Effects of Ecological Water Diversion on Internal Nitrogen and Phosphorus Release in a Typical Small Shallow Lake in China

Abstract

Ecological water diversion is an important method to improve water quality in lakes and reservoirs. But the environmental effects, from the ecological water diversion project (EWDP) to the internal release of sediment nutrients, remain unclear. In this study, an indoor simulation of an EWDP with different treatment scenarios with water transfer proportions of 25%, 50%, 75% and 100% was conducted to study the effects of water diversion on sediment nitrogen and phosphorus release in Lake Wanshandang. Our results showed that the flux of NH3–N released from the sediments in the western and eastern areas of Lake Wanshandang was significantly reduced after water transfer treatment, and the degree of reduction increased with increased water transfer. Specifically, the release flux of NH3–N in the sediment in the western area decreased from 18.02 mg/(m2/d) to −2.25 mg/(m2/d) when the transferred water reached 100% replacement of the original overlying water. The effect of water transfer treatment on the release flux of SRP from sediment varied greatly throughout the lake. After treatment, the SRP release flux in the western and central areas increased significantly, while it decreased in the eastern area. The NH3–N and SRP concentrations changed from 0.12–0.27 mg/L and 0.02–0.049 mg/L to 0.28–0.84 mg/L and 0.01–0.066 mg/L before and after the water transfer treatment. Our statistical analysis showed that the change in NH3–N and SRP release fluxes after treatment was significantly negatively correlated (p < 0.05) with concentrations of NH3–N or SRP in the overlying water before and after water transfer. We suggest the increase in NH3–N and SRP concentrations in the overlying water after the water transfer treatment led to the subsequent decreased NH3–N or SRP release flux, while the decrease in SRP concentration in overlying waters enhanced SRP release from the sediment. The differences in the concentrations of nitrogen and phosphorus between the original overlying water and the transferred incoming water are important factors affecting the release of nutrients from sediment.