Influence of Thermal Stratification on Nitrogen and Phosphorus Profile Distribution in a Deep Subtropical Reservoir of China

Nitrogen and phosphorus stratification is crucial for water quality management and ecological processes in deep lakes and reservoirs and can be substantially affected by hydrological and meteorological processes in the catchment. To understand the response of nitrogen and phosphorus stratification when changing physical stratification processes is crucial important for water quality management in reservoirs using drinking water source. Detailed depth profiles of water temperature (WT), dissolved oxygen (DO), and other high-frequency sensors data were recorded daily in Qiandaohu reservoir, a large, deep subtropical reservoir in China, during 2017 to 2019. In addition, high-frequency measurement of total nitrogen (TN) and total phosphorus (TP) were undertaken once every three days, and the empirical estimation equations of TN and TP were established through multiple regression analysis. It was found that the nitrogen and phosphorus in water column was periodically stratify. Thermal structure stratification is the main driving force of nitrogen and phosphorus stratification. However, in the monthly scale, rainfall is the main factor to affect the stability of nitrogen and phosphorus vertical profile. This research indicated that physical processes play an important role on the biogeochemical process of nutrient in deep reservoirs. To evaluation the water quality or simulating the ecological processes of reservoirs, climatic and meteorological factors must be considered.