It is an increasing concern that the depletion of dissolved oxygen (DO) concentration at the bottom layer of the northern-part of Lake Biwa with 100m in maximum depth has been worsen in recent years not only due to eutrophication in water but also the effects of global warming such as the raise of air temperature during winter. Many research have been carried out by various organizations to clarify the mechanism of DO depletion near the lake bottom and to propose the countermeasures. This article firstly describes the seasonal variations of the vertical water temperature and water quality distributions to explain the fundamental features of heat and mass transfer occurring in the northern part of Lake Biwa throughout the year. Since it is well known that the thermal convection during the cooling period is the main mechanism of DO transfer from the surface to the lake bottom, the mixing processes caused by thermal convection are shown using the results of a 3D CFD simulation. Then, the electrolysis of water at the lake bottom for the restoration and DO recovery is introduced as one of the countermeasures proposed by Lake Biwa Environmental Research Institute, Shiga Prefecture. The simulation results of O2 bubble plumes with the dissolution into lake water are also shown under the conditions of laboratory and field experiments.