三峡水库泄、蓄水过程对小江浮游植物群落结构的影响

Impact of Three Gorges Reservoir Impoundment and Discharge on the Phytoplankton Community Structure of Xiaojiang River

为阐述水位升降时三峡水库支流浮游植物生态学特征的变化,分别于2013年6月和10月对小江泄、蓄水过程的浮游植物野外监测数据进行对比分析。结果表明,三峡水库泄、蓄水过程调查期,在小江分别鉴定出浮游植物6门、70属、136种（变种）和7门、70属、136种（变种）,以绿藻、硅藻、蓝藻为主,共计占总种类数的87.49%和80.87%,但2个时期种类组成略有差异;浮游植物细胞密度变化范围分别为8.1×104～127.2×104个/L和24.4×104～99.9×104个/L,泄水过程略高于蓄水过程,且随着蓄水过程的推进持续减少,随着泄水过程的推进显著增加;浮游植物数量组成分别以硅藻和蓝藻占优势,占浮游植物总细胞密度的比例均在50%以上;蓄水过程Shannon指数和Margalef指数变化范围分别为1.80～3.68和0.09～0.61,与泄水过程相比,此2项指数明显较高且变化幅度大;泄水过程和蓄水过程的浮游植物群落分别聚在一组内,但各组内变化情况不同;在三峡水库泄、蓄水过程中,水温和营养盐对浮游植物的生长均有很重要的影响,而水文特征变化对浮游植物生长影响作用十分有限。三峡水库泄、蓄水过程中小江浮游植物群落结构特征存在差异,水温、营养盐是主要影响因子,其中泄水后期浮游植物细胞密度会显著增加,且随着水位的进一步降低会持续增加,水华暴发潜在风险增大,需引起关注。

The water level fluctuation（ from 145 m- 175 m） in Three Gorges Reservoir,due to impoundment and discharge,is inverse to the natural wet and dry season and significantly affects the phytoplankton community and eutrophication status of tributary bays. The Xiaojiang River is an important tributary located on the north side of Three Gorges Reservoir and algae blooms in the backwater zone occur frequently in late spring and early autumn.As a result,this area has been targeted for monitoring by the Water Environment Management Department and Three Gorges Reservoir Management Department. To study the effect of rapid water level fluctuations in Three Gorges Reservoir,the phytoplankton community of Xiaojiang River was investigated in June and October of 2013.Three sampling sites were established in the Gaoyang section,where algae blooms frequently occur,and located in the middle and on each side of the tributary bay. Samples from the upper,middle and lower layers were collected for quantitative and qualitative phytoplankton analysis on June 1,8,15,22 and 29 during the discharge period of Three Gorges Reservoir,and on Oct. 4,11,18 and 25 during the impoundment period. Water quality parameters were measured when phytoplankton samples were collected. A total of 136 phytoplankton species（ including varieties） belonging to 70 genera and 6 phyla（ during the discharge period） and 7 phyla（ during the impoundment period） were identified in Xiaojiang River. Phytoplankton species were dominated by Chlorophyta,Bacillariophyta and Cyanophyta,together accounting for 87. 49%（ discharge） and 80. 87%（ impoundment） of the total phytoplankton species,and displayed little difference in species composition between the two periods. The phytoplankton density range was 8. 1 × 104 cells / L- 127. 2 × 104 cells / L during the discharge period and 24. 4 × 104 cells / L- 99. 9 ×104cells / L during the impoundment period,slightly lower and with less variation than during the discharge period.Cell density increased significantly with time during the discharge period,while it gradually decreased with time during the impoundment period. Phytoplankton density was dominated by diatoms during discharge period and by cyanobacteria during the impoundment period（ 50% in each case）. Both the Shannon index range（ 1. 80-3. 68） and Margalef index range（ 0. 09- 0. 61） during the impoundment period were significantly higher,and larger,than the range during discharge period. Cluster analysis shows that the phytoplankton community during the two periods was clustered into different groups,which changed differently over time. Correlation analysis shows that temperature and nutrient levels played critical roles in phytoplankton growth,but the hydrologic index had very little impact during either period. To summarize,the discharge-impoundment cycle of Three Gorges Reservoir strongly affects phytoplankton community structure in Xiaojiang River and temperature and nutrient levels are the primary influences. Phytoplankton density increases significantly late in discharge period,when the decrease in water level leads to a high risk of algal bloom.