Study On The Phytoplankton Community Structure Dynamic And Their Relationship With Environmental Factors In Lake Dishui

Dishui Lake is an important part of Shanghai eco-city construction, which also has great value in Lin’gang New District landscape building and ecological system maintenance. In order to investigate the lake’s phytoplankton community structure, spatial and temporal dynamic, major factors and their differences between2different depths,8sampling sites including water inlets, sluice gate, the lake center and islands etc. were set and, chemical and physical indicators and phytoplankton samples were collected monthly in2different depths of these sites in Dishui Lake during Jan. to Dec. in2011. The phytoplankton species composition, density variation, dominated species, bio-diversity index and chlorophyll-a content were studied and analyzed by one way ANOVA method in order to evaluate current water quality and eutrophication situation of Dishui Lake. Meanwhile, with Canonical Correspondence Analysis (CCA) and Statistic Package for Social Science (SPSS) methods, correlation among phytoplankton, chemical and physical indicators, and sampling sites were studied by different seasons and depths, for better investigation of the main factors of influencing phytoplankton development. The main results are as follows:1. A total of6phylums,61genera,128species of phytoplankton were identified during the whole year investigation. Of which, Chlorophyta accounted for the most species number with27genera53species, followed by Bacillariophyta, Euglenophyta, Cyanophyta, Cryptophyta and Pyrroptata in sequence. The variation of different phytoplankton species numbers was not significant. The distribution of both Chlorophyta and Bacillariophyta species number of the whole year showed bimodal shape changes. Chlorophyta had highest species numbers in early spring, late summer and early autumn, and Baillariophyta had most species in autumn and winter. The phytoplankton dominant species were of Chlorophyta, Crytoplyta and Bacillariophyta. The whole year’s dominant species were Chroomonas acuta, Chlorella ellipsoidea, Chamydomonas snowiae, Cryptomonas ovate, Melosira ambigua and Coelastrum microponim. Dominant species componsition varied among different time and sites. On analysis of8different sampling sites in4different seasons, the frequencies of Chroomonas acuta, Chlorella ellipsoidea, Chamydomonas snowiae becoming dominant species were larger than75%. The dominant species number in spring and autumn were higher, which indicated the community structure in spring and autumn were more complicate than in summer and winter. One way ANOVA was carried out on phytoplankton species and density separately of different depths and sampling sites, which indicated the phytoplankton species and density variation of different months were both more significant than those of sampling sites.2. The phytoplankton cells density of Dishui Lake was in the range of1.17×106-36.65×106ind/L, with the average density of7.36×106ind/L. The density of phytoplankton cell was highest in the north of the lake, followed by the center, the east and the northwest. And for seasonal changes, the phytoplankton cell density was highest in winter, followed by autumn, summer and spring. The phytoplankton cell density in the north of the lake was higher in autumn and winter, and lower in spring and summer. And for the east of the lake, it was higher in spring and winter and lower in summer and autumn.3. The seasonal variation of different depths was significant. For0.5m below water surface, the phytoplankton numbers were higher in winter and lower in summer, with spring and autumn almost the same. For1.5m below water surface, the phytoplankton number in spring was highest, followed by autumn, summer and autumn. The main reason of seasonal phytoplankton structure variation in different depths was the water combination under impact of wind, besides of other factors like light, temperature and substrate variation of different depths. One way ANOVA showed that the phytoplankton density variation in different depths was significant. 4. The Shannon-Wiener, Margalef and Pielou index of Dishui Lake were1.23±0.46、3.97±0.66、0.83±0.29separately. The Shannon-Wiener index showed the lake’s water quality was in Alpha level of pollution with the number a little lower in summer and winter. Margalef and Pielou index showed that the water quality was in light-medium pollution with the number lower in summer and winter as well. The3diversity index average value showed to be slightly higher in the lake center and sluice than other sites’ average level. Eutrophication evaluation based on chlorophyll a level indicated that all sites of4different seasons were in mesotrophic level. According to the phytoplankton cell density evaluation, most sites were in oligotrophic-mesotrophic status. On analysis of phytoplankton structure and phytoplankton dominant genera (Chroomonas, Chlorella, Cryptomona and Cyclotella). Dishui Lake was currently in mesotrophic or eutrophic condition.5. The average value of total nitrogen (TN) concentration of the whole lake was3.23mg/L, and that of total phosphorus (TP) was0.531mg/L. TN and TP concentration of1.5m under water surface was slightly higher than that of0.5m. TN and TP concentration of spring and autumn all showed that the water qulity of most sampling sites were below Class Ⅴ, and summer and autumn water quality were slightly better than the other seasons. Nutrient levels indicated the lake water quality was not optimistic, and lacking of aquatic plants, low water quality of the lack source, high nutrient release of sediment as well as islands and lake development might be some key factors which impact the lake water quality..6. SPSS analysis indicated that major environmental factors which impact phytoplankton were:pH, chlorophyll a, dissolved oxygen (DO) and water temperature. CCA analysis on phytoplankton of different depths indicated that most alage had positive correlation with TP. pH and chlorophyll a. CCA analysis on phytoplankton of different seasons indicated that most main phytoplankton species had positive correlation with TN, pH, chlorophyll a in spring and summer. In autumn, most phytoplankton species had positive correlation with DO, chlorophyll a and pH, while in winter, phytoplankton were more likely to be impacted by wter temperature and transparency.