Trophic status observations for Honghu Lake in China from 2000 to 2021 using Landsat Satellites

Eutrophication of lakes harms aquatic organisms, changes the function of lakes and causes water pollution, which has affected global public health. As an internationally important wetland reserve, Honghu Lake is polluted to varying degrees and the eutrophication trend has accelerated. The objective of this study is to quarterly monitor and assess the trophic status of Honghu Lake from 2000 to 2021 using the trophic level index (TLI). The TLI was retrieved by developing a semi-empirical model based on radial basis function neural network (RBFNN), air temperature and water level data were added to the model as input parameters, forming a control group with the model that did not include air temperature and water level data. Based on 178 Landsat images from 2000 to 2021, the eutrophication status of the Honghu Lake over the last 20 years was successfully determined. The main findings are as follows: 1) The accuracy of TLI retrieval model, with air temperature and water level data included (R2 = 0.723, RMSE = 4.971), is significantly higher than that without air temperature and water level data (R2 = 0.197, RMSE =10.453); 2) The eutrophication level of the waters in the northwestern part of Honghu Lake was higher than those observed in other lake water; 3) There were significant seasonal fluctuations in the eutrophication of Honghu Lake. The highest TLI values were observed in summer and autumn, while the lowest values were observed in winter and spring; 4) The eutrophication trend of the Honghu Lake has significantly accelerated from 2013 onwards, showing a change in eutrophication status, from light eutrophic to mid-eutrophic. Our results indicate that the TLI retrieval method with air temperature and water level is more predictive. Moreover, the eutrophication status for the entire lake from 2000 to 2021 was determined, which may contribute to the protection of the water environment.