A field observation on hydrodynamic and thermal environments of a fringing reef at Ishigaki Island under typhoon and normal atmospheric conditions

Abstract

To investigate the hydrodynamic and thermal environments of a fringing coral reef under possible influences from both inland and offshore, we have conducted a field survey at Shiraho reef on Ishigaki Island, Okinawa, Japan, by deploying 16 moored buoys inside and outside the reef, on which various sensors for continuous measurements of water temperature, salinity, and turbidity concentration were installed. Several bottom-mounted current meters and wave gauges were also deployed. The results show the abrupt decrease and increase of the water temperature during a typhoon, with the resultant temperature being about 1 degree lower than before. The main cause of this abrupt change and the difference between inside and outside the reef in the thermal response to the atmospheric agitation are investigated. For normal atmospheric conditions, the overall characteristics of currents in the coral reef are found to be governed by the dynamic balance among tide, waves, and wind effects. The salinity and turbidity variations near the river mouth and their cause are also investigated. Thermal environment in the reef is examined by a heat-budget analysis, indicating that it is influenced by both the atmospheric conditions and the temperature difference between inside and outside the reef.