Evaluation of eutrophication in freshwater lakes: A new non-equilibrium statistical approach

Lake eutrophication has become a global problem of water pollution, while factors such as Chlorophyll-a (Chl-a), total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD) and secchi depth (SD) are the main indicators to evaluate lake eutrophication level. Considering the interaction among these factors, this paper applies a new dynamic model to investigate the changes of the lake's trophic level. Taking Lake Fuxian, Lake Yilong and Caohai of Lake Dianchi in China as examples, we built a lake eutrophication simulation model based on maximum flux principle (MPF) and self-organised map (SOM) and analysed the relationship between environmental factors and lake eutrophication level using monthly data. The model calculates the monthly eutrophication level of each lake, which is consistent with the actual situation of these lakes, and this consistence verified the veracity of our model results. In addition, according to the relationship between environmental factors and eutrophication, the different simulation results and the original results of Lake Fuxian and Lake Dianchi, we found that the physical factors such as SD can be the accurate and intuitive observations of the trophic status in clean-state lakes with lower-mesotrophic level, while the trophic level of turbid-state lakes with hypereutrophic level are susceptibly driven by the nutrient such as N and P.