Passive acoustic monitoring of ship sounds in Lake Superior

Monitoring freshwater ecosystems using passive acoustics is a largely unexplored approach, despite having the potential to yield information about the biological, geological and anthropogenic activity of a lake or river system. Minnesota, nicknamed “land of 10 000 lakes,” provides an interesting case study, because of the opportunity to compare the soundscape during winter with up to 100% ice cover and no vessel transits, to the open water soundscape during busy summer shipping season. Passive acoustic monitoring was conducted in the coastal waters of the western arm of Lake Superior, close the Duluth port, from October 2018 to September 2019. By combining long-term acoustic monitoring data with AIS vessel-tracking data and acoustic propagation modelling (bathymetry, sound speed profiles and seafloor properties) a quantitative method for determining the impact of vessel noise on the soundscape has been established. Median broadband sound pressure level (100–12 000 Hz) was significantly lower in winter compared to summer. However, during ice free months tankers and cargo vessels played a key role in the daily soundscape increasing the intensity by >30 dB between 300 and 1,000 Hz. Moving forward, baseline sound levels provide vital evidence for scientists and governing bodies to make proactive decisions for soundscape conservation.