Temperate Mesophotic Soundscape Diversity and Pollution

Abstract

Temperate Mesophotic Ecosystems have both economic and ecological importance, however, they are currently understudied and scarcely protected, leaving them vulnerable to anthropogenic and environmental disturbances. In recent years, novel methodologies have facilitated the study of these ecosystems. One of these methods is acoustic monitoring, which can measure the diversity and health of ecosystems and can reach depths and species that are not easily accessible using other methods. Several acoustic indices have been developed to measure diversity using the soundscape, however these have mostly been used in terrestrial ecosystems. In this study, we analysed the soundscape of two mesophotic ecosystems in the central coast of Chile to assess the reliability of acoustic indices in marine ecosystems by comparing them with diversity measures based on environmental DNA (eDNA). In addition, the effects of the emergence of Oxygen Minimum Zones (OMZ) on the soundscape and acoustic indices was explored.

To this end, first fish sounds from the soundscape were characterised and counted. Next, from the eDNA data, species richness was calculated and based on the acoustic data, eight indices and a soundscape metric, Sound-Pressure Level (SPL), were measured for two frequency bands. From this, a principal component analysis was performed. The first principal component was subsequently used for further analysis. Both qualitative and quantitative analyses were conducted to compare the indices and soundscape to the eDNA data and study the effect of hypoxic conditions on the acoustic indices and SPL.

It was found that there was substantial difference between indices, showing that there is a lot of variability in what they measure. Furthermore, there were similarities in the diversity based on the eDNA and some acoustic indices, and in particular a combination of indices reflected the species richness well. Both sites showed significant variability, with a higher diversity observed near a Marine Protected Area (MPA). Moreover, similar results were found in terms of number of sound-producing species and number of signals detected in the soundscape, both being higher in Algarrobo. Finally, the results showed that there was no significant difference in the results of the indices between oxygen categories.