Monthly Trophic Dynamics of Lakes and Reservoirs in Eastern China Based on Harmonized Landsat–Sentinel Observations

Eutrophication in shallow plain lakes is highly dynamic and often characterized by short-term trophic fluctuations that are difficult to resolve using annual or single-sensor satellite observations. The Eastern Plain Lake Zone (EPL) of China contains numerous shallow lakes and reservoirs embedded in densely populated and intensively cultivated lowland catchments, making it a representative region for high-frequency eutrophication monitoring. Here, we developed a monthly Trophic State Index (TSI) monitoring framework for lakes and reservoirs in the EPL using Harmonized Landsat–Sentinel (HLS) observations. The XGBoost model using combined spectral features achieved the best validation performance (R2 = 0.90, RMSE = 5.23), and provided the highest trophic-state classification accuracy, with an overall accuracy of 0.73 and a Kappa coefficient of 0.66. Lakes showed substantially higher trophic levels than reservoirs, with mean TSI values of 57.32 ± 6.89 and 45.00 ± 9.22, respectively. Temporally, significantly decreasing and increasing TSI trends accounted for about 15% and 5%, respectively. Natural lake TSI reflects integrated climatic, anthropogenic, and hydro-morphological influences, whereas reservoir TSI variability may be more closely associated with hydrological regulation and morphometric conditions. Our results show that HLS observations provide a robust basis for monthly eutrophication monitoring in optically complex shallow lake regions. The EPL-focused framework highlights the value of virtual satellite constellations for detecting short-term trophic deterioration and supporting region-specific lake and reservoir management.