汛末蓄水期神农溪库湾倒灌异重流特性及其对水华的影响

Effect of Reverse Slope Density Flow on Algal Blooms in Shennong Bay during Three Gorges Reservoir Impoundment

为研究三峡水库神农溪库湾倒灌异重流特性、光混比特性及其对水华的影响,根据神农溪库湾的地理特性及库湾区域性水华特性,从神农溪河口至回水末端,沿渡河镇河道中泓线设置7个监测点位（SN01~SN06）,于2014年汛末蓄水期对神农溪库湾的水温等指标进行跟踪监测分析。结果表明,三峡水库蓄水过程通过倒灌异重流作用,最大单日水位变幅为1.04 m,干流水体分别从表层、中上层和底层倒灌潜入库湾;倒灌异重流增强了干支流间水体交换,缩小了干支流间水温差,打破支流水温分层,使支流混合层深度逐步增大。库湾叶绿素a浓度呈现时空差异性,整个蓄水周期内,SN02受干流水体影响明显,叶绿素a浓度始终维持在低水平,均小于20 mg/m^3,蓄水过程中,SN05和SN06断面混合层深度由6.0 m上升至22.5 m,叶绿素a浓度由高于30 mg/m^3降低至10.30 mg/m^3,导致神农溪库湾叶绿素a浓度降低的主要原因是蓄水过程加强了干支流水体交换程度,减小光混比至0.2,库湾叶绿素a浓度均值降低为11.00 mg/m^3,水华情势最终得到有效控制。

Shennong Stream is a typical tributary of Three Gorges Reservoir（TGR） and the backwater,forming Shennong Bay（SNB）,extends 20 km when the TGR impoundment is at 175 m（full pond）.At present,there are few reports on the aquatic ecology of SNB.In this study,we investigated the effect of reverse slope density flow on the ratio of euphotic depth to mixed-layer depth and its influence on algae blooms in SNB.Water depth,velocity,temperature,chlorophyll-a（Chl-a）,euphotic depth and mixing depth in SNB were monitored in 2014 during the impoundment period of TGR that follows the flood season.The study aimed to explore the relationship between hydrodynamic conditions and algae blooms and test the universality of the effect of reverse slope density flow on algae blooms.The study provides theoretical evidence for the control of algae blooms by ecological means.The impoundment in 2014 was divided into two periods： from September 15 to October 8,with water level rising from 165.0 m to 172.4 m,and from October 9 to October 31 with water level rising from 172.4 m to 175.0 m.During impoundment,hydrological parameters were monitored at seven sampling sections（SN01-SN06,located sequentially from the beginning to the end of the backwater,and CJBD,located 0.5 km above the confluence of Shennong Stream with the main stem of the Yangtze River）.In 2014,the maximum variation of daily water level in TGR was 1.04 m during the impoundment period.Water from TGR flows into SNB when there is a large rise in TGR water level and density currents result in different plunging depths into SNB： first at the surface,then at mid-depth（interflow） and finally at the bottom.The reverse slope density flow strengthens mixing in SNB,increasing the mixing depth and decreasing the difference in water temperatures between TGR and SNB.During the entire impoundment period,the Chl-a level in Shennong Bay varied spatially and temporally.The Chl-a at SN02 was clearly affected by TGR intrusion and the concentration of Chl-a remained low,always 20 mg/m^3.The mixing depth at SN05 and SN06 increased from 6 m to 22.5 m,resulting in a decrease in chlorophyll-a from 30 mg/m^3 to 10.3 mg/m^3.Overall,the average chlorophyll-a content in Shennong Bay decreased to 11.0 mg/m^3.The primary reason for the decrease in Chl-a was the water exchanged between TGR and SNB.Additionally,the ratio of euphotic depth（Zeu） to mixing depth（Zmix） was significantly influenced by the adverse slope density flow.The ratio,Zeu/Zmix,decreased to 0.2 and the algal blooms were effectively controlled in SNB,indicating that control of algal blooms in Shennong Bay by ecological means is possible.