Human-altered soundscapes in large European rivers: Widespread potential for masking by boats

Anthropogenic noise is increasingly recognized as a stressor to marine animals, but is rarely considered as a pollutant in freshwater ecosystems. We therefore characterized underwater soundscapes in three major European rivers (Rhine, Elbe, Gironde). We evaluated how natural soundscapes are shaped by biotic sounds (biophony) and river hydro-geomorphology (geophony). Furthermore, we assessed level and duration of boat noise to provide insight into disturbance and masking potential. We (1) characterized biophony in 24-h recordings; (2) assessed relationships between hydro-geomorphological features and the soundscape using drifting hydrophone recordings (geophony); (3) quantified boat noise in 24-h recordings; (4) evaluated masking potential through sound spectrum comparisons between natural sounds and boat noise; and (5) assessed the value of Automatic Identification System (AIS)-based shipping density data to predict noise impact from commercial vessels in large rivers. River soundscapes contained biophony from diverse taxa. Water velocity and sediment hardness explained a significant proportion of the variation in background sound levels at frequencies above 350 Hz, and sound levels sharply increased above 1.4 m/s. In the presence of boat noise, most natural sounds have high potential to be masked. Boat noise occupied 42-86% of recording time in the Rhine, 2-38% in the Elbe, and 0-20% in the Gironde. We found a significant relationship between AIS boat density and recorded and extrapolated boat noise along each river. We conclude that: (a) river soundscapes reflect information on nearby physical river features and biota, providing environmental cues to aquatic animals, and (b) that boat noise is widespread and persisting for long periods, potentially disturbing aquatic fauna and masking natural auditory cues in large rivers.