Distribution of CH₄ in the north basin of Lake Biwa and tributary rivers

Abstract

To determine the origin of CH₄, the vertical distribution of CH₄ around the thermocline in the north basin of Lake Biwa and the horizontal distribution in the Yasu and Ado rivers were measured. In 1995, CH₄ concentrations (ranging from 200 to 1000 nM) exceeding the saturation level were observed just above or in the thermocline in the pelagic region off the Yasu River. These values were higher than those just below the thermocline and in the hypolimnion. CH₄ concentrations in the mouths of the Yasu and Ado rivers were much higher than at other stations, around 2500 and 2000 nM, respectively. Due to the drought in 1994 when there was no water from the Yasu River flowing into the north basin, CH₄ concentration just above or in the thermocline in the pelagic region off the Yasu River ranged from 49 to 74 nM. It is thus concluded that the high level of CH₄ observed in the mouth of the Yasu River is one of the sources of the high CH₄ concentrations in the pelagic region off the Yasu River. On the other hand, the CH₄ concentration in the pelagic region off the Ado River was about 50 nM. It is thought that water flowing in from the Ado River diffused readily into the lake water because the depth of the lake in the region off the Ado River declines steeply. The maximum concentration of CH₄ in the river mouths indicates that these areas are important sites for clarifying the mechanism of the decrease in dissolved oxygen in Lake Biwa and the Yodo River watershed.