Acoustic comparison of a patchy Mediterranean shallow water seascape: Posidonia oceanica meadow and sandy bottom habitats

Soundscapes are strongly linked with the physical structure and biological features of the habitats and their study can reveal ecological processes of the underwater environment. Objective of this study is to characterize two Mediterranean habitats, the Posidonia oceanica meadow and the sandy bottom, and demonstrate their acoustic diversification basing on their soundscapes. Firstly, the habitats have been compared using two different acoustic metrics, the Power Spectral Density (PSD) and the Acoustic Complexity Index (ACI), measured in different frequency band. Then, the acoustic biological component of the habitats has been identified and characterized: five biological signals were described and their acoustic properties and temporal patterns were defined. Finally, the geophonical and anthropogenic components of the two habitats have been compared. In the low frequency (<0.5 kHz) the sandy habitat showed higher values of PSD and lower values of ACI. From 0.5–24 kHz the greatest values of both parameters were recorded in the Posidonia habitat due to the acoustic activity of snapping shrimps and fishes. The wind speed resulted significantly correlated with PSD from 0.1 to 2 kHz for both habitats, but the correlation is less intense in Posidonia habitat suggesting a noise attenuation phenomenon. The two habitats present biophonical component belonged to different fish species and invertebrates; they showed alternated temporal pattern and different frequency allocation. The Posidonia habitat resulted acoustically richer than sandy habitat, confirming the importance of ecoacoustic method to study ecological processes. Finally, a strong acoustic impact from the anthropogenic component was revealed: it achieves 60% of daytime during the summer, especially in sandy habitat. Results demonstrated not only the possibility to discriminate habitats through the sound information but also the need to protect marine ecosystems from the human noise.