Evolution of Trophic Status and Its Driving Factors in Danjiangkou Reservoir

Understanding the changes in trophic status within the Danjiangkou Reservoir, the primary water source for the Middle Route of the South-to-North Water Diversion Project, is essential for effective water quality management. Accordingly, this study integrated long-term monitoring data and published research to analyze the current characteristics and seasonal variations in the trophic status of the Danjiangkou Reservoir, while also examining the evolution and driving factors influencing this status since the construction of the reservoir. Our findings indicate that the reservoir predominantly exhibits a mesotrophic state, with comprehensive trophic level index (TLI) ranging from 30 to 48, and the annual average TLI of the Han Reservoir (38~42) is consistently higher than that of the Dan Reservoir (32~36). Furthermore, specific areas, such as reservoir bays and tributary confluences, were identified as high-risk zones for eutrophication. The trophic status of Danjiangkou Reservoir can be divided into four stages: oligotrophic (1968–1990), mesotrophic (1990–2006), mesotrophic (2006–2014), and fluctuating (2014–2024). Key factors contributing to changes in the trophic status of the reservoir include variations in pollution discharge within the basin, alterations in land-use, reservoir operation and regulation practices, and extreme climatic and hydrological events. In light of these findings, this study proposes management strategies focused on zoned and classified control, coupled with integrated basin management, to provide scientific support for maintaining the water quality safety of the Danjiangkou Reservoir.