Vulnerability of a large monomictic lake (Lake Biwa) to warm winter event

Detailed cross-sectional temperature and oxygen distributions during winter were examined over 5 years (2003–2007) in large, warm, monomictic Lake Biwa. The distribution patterns showed year-to-year variation in the degree of spatial heterogeneity and indicated that both convective mixing and gravity currents contributed to the delivery of oxygen to the profundal zone. We encountered a warm winter in 2007, when deep-water oxygenation was delayed by more than a month relative to normal years. Data obtained during the period of oxygenation in 2007 suggest that the timing of complete oxygenation of the bottom water (or lower layers of the hypolimnion) was controlled by physical processes responsible for destruction of the weak thermal stratification near the bottom. This study revealed that Lake Biwa is sensitive to year-to-year variations in winter meteorological conditions. Specifically, the bottom environments are highly susceptible to a warm winter, especially when it follows a cold winter.