向家坝水库营养盐时空分布特征及滞留效应

向家坝建库后改变了河流原有的水动力、营养盐分布及输移条件.为研究向家坝水库营养盐分布特征及滞留效应,通过2015～2016年分季度水库水质监测结果,分析向家坝水库水体总氮(TN)、总磷(TP)和溶解性硅(SiO_3~(2-)-Si)营养盐时空分布特征、滞留量、滞留效率.研究发现,向家坝水库TN、TP和SiO_3~(2-)-Si营养盐质量浓度均值分别为0. 905、0. 034和7. 98mg·L~(-1).其中,TN质量浓度在城镇人口密集区偏大,分布主要受点源影响;磷营养盐以颗粒态磷为主,TP质量浓度在水库中自上而下沿程降低,SiO_3~(2-)-Si质量浓度分布在时空上差异较小.向家坝对TN、TP和SiO_3~(2-)-Si营养盐滞留量为2. 30×10~4、0. 146×10~4和-2. 4×10~4t·a~(-1).在不同季度,TN和SiO_3~(2-)-Si滞留量有正有负,而TP则始终表现为正滞留. TN、TP和SiO_3~(2-)-Si月平均滞留效率分别为17. 5%、32. 8%和-2. 14%.整体上实际滞留效率表现为丰水期高于枯水期,并且TP的滞留作用更为显著. TN滞留量主要受反硝化作用,以及外源负荷输入影响; SiO_3~(2-)-Si输送通量主要受径流量影响;水库运行周期以及磷的颗粒形态则是TP滞留的主要因素.向家坝水库对营养盐的滞留效应与TN和SiO_3~(2-)-Si质量浓度变化无明显相关性,而水库对TP的滞留效应使TP质量浓度在水库纵向上沿程减小,在各监测样点垂向水深上TP质量浓度则有增大的趋势.

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.