Scale invariance in the high-frequency marine soundscape: diel oscillations as a proxy for ecosystem state

Abstract

Life generated in the oceans. Human activities rapidly and drastically altered marine environments. Scientists are struggling in defining an effective strategy to assess the anthropogenic impacts into seas and oceans. In fact, marine ecosystems are complex systems, with a multitude of interconnected aspects that makes the understanding of its functioning a challenge. Acoustic waves are the most efficient mechanism for long-range energy transport and communication in the marine environment. Consequently, the marine soundscape integrates information from multiple biological and physical processes and offers a promising framework for ecosystem monitoring. However, most acoustic studies have focused on low-frequency signals associated with anthropogenic noise or species-specific acoustic activity. In this study, we analyse the spectral properties of underwater soundscapes recorded using different instruments and across diverse seasonal conditions in the Adriatic Sea. The analysis focuses on the power-law behaviour of acoustic spectra between 13 and 37 kHz and on the temporal evolution of their spectral slopes. By concentrating on the dynamics of the spectral slopes rather than absolute sound-pressure levels, the proposed approach provides an indicator that is independent of hydrophone calibration and instrument sensitivity offsets. For the first time, we report a persistent diel oscillation in the spectral slope of high-frequency acoustic spectra, consistently observed across winter and late-spring deployments. Statistical analyses demonstrate that day-night differences are robust and reproducible across daily cycles. Comparisons with environmental variables, including irradiance, temperature, tides, and wave dynamics, suggest that the observed oscillation cannot be explained solely by physical forcing. We therefore hypothesize that the diel modulation may reflect temporal variations in the biological component of the ecosystem, potentially associated with collective biological rhythms rather than isolated species-specific acoustic emissions. These results suggest that high-frequency spectral slopes may constitute a novel calibration-independent acoustic indicator for marine ecosystem monitoring. The recurring diel oscillation further indicates that this indicator may provide information on the temporal organization and state of biological activity within marine ecosystems, opening new perspectives for passive-acoustic assessment of ecosystem dynamics.