Dynamics of nitrogen and phosphorus profile and its driving forces in a subtropical deep reservoir

Abstract

Nitrogen and phosphorus stratification is crucial for ecosystem dynamics in deep lakes and reservoirs. It is critical for water quality management to understand the characteristics of nitrogen and phosphorus stratification and its driving forces. In this study, high-frequency total nitrogen (TN) and total phosphorus (TP) from January 2017 to October 2019 were estimated using the datasets of high-frequency buoy parameters, including water temperature, pH, chlorophyll-a, oxidation-reduction potential, dissolved oxygen, and fluorescent dissolved organic matter. The results revealed that both nitrogen and phosphorus in water column were periodically stratified. Specifically, the stratification of nitrogen and phosphorus occurred from April to December or January of the following year. Moreover, indices of TN stratification (IC-TN) and TP stratification (IC-TP) were - 0.29 ~ 0.05 and - 0.78 ~ 0.28, respectively. Significant (P < 0.01) positive correlations were observed between RWCS (an index of thermal stability) and IC-TN (or IC-TP), indicating thermal stratification may be the main driving force of nutrient stratification at inter-month scales. Further analysis indicated that the thermal stratification may affect nitrogen and phosphorus stratification though (1) influencing algal growth and (2) affecting the release of internal sources and the material exchange between water columns. Furthermore, precipitation is also suggested as an important factor affecting the stability of nitrogen and phosphorus vertical profile in the flood season. These findings may provide important information for optimizing water quality management efforts in Qiandaohu and other subtropical deep reservoirs. In addition, the knowledge of the effect of temperature and precipitation on nutrient stratification are essential to understand future ecosystem dynamics of deep reservoirs.

Keywords: Climate changes; Deep reservoir; Eutrophication; Nitrogen and phosphorus; Thermal stratification.