Norra Midsjöbanken is a Natura 2000 area situated approximately 50 km east of the southern tip of Öland island in the Swedish Exclusive Economic Zone (EEZ). The second of the two Nord Stream pipelines will pass approximately 4 km south of this protected area. The aim of this study was to measure and quantify the noise during Nord Stream’s construction and trenching activities as well as the ambient noise including commercial shipping noise. This kind of comparison has not been performed before in this area and is in line with the European Marine Strategy Framework Directive (2008/56/EC). During the winter of 2012, autonomous hydrophone buoys were placed at two locations in the Norra Midsjöbanken area. Location “A1” was situated approximately 1.5 km from one of the main shipping lanes in the Baltic Sea. Hydrophones at this location recorded ambient noise dominated by shipping noise, and were undisturbed by Nord Stream’s activities. The other location, “B1”, was situated approximately 1.5 km from the route of Nord Stream’s second pipeline. Here, hydrophones recorded ambient noise and noise pollution caused by laying the Nord Stream pipeline and by post-lay trenching. Careful consideration was given to the design of the hydrophone rig, ensuring low self-noise and contributing to the successful retrieval of all buoys. The recorded noise data was analysed at frequencies up to 3500 Hz. Third octave band spectra and sound pressure level evolutions as well as statistics were calculated for each of the hydrophones and during different conditions. Average ambient noise levels of 116.5-116.6 and 110.9-111.5 dB re 1 µPa were estimated at locations A1 and B1, respectively. Compared to previous results and predictions made by ambient noise models, we find that the noise levels at Norra Midsjöbanken are consistently higher. We speculate that this is due to the proximity of shipping lanes and the large numbers of passing ships. The mean noise level estimated at location B1 during trenching was 126.0 dB re 1 µPa. The trenching was performed by the vessel Far Samson. The source level of this vessel during trenching was estimated to 183.5 dB re 1 µPa @ 1m using AIS information on its position. In a similar manner, we estimated the source levels of three commercial vessels that passed close to location A1, and obtained results of 178.6 to 184.6 dB re 1 µPa @ 1m. We conclude that the source level of the vessel Far Samson during trenching is not greater than that of a commercial vessel. The mean noise level estimated at location B1 during pipelay was 130.5 dB re 1 µPa. Compared to the noise level during trenching, the level during pipelay was 4.5 dB higher. At this location, the pipelay fleet consisted of nine vessels of different characteristics. It is not possible to estimate the source level of each individual vessel. However, we show that the 4.5 dB increase over the trenching level is expected for such a large fleet, and indicates that the source levels of the vessels in the pipelay fleet are probably similar to those of the commercial vessels. Finally, we conclude that the vessel traffic in the Norra Midsjöbanken area is so heavy that it is difficult to measure a noise level undisturbed by shipping. The funding for this study was provided by Nord Stream AG and Naturvårdsverket (Swedish Environment Protection Agency).

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