三峡库区澎溪河与磨刀溪电导率等水质特征与水华的关系比较

Comparison of Relationship Between Conduction and Algal Bloom in Pengxi River and Modao River in Three Gorges Reservoir

三峡库区北岸最大一级支流自三峡大坝2003年蓄水以来,频繁暴发水华,而毗邻的一级支流磨刀溪却少有水华暴发.本文以澎溪河和磨刀溪作为研究对象,于2014年春季和夏季三峡库区水华高发期对两条河流同时采样,对比分析两条河流水体水质以及叶绿素a(Chl-a)含量的时空变化,探索澎溪河水华暴发机理.结果表明:澎溪河Chl-a含量较磨刀溪高,澎溪河春季Chl-a最大值为60.5μg·L^(-1),夏季Chl-a最大值仅7.8μg·L^(-1);磨刀溪Chl-a变化趋势与澎溪河相反,磨刀溪春季Chl-a含量为2.92μg·L^(-1),夏季Chl-a达到7.48μg·L^(-1).澎溪河与磨刀溪春季和夏季节水体温度分层,为温跃层+滞温层模式,而没有混合层;两条河流Chl-a含量均位于水深10 m温跃层.澎溪河春季总氮(TN)、总磷(TP)平均值为2.305 mg·L^(-1)和0.053 mg·L^(-1),夏季为1.673 mg·L^(-1)和0.097 mg·L^(-1);磨刀溪春季为1.875 mg·L^(-1)和0.075 mg·L^(-1),夏季为1.79 mg·L^(-1)和0.054 mg·L^(-1).TN、TP水平均超过了国际公认发生富营养化的阈值;水体氮磷含量与Chl-a浓度并无显著相关性,营养盐并不是藻类生物量的限制性因素.然而在水体电导率的规律方面,两条河却存在很大的差异;春季,磨刀溪上游上层水体(0~10 m)电导率只有下游和长江干流的75%,来自长江干流的回水可覆盖至磨刀溪中游(断面MD03),与Chl-a在此处密集保持一致;夏季电导率和回水区分布与春季相似.与磨刀溪不同,澎溪河春季上游电导率为下游和干流的150%,长江干流回水可到PX04与PX05之间,上游高电导率对应着高Chl-a含量;澎溪河电导率与藻类生长分布表现出显著正相关关系,水体中除N、P营养盐外的其它离子对澎溪河水华暴发起重要作用.

Pengxi River is the largest tributary in the northern bank of Yangtze River in the Three Gorges Reservoir （TGR） region, and serious algal bloom has been reported since 2003 when the TGR dam started impounding water. On the other hand, Modao River, a tributary in TGR locating in the same county with Pengxi River, was rarely reported with algal bloom in the same time period. In this study, water samples were collected on the same day in both rivers in spring and summer, the most likely blooming seasons in TGR, to compare the water quality parameters and Chlorophyll-a（Chl-a） concentration dynamics in order to understand the mechanism of algal bloom in TGR. The results showed that the maximum value of Chl-a in Pengxi river reached 60.5 μg·L-1 in spring, and was only 7.8 μg·L-1 in summer, while in Modao river the Chl-a content was 2.92 μg·L-1 in spring and 7.48 μg·L-1in summer. Both rivers stratified during samplings, but having layers of thermocline and hypolimnion with the lack of epilimnion, and most of Chl-a content was located in thermocline. Also in both rivers, TN and TP exceeded the international threshold for eutrophication, with the average TN and TP values of 2.305 mg·L-1 and 0.053 mg·L-1in Pengxi River in spring, and 1.673 mg·L-1 and 0.097 mg·L-1 in summer, respectively, while in Modao River the values were 1.875 mg·L-1 and 0.075 mg·L-1 in spring, and 1.79 mg·L-1 and 0.054 mg·L-1 in summer respectively. Nutrients concentrations showed no significant correlation with Chl-a. On the other hand, conductivity value and trend were totally different between the two rivers：in Modao River in spring, the conductivity in upstream was only 75% of that in the main stream of the Yangtze River, and the backwater from the main stream reached to the middle in Modao river, where the highest Chl-a among all the river sampling sites was detected; summer conductivity distribution was similar with that in spring. Different from Modao River, the conductivity in upstream of Pengxi River in spring was 150% of that in the main stream of Yangtze, the backwater from the main stream reached area between sampling sites of PX04 and PX05 （upper than the middle reach）; its upstream had significantly high content of Chl-a and conductivity, and these two factors were significantly positively correlated. The study showed that other than N and P, other ions in the Pengxi River played an important role in bursting ＂bloom＂, and need to be considered regarding bloom control.