Lake Ikeda is a caldera lake, located 40 km south of Kagoshima city hall. It has a surface area of 11 km² and a shoreline length of 15 km. The maximum depth amounts to 233 m. Together with Mt. Kaimon on its south side, it forms one of the most scenic spots of Southern Kyushu tourist zone, Kirishima-Kinkowan National Park.

The lake water quality was excellent until some years ago, but has been deteriorated due to the development of the surrounding areas since around 1955. The transparency of the lake measured 26.8 m in 1929, but decreased to about 5 m in 1955-2007. Freshwater red tides have also been observed in 1980-1981 and 1991-1993. There are resort facilities in the northeastern part of the lake, which attract 2.8 million tourists annually. The lake also serves for fish culture and irrigation. An irrigation project for 6,000 ha of agricultural field and 10,000 households was initiated in 1965 and the courses of three nearby rivers were diverted into the lake. The irrigation system has been in operation since 1982, resulting in a considerable amelioration of the lake water quality (Q). (1987, revised in 2018)

In March 1983, the Kagoshima Prefectural Government drew up the plan to preserve the water environment of the Lake; The Ikeda Lake Water Quality Control Plan (for the period from April 1983 to March 1991). The plan went through the update a couple of times (II: 1991.4~2001.3, III: 2001.4~2011.3) and the fourth plan (IV: 2011.4~2020.3) was prepared in March 2011. It aims to keep the water quality of the Lake Ikeda good for the foreseeable future by avoiding eutrophication due to T-N and T-P, and by preventing the adversary effect of incomplete lake water overturn.

Owing to the activities following the plan, COD and the total concentration of phosphorous are achieving the target of the plan. Regarding the total concentration of nitrogen, the value was going over the target value between 1998and 1999 due to the increased inflow by the Nansatsu Field Irrigation Project. Recent year, however, it keeps roughly the same level as the target of the plan.

Lake water overturn was incomplete and it made the bottom layer (~200m) devoid of oxygen. As a result, COD, the total concentration of nitrogen and phosphorous was increasing since 1992, peaking during 2003 and 2005. In February of 2011 and 2012, whole overturn of the lake water occurred in a row and made the density of the resolved oxygen high, which in turn reduced the total phosphorous concentration at the bottom layer significantly. In winters during 2013 and 2017, whole overturn of the lake water did not occur and the density of the resolved oxygen at the bottom layer was showing decreasing trend. (March 2018)