Freshwater ecoacoustics:listening to the ecological status of multi-stressed lowland waters

A major challenge in water quality assessment is to identify suitable indicators to monitor and assess the effects of anthropogenic stressors on the ecological status of freshwater ecosystems. Passive acoustic monitoring is a novel approach that could potentially be used to detect invertebrate species and ecological processes such as dissolved oxygen dynamics in freshwater environments. The aim of the present study was to evaluate to what extent sounds can be used for water quality assessment. We performed a field study to relate acoustic indices to the intensity of several stressors, the invertebrate community composition and the dissolved oxygen dynamics in 20 temperate lowland streams and drainage ditches impacted to a varying degree by agricultural activities and discharges from waste water treatment plants. Our results showed that the recorded acoustic patterns were primarily associated with the fluctuation in dissolved oxygen saturation, while specific frequency bands could be related to the sound-producing invertebrate community. We observed that acoustic indices do not allow to detect the adverse effects of anthropogenic stressors on the invertebrate community composition, presumably due to the prevalence of Heteroptera which are relatively insensitive to stressors, but make a lot of sounds. A strong relation between acoustic indices and oxygen fluctuation indicate that passive acoustic monitoring may be used to estimate metabolism in water bodies. We suggest that the next step in freshwater ecoacoustics will be to precisely characterise each source of sound emitted during the processes of primary production, respiration and re-aeration, in order to distinguish these parameters. This may overcome some of the challenges encountered in the estimation of metabolism from diel dissolved oxygen curves.