澜沧江梯级筑坝下水体氮磷分布特征及其形成机制

Distribution characteristics and controlling mechanisms of nitrogen and phosphorus in water under cascade dam construction of Lancangjiang River

水库建设改变了河流水文情势及物质迁移转化过程,从而影响水环境质量。为探究梯级筑坝影响下河流氮、磷的空间分布特征及其形成机制,以澜沧江为研究对象,于2016年和2021年分别开展了沿程水环境监测,对比分析水体中氮、磷及其形态浓度在水库建成前后的变化及沿程分布特征,探究氮、磷变化及其沿程分布的主控因子和影响机制。结果表明:由于河流建库蓄水淹没的土地释放大量土壤有机氮,新建水库段(2021年)水体总氮(TN)浓度相比于建库前(2016年)显著上升;由于建库后水流流速减缓而促进颗粒态磷沉降,水体总磷(TP)浓度显著下降。此外,河流建库蓄水后原自然河道的水环境特征改变且利于沉积物磷的释放,筑坝后水体磷酸盐(PO^(3-)_(4)-P)占生物可利用磷(Bio-P)的比例显著上升。受沿程土地利用的影响,从上游到下游水体TN浓度总体上逐渐升高,而水体TP浓度由于水库的截留效应逐渐降低。筑坝增加的水力停留时间为水库氮、磷转化提供了有利条件,主要表现为溶解性无机氮以硝态氮为主转变为以氨氮为主;同时,Bio-P中PO^(3-)_(4)-P的占比也有所上升。研究揭示了澜沧江梯级筑坝对水体氮、磷分布特征的影响及机制,研究结果可为澜沧江梯级水库建设下河流生态环境保护提供科学依据。

Reservoir changes the hydrological characteristics of rivers and the process of materials transport and transformation,thus affects river water quality.To investigate the spatial distribution characteristics of nitrogen(N)and phosphorus(P)as well as the associated affecting factors under the influence of cascade dams,two field campaigns were conducted in 2016 and 2021,respectively,along the Lancangjiang River.We compared the longitudinal changes and characteristics of different forms of N and P before and after reservoir impoundment in the upstream of the Lancangjiang River,and investigated the dominant factors affecting the observed variations in N and P along the river.Significant increase of total nitrogen(TN)concentration in the water column of the new reservoirs in 2021 compared with that in 2016 before the reservoir construction was observed,which was mainly due to the release of large amounts of soil organic N from the ambient inundated land.The results showed that the significant decrease of total phosphorus(TP)concentration in the water column was due to the slowdown of the flow velocity after the reservoir storage,which promoted the deposition of particulate P.In addition,the proportion of orthophosphate(PO^(3-)_(4)-P)to bioavailable P(Bio-P)in the water column increased significantly after damming,mainly due to the change in the water environment characteristics of the natural river channel,which facilitated the release of sediment phosphorus.Along the cascade reservoirs,the overall TN concentration in the water column increases gradually due to the influence of land use along the Lancangjiang River,while the TP concentration in the water column decreases gradually due to the retention effect of the reservoir.Furthermore,the increased hydraulic residence time due to dam construction provided favorable conditions for the transformation of N and P in the reservoir,mainly in the form of dissolved inorganic N gradually transitioned from being dominated by nitrate to ammonium;meanwhile,the proportion of PO^(3-)_(4)-P in the Bio-P fraction also increased.This study initially elucidates the effects of damming on the distribution characteristics of N and P in the water column and its influencing factors in Lancangjiang River,and the results can provide valuable reference for ecological protection in Lancangjiang River under the cascade reservoirs.