分层异重流对香溪河浮游植物叶绿素a空间分布的影响

基于三峡水库香溪河库湾汛期（2017年8月4—10日）水动力、气象和藻类漂移监测数据，运用数理统计方法，分析汛期香溪河浮游植物叶绿素a（Chla）浓度空间分布特征及其影响因素，探究分层异重流背景下藻类的水平输移和垂向掺混作用过程。结果表明：1)汛期香溪河库湾水动力具有明显的分层异重流特性，监测期间雷诺数均大于4 000，水体呈现紊流状态，对藻类的运动输移产生水平输运和垂向掺混2种影响。2)表层藻类水平漂移速度受到风速和水流流速的影响，响应关系式为V_藻=0.035V_风+0.461V_水+0.034 (R²=0.917，P<0.01)。3)分层异重流对高浓度藻细胞库湾原水的稀释作用和环流对高浓度藻细胞的携带作用，导致水柱Chla总浓度呈直线下降趋势，此时干流水体从中上层倒灌进库湾，上游来水以底部顺坡异重流方式流向河口，在中上游相遇形成环流，在库湾中部倒灌水体一分为二，分别从底部和表层输出库湾，Chla浓度分布特征为表层>底层>中层；当倒灌水体与库湾原水形成环流并从底部输出库湾时，Chla浓度分布特征为表层>中层>底层或者中层>底层>表层。因此，Chla浓度的水平分布主要受到风速和异重流的水平流速的影响，而垂向分布受到异重流所形成的水体循环模式的影响。

Effects of stratified density flow on the spatial distribution of chlorophyll-a in phytoplankton in Xiangxi River

The spatial distribution characteristics of chlorophyll-a (Chla) in phytoplankton in the Xiangxi River during the flood season and their relevant affecting factors were analyzed by applying mathematical statistics methods and based on the monitoring data of hydrodynamics, meteorology and the drifting of the algae in Xiangxi River reservoir of the Three Gorges Reservoir during the flood season (August 4-10, 2017). The process of horizontal transport and vertical mixing of algae under the background of stratified density flow was explored. The results showed that: 1) The hydrodynamic data of Xiangxi River reservoir was featured with the obvious density-stratified flow. Reynolds numbers during the monitoring period were all above 4 000, and the water body got into the turbulent state, which produced two effects on the movement of the algae, i.e. horizontal movement and vertical mixing. 2) The horizontal drift velocities of the algae on the water surface was affected by wind speeds and water velocities with the response formula Valgae=0.035Vwind+0.461Vwater+0.034 (R2=0.917, P<0.01). 3) The dilution effect of density-stratified flow on the raw water of high-concentration of algal cells in the reservoir and the carrying effect of circulations on high-concentration algal cells led to a linear downward trend of the total Chla concentrations of the water column. Meanwhile, the mainstream poured back into the reservoir at the upper and middle layers, and the upstream inflow flowed to the river mouth in the form of density current at the bottom along the slopes. The two flows met in the middle and upper sections of the river and thus a circulation was formed, and the pouring-back flow was divided into two parts in the middle of the reservoir, which were respectively discharged from the surface and the bottom. Chla concentrations presented such a sequence: surface > bottom > middle. When the circulation was formed as a result of the clash between the flow poured back and the raw water of the reservoir and discharged from the bottom to the reservoir bay, Chla concentrations presented this sequence: surface > middle > bottom, or middle > bottom > surface. The results suggested that the wind speeds and water horizontal velocities of density flow were key factors for the horizontal distribution of Chla, while the vertical distribution was affected by the water circulation mode formed by density flow.