Direct In situ Measurements of Sound Speed Profile in Thermocline of Lake: The Example of North Lake Biwa in Autumn 2008

Abstract

In recent years, the rise in water temperature has had an adverse effect on the ecosystem in Lake Biwa. Thus, it is very important to continuously monitor the water temperature. We previously proposed the use of acoustic tomography to monitor the water temperature in Lake Biwa. In autumn, a thermocline develops in Lake Biwa. During this season, the water temperature changes considerably between the warm surface and the cold bottom of the lake, and a circular current develops when the thermocline is formed. It is necessary to ascertain the relationship between the thermocline and the speed of sound. In this study, to determine the relationship between the speed of sound and the water temperature when a thermocline exists, the speed of sound was measured in the north of the lake in autumn 2008. We report the estimated water temperature and the measured speed of sound in Lake Biwa. The speed of sound measured directly was greater than the calculated value for pure water. The existence of the thermocline was also verified at the same time, in which the water temperature changed by about 7.5 °C from the lake surface to a depth of 20 m. The estimated water temperature of the lake assuming nonsaline water was about 0.2 to about 0.5 °C higher than the measured water temperature. To improve the accuracy of the estimated water temperature, the temperature was corrected using a salinity conversion value, which was calculated from the speed of sound measured in situ. The salinity conversion value was used to estimate the water temperature from the speed of sound measured directly in situ in Lake Biwa within an accuracy of 0.1 °C.