Abstract: Lake Hongfeng is an important drinking-water source for Guiyang City, where eutrophication is a potential risk to water quality. It is of great importance to understand the trophic level in the lake to ensure the drinkingwater safety. Combined with statistical analysis, we analyzed the trophic level and its evolution in the lake using the comprehensive trophic level index, based on the water quality monitoring data between 2004 and 2018. The results showed that the trophic level of the lake was largely between mesotrophication and slight eutrophication during the fifteen years. Although the trophic level has been alleviated since 2009, the probability of slight eutrophication was significantly increased in 2018. It is difficult to evaluate the trophic level accurately based on the national unified method, indicating the necessity for improving the method of trophic level assessment. The risk of eutrophication would be a result of multiple factors, including nutrients and their cycling conditions, hydrodynamic and energy conditions. More attention should be paid to the changes in water hydrodynamic conditions and the internal circulation of nutrients. Developing prediction and early warning technologies is of importance to ensure the safety of water quality in Lake Hongfeng.

Key words: eutrophication, comprehensive trophic level index, sub-deep lake, influencing factor.