Beneath the surface, the sea is never truly silent. Waves, currents, fish, crustaceans, and ship propellers: everything contributes to a continuous background of sound that scientists call a “soundscape”—the soundscape generated jointly by physical, biological, and human processes. It is precisely within this interweaving of sounds that a group of researchers from the National Research Council has sought a signal capable of revealing something about the health of the marine environment. The work, which originated within the PRIN SEAmPhonia project, was published in the international journal *Ecological Indicators*.
Listening from the Acqua Alta platform
The study was authored by Pier Francesco Moretti (CNR-DSSTTA), Angela Pomaro (CNR-ISMAR), and Silvano Buogo (CNR-INM). To collect the data, the team installed several hydrophones—microphones designed to record underwater—on the Acqua Alta oceanographic platform during several experimental campaigns carried out with the support of the Underwater Acoustics Laboratory at the Tor Vergata Research Area.
The innovation lies in the method. Usually, underwater noise is broken down into individual sources or studied based on the low frequencies produced by ship traffic. The researchers, however, chose to observe the acoustic field as a whole in order to derive a characteristic that describes the ecosystem as a single system. “The goal is to identify a new indicator capable of assessing the ecosystem’s condition through the acoustic signals it produces,” explains Pomaro.
A Rhythm Between Day and Night
Upon analyzing the recordings, the team observed a persistent oscillation between day and night in the high-frequency signals, which cannot be fully explained by environmental factors alone. In particular, the spectral slope at high frequencies varies: a portion of the acoustic spectrum that follows a power-law distribution, typical of complex systems and linked to the concept of scale invariance. According to the authors, this pattern could reflect the collective rhythms of marine life—a sort of “heartbeat” of the sea. Rather than the behavior of a single species or a single physical phenomenon, this signal would therefore describe the state of the ecosystem as a whole.
Like a fever for the human body
To explain the significance of the discovery, Moretti uses a medical analogy: the indicator works like body temperature, which signals that something is wrong without describing every process in the body. If the value changes, it means the system has changed; to understand why, targeted analyses will then be needed.
The method also has practical advantages. Monitoring is continuous and non-invasive, the results are comparable across different sites, and the indicator does not depend on instrument calibration. This is a significant point, given that the European Marine Strategy Framework Directive (MSFD) currently relies primarily on absolute sound intensity levels and the low frequencies of maritime traffic. Listening to the soundscape, the researchers argue, would instead allow for the early detection of changes in ecosystems.
