Exploring the soundscape of small freshwater lakes

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. The state of Minnesota, located in the upper Midwest of the USA and nicknamed ‘land of 10,000 lakes’, provides an interesting case study for soundscape research, because lakes offer ecological, recreational and economic value throughout the area. The underwater soundscape was monitored at fifteen small lakes <10 km² on representative days in winter (during 100% ice cover) and summer 2018 using a hydrophone suspended 2 m below the water's surface. Median broadband sound pressure level (100–12,000 Hz) was significantly lower in winter (57.2 dB re 1μPa) compared to summer (66.7 dB re 1μPa), possibly because low frequency wind sounds were reduced in winter. Recordings suggest small freshwater lakes in Minnesota have a relatively pristine soundscape, where vocalizing aquatic animals may hold acoustic niches. However, sound from anthropogenic activity was also present in the study lakes. Ice auger and motorboat sound increased the intensity of the soundscape by 10 dB and overlapped the frequency range (300–1000 Hz) of biological sounds in the environment, that may be important to aquatic life. Understanding current baseline sound levels in ecologically significant freshwater lakes, like those in this study, is the first step in determining any potential consequences of anthropogenic sound. Moving forward, baseline sound levels provide vital evidence for scientists and governing bodies to make proactive decisions for soundscape conservation.