First observation of incomplete vertical circulation in Lake Biwa

Vertical circulation reaches the bottom of the deepest (> 90 m) part of the northern basin of Lake Biwa, a warm monomictic lake, during winter (January to March) every year. In winter 2019, we found non-circulated water masses at the bottom of the lake where the basin is 90 m deep. This is the first observation of an incomplete vertical circulation event since we started monitoring Lake Biwa in 1979. Analysis of water quality monitoring and meteorological data shows that weaker cooling and weaker winds during winter 2019 affected vertical circulation. Dissolved oxygen concentrations in the bottom layer recovered even after the appearance of a seasonal thermocline. However, the concentrations remained below that in the surface layer.