Observation of dissolved oxygen depletion at the bottom of Lake Biwa using the autonomous underwater vehicle “Tantan”

Lake Biwa is the largest lake in Japan with a surfaee area of670 km, maximum depth of 104 m, anda mean depth of 41 m. lt is loeated about 34°58'-35°31 'N; 135°52'1360 17'E in the Shiga Prefeeture at the eenter o f the Japanese arehipelago. Beginning in the 1960s, the lake began to show signs of eutrophieation and oxygen depletion in the hypolimnion before winter vertieal mixing (NAKA 1973). Over the past 30 years the average air temperature of Shiga Prefeeture has inereased by about 1-1.2 oe, and Lake Biwa bottom water temperature has inereased by 1.4 oe (HAYAMI & FUJIWARA 1999); mean annual preeipitation (mainly winter snow) over the same area has deereased sinee 1980 (KUMAGAI et al. 2006). These climatie changes have likely eontributed to the weakening of winter vertieal mixing and dissolved oxygen (DO) depletion at the bottom o f the lake. Lake Biwa is the souree of drinking water for 14 million people in the Kansai area; therefore, preserving the quality of the lake water quality and its ecosystem is a serious eoncern. With the help of a highly specialized research vessel and underwater robots as well as the implementation of a routine monitoring program, an advanced monitoring system of the lake has been developed (KUMAGAI et al. 2002). Because monitoring ehanges in the deeper (80-90 m depth) areas of the lake is diffieult, an underwater robot has recently been used because it can maintain a constant distance from the lake bottom (URA 2002, FoNG & STACEY 2003). In this study, high-resolution digital images and environmental sensors of the underwater robot were used to evaluate DO depletion at the bottom of the lake and its environmental effects.