西江下游浮游植物群落周年变化模式

西江肇庆段是珠江干流汇入珠三角河网水域的咽喉通道,对2009年该江段的浮游植物群落组成及变化进行系统阐析.调查期间共发现浮游植物7门,245种(包括变种、变型),其中硅藻和绿藻是浮游植物群落组成的最主要类群,分别占总种数的42.44％和34.69％,其次是裸藻和蓝藻.PCA分析结果显示,浮游植物各类群的种类丰富度和生物量的周年变化趋势主要受水温和径流量的影响.浮游植物种类丰富度的周年变化呈现明显的高温季节高,低温季节低的特征,除因大多数藻类物种直接喜好高温之外,径流量的增大有助于真浮游绿藻种类的外源注入及半浮游和偶然性浮游硅藻种类的增加.浮游植物生物量的周年变化呈现明显的双峰型,峰值分别出现在8月和11月.然而8月份第1个高峰出现之前,生物量波动不大,尽管水温的升高有助于生物量的增加,但是径流量增大所带来的稀释作用掩盖了水温上升对浮游植物生长的促进作用.真浮游植物生物量的变化趋势和数值与总浮游植物极其接近,主要得益于真浮游硅藻物种颗粒直链藻在总种群中的绝对优势地位.综上,水温升高对浮游植物种类丰富度和生物量的增长均有促进,径流量的增大虽然有助于种类丰富度的增加,但不利于生物量的增加.

Annual variation pattern of phytoplankton community at the downstream of Xijiang River

The study on population dynamics of phytoplankton was still an important basic work for ecological investigations in large rivers. Through continuous sampling at the downstream of Xijiang River in 2009, the annual variation pattern of phytoplankton community structure and its correlation with environmental factors were demonstrated in this study. During the investigation, the 245 taxonomic species (including variant and derivative) in 7 groups were identified. The results showed that the diatom (Bacillariophyaceae) and chlorophytes were most abundant and they were 42.44% and 34.69% to total numbers respectively, and the Euglenophyta and Cyanobacteria were subsidiary to them. The above structure pattern was the similar to those of other large rivers in the world. 15 main species (dominant and frequent species) were listed in table 1, but only 3 species showed both high biomass contribution and high occurrence. For all the species, Aulacoseira granulata var. granulata was the predominant one because it showed continuous dominance except for its biggest occurrence. The annual variation pattern of total species abundance was high in warm seasons and low in cold seasons, which was also consistent with the diatom and green algae group. One reason is that many species favorite the high water temperature and the other is that the high discharge of warm seasons could also promote the inoculations from out-of-channel sources of true planktonic green algae and the complement of tychoplanktonic diatom species. This result has been confirmed by the annual pattern of true planktonic species richness, since the true planktonic diatom showed almost steady during the whole year, and the PCA analysis confirmed the two key factors. The annual phytoplankton biomass showed apparent bimodal pattern, and the two peaks appeared after the flood period. In addition, the biomass values showed almost no fluctuations besides the two peaks, which was because that the positive role of rising water temperature was covered by the negative role from the dilution impact of high discharge. According to the previous studies, self-sustaining populations of riverine algae were primarily based on meroplanktonic diatom species, whereas true planktonic populations, mostly chlorophytes depended on periodic inoculations from outer sources. Therefore, diatom would apparently precede chlorophytes when water discharge increased. In conclusion, the diatom and green algae were the most important groups of phytoplankton community in Xijiang River, and their annual variation pattern could strictly affect the total phytoplankton community. The phytoplankton species abundance was mainly determined by the true planktonic green algae and tychoplanktonic diatoms, and both water temperature and water discharge were the main reasons for their increments, and Si concentration was only beneficial for diatoms. The phytoplankton biomass was mainly determined by several dominant species (especially the true planktonic diatom species), and water temperature and discharge played important roles for the phytoplankton biomass during the seasons.