Soundscape around the Dongsha reef atoll at the northern South China Sea

Abstract

The Dongsha reef atoll is on the direct path of trans-basin large internal solitary waves (ISWs) at the northern South China Sea. The ISWs bring nutrient-rich bottom water to the surface enriching marine life around the atoll. This is an important ecosystem for the ambient noise study to categorize the source of underwater sound and general soundscape. Four Passive Miniature Acoustic Recorders (PMARs) developed by the Applied Physics Laboratory, University of Washington were deployed in 2018 and 2019 within and near the outer bank of reef ring at the depth of ∼10 m. The PMARs collected the ambient noise from 100 to 50 kHz about 2–3 months each year. There are clear diurnal signals in pre-dawn and dusk periods with a peak frequency at 10 kHz. The noise level is about ∼5 dB higher than the background noise. The outer reef has a higher noise level (∼8 dB) than the inner reef due to stronger biological activities. In general, biological noise is the dominant source in this reef ecosystem, yet during the typhoon Mangkhut the wind noise is much higher at the lower frequency (below 3 kHz) and the near-surface bubbles layer generated during the typhoon inhibited the sound transmission which causes the lower noise level above 5 kHz.