Numerical analysis of the relationship between mixing regime, nutrient status, and climatic variables in Lake Biwa.

As awareness of climate-related freshwater quality problems increases, more research is needed to better understand how climate impacts water quality. Climate has significant impacts on the mixing regime and nutrient status of lakes. However, the relationship between climatic variables, mixing regime, and nutrient status in large monomictic lakes requires further study. Here we applied a three-dimensional ecosystem model to the large monomictic Lake Biwa, where hypoxia had recurred over the past 60 years. The model was validated using monitoring data, and the relationships among mixing regime, nutrient status, and climatic variables have been investigated. The turnover timing, which represented the mixing regime, varied by 36 days and depended most on wind speed but least on air temperature. In the early period prior to waste-water treatment there was a strong linear relationship between phosphorus and deep-water dissolved oxygen (DO) concentrations. Following this period, the relationship weakened but remained significant. In addition, we find a weak relationship between turnover timing and deep-water DO concentrations. We suggest that later turnover timing may favor lower deep-water DO concentrations, which in turn may favor release of legacy nutrients from sediments. Although waste-water treatment has improved conditions in the lake, climate change induced alteration of turnover timing may adversely influence water quality. Maintaining water quality under continued warming may require more rigorous controls on phosphorus loading to the lake.