[Spatial and Temporal Distribution Characteristics and the Retention Effects of Nutrients in Xiangjiaba Reservoir]

After the construction of the Xiangjiaba Dam, the hydrodynamic conditions, nutrient distributions, and transport conditions of the Jinsha River were changed. Here, the nutrient distribution characteristics and retention effects of Xiangjiaba Reservoir were investigated according to the results of water quality monitoring from 2015 to 2016. Spatial and temporal variations in TN, TP, SiO₃^2-Si, and other nutrients, and retention flux and retention rate were analyzed. The results showed that the nutrient mass concentration of TN, TP, and SiO₃^2--Si was 0.905 mg·L^-1, 0.034 mg·L^-1, and 7.98 mg·L^-1, respectively. The distribution of TN was affected by point sources and the concentration of TN was large in urban areas. This distribution of TP was mainly granular and the mass concentrations decreased along the river path. The mass concentration of SiO₃^2--Si did not significantly vary over time and space. Furthermore, Xiangjiaba Reservoir had a persistent effect on nutrient salts; the average annual retention of TN, TP, and SiO₃^2--Si was 2.30×10⁴ t·a^-1, 0.146×10⁴ t·a^-1, and -2.4×10⁴ t·a^-1, respectively. During different seasons, the retention of TN and SiO₃^2--Si varied between positive or negative; however, TP appeared to be consistent. The average monthly retention efficiency of TN, TP, and SiO₃^2--Si was 17.5%, 32.8%, and -2.14%, respectively. Overall, retention efficiencies were higher during the dry season than that wet season, and phosphorus retention was most pronounced. The retention of TN in the reservoir may be related to denitrification and the input of external load; the flux of SiO₃^2--Si was mainly affected by runoff; and the particle morphology of phosphorus, as well as reservoir period, were the main factors affecting TP retention. There were no clear correlations between nutrient retention and the mass concentrations of TN and SiO₃^2--Si, but the nutrient retention effect of Xiangjiaba Reservoir reduced TP concentrations along the river path and increased TP concentration with vertical depth.