The Nord Stream Pipeline (a two-pipeline system) from Russia to Germany through the Baltic Sea was constructed during 2010–2012. In Swedish waters, subsea post-lay trenching by ploughing was carried out at minimum 4 km distance to two Natura 2000 areas1, which was considered by the environmental authorities to be relatively close. The Swedish construction permit required that the excess Suspended Sediment Concentration (SSC; often the indirect measure turbidity is used) caused by sediment spill as a result of the construction works (particularly post-lay trenching by ploughing) never exceeded 15 mg/l above natural background at the borders of the Natura 2000 areas. Because natural turbidity levels were not well-described in this area, a monitoring programme combining passive, real-time turbidity monitoring and active, vessel-based spill monitoring combined with a feedback mechanism was established. The method proved successful in documenting compliance with permit requirements with regard to threshold limits for excess SSC caused by construction activities during construction of Line 1. This ensured that trenching activities could be completed as planned, and it also enabled a significant reduction in the scope of the monitoring programme for Line 2. The monitoring was also successful in verifying that numerical modelling of sediment spreading carried out as part of the pre-construction impact assessment was correct. Finally, the new knowledge on spill rates acquired from the vessel-based monitoring will enable a more accurate assessment of the environmental impact of future subsea ploughing operations.

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