Abstract: Abnormally high pH levels (pH > 9) have emerged as a significant water environment issue in Dianchi Lake and other highland lakes (reservoirs). By using correlation analysis and multiple linear regression, we systematically examined seven water quality indicators at eight sampling locations in the south part (which is called “Waihai”, the outer waters) of Dianchi Lake during 2016-2019. Our aim was to investigate the causes of abnormally high pH values in Dianchi Lake in recent years, and to identify its primary driving factors. Findings reveal that the abnormally high pH values in Dianchi Lake primarily occurred in summer, autumn, and winter. Specifically, during summer and autumn, no water quality indicators exhibited significant positive correlation with pH value; however, during winter, pH value was significantly positively correlated with dissolved oxygen (DO) and chlorophyll-a (Chl-a) concentration. In particular, Chl-a contributed remarkably to the abnormally high pH values in summer, autumn, and winter, with standardized coefficients of 0.260, 0.231, and 0.444, respectively. Such contribution surpassed those of other physicochemical factors during autumn and winter, with larger significance. Multiple linear regression models for summer, autumn, and winter were statistically significant, without notable issues regarding variable's collinearity or regression residual bias. These results clearly indicate that the abnormal proliferation of phytoplankton is the main cause of the abnormally high pH values in Dianchi Lake. These findings can guide efforts in identifying and scientifically treating the abnormally high pH levels in highland lakes and reservoirs.

Key words: abnormally high pH value, correlation analysis, multiple linear regression, influence factor, driving mechanism, Dianchi Lake