Physicochemical and acoustic conditions of habitats surrounding Kueishan Island, Yilan, Taiwan

In the past, research on hydrothermal vents has primarily focused on deep sea, but shallow-water ones have gained increasing attention in recent years. Shallow-water hydrothermal systems experience greater fluctuation due to various epipelagic factors such as insolation, tides, currents, and seasonality. While existing studies have explored geochemical and biological features of hydrothermal vents mostly through snapshot investigations, little is known about their spatio-temporal effects on the surrounding environments. In the present study, we monitored the physicochemical and audio conditions around Kueishan Island, Taiwan. Within a vertical distance of less than 4 km, Kueishan Island possesses both hydrothermal vents and coral ecosystems, making it an ideal platform to address these issues. Our results reveal that the impacts of hydrothermal vents on the ecological habitats surrounding Kueishan Island were not constant, which is attributed to the intermittent discharge of vent fluids. Additionally, we observed a potential connection between hydrothermal venting and earthquake activities, hinting at possible geophysical implications. Overall, our research provides insights into exploring these fascinating environments with a greater mastery of them. Prospectively, long-term, and continuous monitoring could broader our knowledge of hydrothermal vents, including interkingdom evolutionary processes, economic opportunities, and developing strategies for safeguarding these precious natural resources for future generations.

Methods
Seawater samples and audio data were collected and recorded by SCUBA diving at submarine areas surrounding Kueishan Island during the daytime from August 2020 to October 2022.

Seawater samples were collected from six sampling sites surrounding Kueishan Island, including VN (near the vents), VF (about 30 m from the vents), C (CO2 vents), S (south), N (north), and T (tail) sites of Kueishan Island. Physicochemical parameters were assessed, including temperature (Temp), pH value, dissolved oxygen (DO), sulfide (S2-), total alkalinity (TA), and dissolved inorganic carbon (DIC). The partial pressure of CO2 (pCO2), bicarbonate (HCO3-) and carbonate (CO32-) concentrations were calculated using a CO2SYS module within Microsoft Excel according to DIC, temperature, and pH level.

Audio data were obtained using a SoundTrap recorder (ST300 STD, Ocean Instruments, New Zealand). To capture most underwater sounds from biological and environmental sources, the built-in hydrophone of recorder has a sensitivity of -175 dB re 1 V/µPa and a working bandwidth of 20 Hz to 60 kHz (±3 dB). Based on the median spectrum, we calculated frequency band-specific sound pressure levels (SPLs) by integrating the power spectral densities from frequencies 20-200 Hz, 200-2k Hz, and 2k-20k Hz.  For each survey, three 90-sec recording fragments were randomly selected from each site to ensure a consistent sampling effort with the physicochemical data.