Increasing risk of water quality deterioration in a typical inland lake of China

Comprehensively evaluating water quality is of great significance to lake environment management. In this study, we integrated field measurements and long-term Landsat-5/8 images to assess the water quality of Nansi Lake, one of the important water transport trunks of the east route of the South-to-North Water Diversion project in China. Results showed that water temperature of the lake in spring has risen by about 4.7 °C from 2013 to 2020. The COD Cr and TP both showed a downward trend while NH 3 N presented an upward trend in spring and autumn after 2014. The TN concentration showed a linear upward trend in autumn after 2014. Comprehensively, the WQI and TLI in both spring and autumn increased slightly from 2013 to 2015, decreased rapidly in 2016, and then presented a linear increase trend, indicating that the degree of water pollution in Nansi Lake has continued to increase since 2016, and the nutrient status has changed from mesotrophic to mild eutrophication. The concentration of COD Mn is highly positively correlated with COD Cr and both closely connect with the content of BOD5 and TP. The NH 3 N, TP and TN are covariant with each other. Remote sensing-based Chl a exhibited a gradual increase from about 10 mg/L in 1990 to 18.3 mg/L in 2005 and then linearly dropped and kept at around 10 mg/L after 2010. The change of water quality in Nansi Lake is highly correlated with the growth of population and the expansion of local fishery size, meaning a prolonged impact of social-economic development.