Rainfall impact on the disappearance of algal blooms in Xiangxi River of Three Gorges Reservoir under stratified density current

Abstract

为探究在三峡水库特殊分层异重流背景下降雨对水华消退的影响,以香溪河为例,对库湾降雨前后水动力、生态环境因子开展连续三维立体跟踪监测。结果表明:降雨对水华的消退作用显著,降雨后香溪河库湾叶绿素a(Chl.a)浓度明显下降。热分层稳定指数(RWCS/H)变化不大,库湾近河口处分层较弱、中上游分层较强的特性并未随此次降雨发生较大变动。受降雨影响,藻类在表层水体聚集程度降低,藻类聚集度指数(MI)、微藻群体平均深度(MRD)下降。库湾流态随降雨发生而变得复杂,库湾水体浊度明显增加,异重流倒灌形式由近表层倒灌向中下层倒灌转变,雨后又逐渐转变为中层倒灌,长江干流水体倒灌进入库湾的影响范围、潜入深度增加。水体水平输移增强,分散下沉的藻类易随水体环流流出库湾,水华消退。雨后库湾入库流量增加,大部分上游来流依旧由上层流向河口,与中层倒灌异重流形成逆时针环流,藻类无法在表层水体稳定生长,库湾Chl.a浓度能在较长时间内保持较低水平,不会再次迅速暴发水华。;In order to explore the impact of rainfall on the disappearance of algal blooms under the background of special stratified density current in the Three Gorges Reservoir, taking Xiangxi River as an example, this study conducted continuous three-dimensional tracking monitoring of hydrodynamic and ecological environmental factors before and after rainfall in the Xiangxi Bay (XXB). Results showed that rainfall had an obvious effect on the disappearance of algal blooms. The concentration of chlorophyll-a decreased significantly after rainfall in the XXB. The stability index of thermal stratification (RWCS/H) changed slightly. Meanwhile the phenomenon which the stratification was weak near the estuary area but strong in the middle and upper reaches of the XXB did not change significantly under the rainfall. Affected by rainfall, the degree of algal aggregation in surface water, the morisita's depth (MI) and mean residence depth (MRD) all decreased. The flow pattern of the XXB became complex with rainfall, with a significant rise in the turbidity of the water body. The pattern of backflow density current changed from near-surface inversion to middle-lower inversion, and gradually changed into middle inversion after rain. The influence range and depth of the backflow of the Yangtze River mainstream into the XXB increased. Besides, the horizontal transport of water body enhanced so that the algae that dispersed and sank were easy to flow out of the XXB with the water circulation, and algal blooms disappeared. After the rain, the inflow of the XXB increased, and most of the upstream inflow still flowed from the upper layer to the estuary, forming a counterclockwise circulation with the middle inverted density current, which made it impossible for algae to grow stably in the surface water. The concentration of chlorophyll-a could be in low for a long time, and there will be no more rapid blooms again.