Hydro-acoustic survey of shallow water seagrass beds using a line array of hydrophones, a case study in Xincun Lagoon, Hainan Island

Sound is considered an important aspect of an ecosystem and acoustic methods have emerged as effective tools for ecosystems research. Xincun Lagoon, Hainan Island, is an important ecosystem characterized by dense seagrass, which has been declining due to increased human activities, raising great concerns. Previous studies have identified various threats to seagrass, including heavy metal pollution, poor quality water, and so on. In this study, we investigate sources and levels of noise in seagrass beds and attempt to point out potential threats from noise pollution. A line array of six hydrophones was deployed over a period of seven days, from January 15 to January 21, 2024. The recordings captured various sounds from marine life, human activities, and natural processes. Biological sounds, such as fish sounds and whale calls, were the most prevalent. Low-frequency noise from wind and tide were often recorded. Xincun Bay hosts more than 1500 fishing vessels; however, due to bad weather conditions that kept most vessels docked during the recording period, only one segment of boat noise was recorded; it lasted for 7 minutes, exhibiting strong energy over a broad frequency band. This event underscores the necessity of long-term monitoring of noise to identify and evaluate not only boat noise but other noise sources, especially ones that are intermittent but strong, that were not encountered during the limited period of observation on which this report is based.