Tradeoff Between Hot spring Use and River Ecosystem: The Case of Beppu City, Oita Prefecture, Japan

In Hirata River at Beppu, Oita prefecture, the inflow of hot spring drainage into the rivers increases the Nile tilapia, an alien species. It is a tradeoff between the hot spring use and river ecosystem. In order to understand details of the mechanisms connecting hot spring use and ecosystem, we investigated the river water quality and the water temperature from the upper reaches to the river mouth in the Hirata River. The results show that the hot spring drainage flowing in a specific section affects the estuary area. Additionally, it is revealed that the current average water temperature of hot spring drainage is very good condition for Nile tilapia in the estuary area. It is suggested that the tradeoffs between the hot spring use and the ecosystems can be resolved by suppressing high-temperature unused hot spring drainage, and/or by collecting hot spring drainage and reduce its average temperature further by using its thermal energy more extensively.