Spatial variation of the underwater soundscape over coral reefs in the Northwestern Hawaiian Islands

Abstract

Coral reefs create a complex acoustic environment, dominated by sounds produced by benthic creatures such as crustaceans and echinoderms. While there is growing interest in the use of ambient underwater biological sound as a gauge of ecological state, extracting meaningful information from recordings is a challenging task. Single hydrophone (omnidirectional) recorders can provide summary time and frequency information, but as the spatial distribution of reef creatures is heterogeneous, the properties of reef sound arriving at the receiver vary with position and arrival angle. Consequently, the locations and acoustic characteristics of individual sound producers remain unknown. An L-shaped hydrophone array, providing direction-and-range sensing capability, can be used to reveal the spatial variability of reef sounds. Comparisons can then be made between sound sources and other spatially referenced information such as photographic data. During the summer of 2012, such an array was deployed near four different benthic ecosystems in the Northwestern Hawaiian Islands, ranging from high-latitude coral reefs to communities dominated by algal turf. Using conventional and adaptive acoustic focusing (equivalent to curved-wavefront beamforming), time-varying maps of sound production from benthic organisms were created. Comparisons with the distribution of nearby sea floor features, and the makeup of benthic communities, will be discussed.