Abstract
This paper describes the natural variability of ambient sound in the Southern Ocean, an acoustically pristine marine mammal habitat. Over a 3-year period, two autonomous recorders were moored along the Greenwich meridian to collect underwater passive acoustic data. Ambient sound levels were strongly affected by the annual variation of the sea-ice cover, which decouples local wind speed and sound levels during austral winter. With increasing sea-ice concentration, area and thickness, sound levels decreased while the contribution of distant sources increased. Marine mammal sounds formed a substantial part of the overall acoustic environment, comprising calls produced by Antarctic blue whales (Balaenoptera musculus intermedia), fin whales (Balaenoptera physalus), Antarctic minke whales (Balaenoptera bonaerensis) and leopard seals (Hydrurga leptonyx). The combined sound energy of a group or population vocalizing during extended periods contributed species-specific peaks to the ambient sound spectra. The temporal and spatial variation in the contribution of marine mammals to ambient sound suggests annual patterns in migration and behaviour. The Antarctic blue and fin whale contributions were loudest in austral autumn, whereas the Antarctic minke whale contribution was loudest during austral winter and repeatedly showed a diel pattern that coincided with the diel vertical migration of zooplankton.
Highlights
Underwater ambient sound is created by the superposition of sounds from countless abiotic, biotic and anthropogenic acoustic sources; it is termed the ‘acoustic environment’ [1] or ‘ambient noise’ [2]
Anthropogenic sound sources rarely enter this region, mainly comprising sporadic research vessels and cruise ships that primarily target the Western Antarctic Peninsula area. The scarcity of such acoustically pristine habitats makes these invaluable in the context of passive acoustic monitoring (PAM) studies, as they can be used as a potential reference for trends in ocean ambient sound and to assess the natural variability of ambient sound
Peaks associated with marine mammal vocalizations (MMCs, table 2) can be found between 15 and 30 Hz as well as 90 and 1000 Hz
Summary
Underwater ambient sound is created by the superposition of sounds from countless abiotic, biotic and anthropogenic acoustic sources; it is termed the ‘acoustic environment’ [1] or ‘ambient noise’ [2]. Anthropogenic sound sources rarely enter this region, mainly comprising sporadic research vessels and cruise ships that primarily target the Western Antarctic Peninsula area. The scarcity of such acoustically pristine habitats makes these invaluable in the context of passive acoustic monitoring (PAM) studies, as they can be used as a potential reference for trends in ocean ambient sound and to assess the natural variability of ambient sound. We analyse the temporal and spectral variation of Southern Ocean ambient sound over a 3-year period and discuss its relation to environmental factors such as wind speed and sea ice
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