showed that deep water sponges had a rapid respiratory response to short exposure of elevated turbidity.Boat activities in harbours are another major cause of sediment and contaminant remobilisation to the water column. A laboratory experiment showed that suspension of sediment with high TBT concentrations induced higher mortality in blue mussels, Mytilus edulis, and faster growth inhibition of the alga Ceramium tenuicorne than unsuspended sediment (Paper 2).This thesis clearly shows that resuspension due to human activities, in particular bottom trawling, is widespread and can have impacts on a range of organisms. However, there are large differences in management of these activities as they are covered by different legislative frameworks. When managing dredging, it is the most cited concern, whereas it is not considered in the management of bottom trawling.