Underwater Acoustics in Estuaries

AEstuaries represent dynamic transitional zones where rivers converge with the sea, giving rise to unique hydrodynamic and geological properties. In recent decades, marine biologists have increasingly turned their attention to the underwater acoustic environment—or soundscape—of these shallow ecosystems. Unlike the deep, relatively homogeneous waters of the open ocean, where acoustic energy can propagate across vast distances with minimal impedance, estuaries create an exceptionally complex acoustic matrix. Shallow water columns, shifting bathymetry, and high concentrations of suspended sediment cause sound waves to rapidly attenuate and scatter. Furthermore, the sharp gradients in temperature and salinity that characterise estuarine mixing zones produce micro-refraction layers, bending acoustic paths and resulting in a localised, rapidly fluctuating acoustic climate.

BA substantial proportion of the estuarine soundscape consists of biological sounds, collectively termed biophony. In temperate and tropical estuaries worldwide, benthic invertebrates, particularly snapping shrimp from the family Alpheidae, dominate high-frequency ambient noise. By rapidly closing an enlarged claw, these creatures generate a high-velocity water jet that creates a transient cavitation bubble, the collapse of which emits an intense acoustic pulse. The collective effect of millions of these snaps resembles the sound of burning timber or frying fat. Simultaneously, numerous fish species contribute lower-frequency vocalisations. Members of the drum and croaker families utilise specialised sonic muscles attached to their swim bladders to generate rhythmic grunts and purrs, predominantly during nocturnal spawning choruses that synchronise reproduction.

CThese biological acoustic signatures serve vital ecological purposes, particularly for the recruitment of juvenile organisms. The pelagic larvae of many marine species, such as crabs, oysters, and various reef fish, develop in the open sea before migrating into sheltered estuarine nurseries to mature. Historically, scientists assumed that settlement was governed almost exclusively by hydrodynamics and chemical plumes. However, field experiments have demonstrated that many larvae exhibit phonotaxis, actively swimming towards the distinctive acoustic signatures produced by healthy estuarine habitats. High-density oyster reefs and saltmarsh channels broadcast acoustic cues across several kilometres, providing early-stage organisms with an effective orienting beacon in waters where poor visibility prevents the use of visual navigation.

DAlongside animal vocalisations, physical processes generate an intricate background soundscape known as geophony. Estuarine waters are governed by the ebb and flow of tides, which mobilise coarse sediments along channel floors. The friction of rolling gravel and coarse sand produces a continuous low-frequency rumbling that intensifies during spring tides when current velocities peak. Additional noise arises from meteorological events; heavy rainfall striking the water surface injects distinct acoustic frequencies, while breaking waves over intertidal flats entrain air bubbles into the water column. As these bubbles resonate and oscillate before dissipating, they contribute a broad spectrum of acoustic energy that varies markedly between tranquil weather conditions and severe seasonal storms.

EHuman activity introduces substantial acoustic disturbance into these sensitive aquatic corridors. Anthropogenic noise, or anthrophony, stems from commercial shipping, maintenance dredging, pile driving, and recreational motorboats. In shallow estuaries, artificial noise is particularly problematic because sound waves repeatedly reverberate between the surface and the seabed, creating persistent acoustic clutter. Such interference frequently causes acoustic masking, whereby background artificial noise obscures natural sounds. Studies indicate that sustained vessel traffic can conceal the settlement cues required by larval invertebrates and drown out the courtship vocalisations of native fish, potentially impairing reproductive success and disrupting navigation in restricted navigation channels.

FTo assess and protect these vulnerable habitats, environmental scientists are deploying passive acoustic monitoring (PAM) as a standard surveying methodology. Estuarine environments are notoriously turbid, with suspended silt and organic matter limiting visibility to a few centimetres, which renders traditional visual observation and camera traps ineffective. Hydrophone arrays, moored autonomously along the seabed, offer a continuous, non-intrusive means of recording underwater audio over months or years. By analysing the temporal patterns, frequency distributions, and overall acoustic complexity of these recordings, researchers can track biodiversity levels, monitor the timing of fish spawning aggregations, and detect the arrival of invasive organisms without physically disturbing delicate benthic communities.

GBeyond passive observation, understanding estuarine acoustics has opened new possibilities for active habitat restoration. In degraded estuaries where historical overharvesting and pollution have destroyed natural biogenic reefs, marine scientists have begun trialling acoustic enrichment techniques. By installing submerged underwater speakers in depleted zones, practitioners broadcast audio tracks recorded from thriving, undisturbed reefs. Initial trials have shown promising results; artificial soundscapes have successfully accelerated the recruitment of oyster larvae and enhanced fish colonisation on newly constructed artificial substrates. This method suggests that acoustic remediation could become a valuable component of broader restoration initiatives, helping to rebuild degraded estuarine food webs.