Identifying the Key Stages, Pollutants, and Driving Mechanisms of Water Quality Variation in Relation to Water Level Dynamics

Distinguishing the response of water quality to water level is of great significance for improving reservoir water quality and ecosystems stability. Present research primarily examines the relationship between water level and water quality from a holistic perspective, with infrequent differentiation among various states such as high water level, low water level, water level decline, and water level rise. The paper focused on Miyun Reservoir as a case study, examining the trends in reservoir water quality, water level, and nutrient input over the past 31 years. It employed hierarchical partitioning analysis to investigate the relative impacts of nutrient input and water level on reservoir water quality and identify the main stages, pollutants, and driving mechanisms by which water level influences water quality. The findings indicated that, in every instance, total phosphorus was predominantly constrained by nutrient input. At high water levels, nutrient input significantly impacted water quality. Conversely, during other stages, water level was the primary factor that affecting the variations in nitrogen-containing substances. Through the analysis of relationships among nitrate nitrogen proportions, hydraulic retention time, and the processes of ammonification, nitrification, and denitrification, the mechanisms by which water levels influence nitrogen concentration variations were determined to differ across the three stages. During the drop of water level, the hypoxia in the reservoir inhibited the nitrification process, leading to the accumulation of ammonia nitrogen. In the low water level stage, the denitrification rate continued to decline after the water level dropped, resulting in increased concentrations of total nitrogen and nitrate nitrogen. During the water level rise stage, the fluctuation zone released significant amounts of ammonia nitrogen into the water, causing a rise in total nitrogen and ammonia nitrogen concentrations. These findings could offer a foundational framework for enhancing water environment management strategies aimed at improving water quality through the regulation of water levels.

Graphical abstract