Study On The Temporal And Spatial Variation Of Phytoplankton Community And Their Relationship With Environmental Factors In Lake Dianshan

Lake Dianshan is one of the drinking water conservation areas in Shanghai with other important functions in ecology, fishery, water transportation, etc. Recently, the aggravation of eutrophication and high frequency of algae bloom severely threaten the aquatic ecosystem security. In order to evaluate the spatial and temporal variation of phytoplankton community structure in Lake Dianshan and their relationship with environmental factors and also to analysis the impact of ecological restoration project on phytoplankton, an investigation was carried out from October 2008 to September 2009. In this period,17 sampling sites were selected and the samples were run once a month (the investigation was interrupted by heavy rain and wind in February 2009).The species composition, density variation, dominated species, bio-diversity indexes and chlorophyll-a content were studied and the main results are as follow:1 A total of 8 phylum,147genera,503 species of phytoplankton were identified(with varietas and metatype). The phytoplankton community was mainly composed of Chlorophyta(65 genera 204 species), Bacillariophyta (32 genera 148 species),Cyanophyta(18 genera 67 species) and Euglenophyta(7 genera 59 species). The dominated species were mainly Chroococcus, Microcystis, Oscillatoria in Cyanophyta, Chroomonas, Cryptomonas in Cryptophyta, Chlorella, Ulothrix in Chlorophyta and Melosira, Nitzschia in Bacillariophyta. In quantity, Chroomonas acuta, Chlorella vulgaris were the dominated species all year round and Microcystis was in the summer for some areas.The group density of phytoplankton in Lake Dianshan was affected deeply by the algal bloom. Before the algal bloom, Chlorophyta, Cryptophyta, Bacillariophyta, Cyanophyta were dominant. However, after the algal bloom, Cyanophyta replaced the Chlorophyta to be the main group with a proportion of 55.61% followed by Chlorophyta, Cryptophyta and Bacillariophyta.2 The seasonal order of phytoplankton species number from high to low was autumn,winter, summer, spring, while the spatial order is north,west and southwest,south and southeast. The phytoplankton density was on average 6.16×10cells·L-1 with double peaks in a year. Moreover, the summer peak was much higher than that in spring. Generally, the phytoplankton density ranged from high to low in the following season order:summer, spring, winter, autumn. In Lake Dianshan, the spatial variation was affected deeply by the algal bloom too. Before the algal bloom, the density in east and north areas were higher than the others, but after the bloom, the density in southwest and west area were higher. With regards to the results of cluster analysis, the west and southwest areas in Lake Dianshan are vulnerable spots for algal bloom.The seasonal order of Shannon-Weaver index from high to low was autumn, winter, spring, summer and the Margalef index was the same. Meanwhile, the Pielou index was in the order of autumn, winter, and spring equal to summer. In space, the bio-diversity indeces of front damming at Da Zhusha and Qian Dunpu were higher than that in the other areas.This observation showed that the abundance, stability and uniformity of phytoplankton species were better than the other areas. The otherness of phytoplankton at different depth was low which showed that the stratification effect of water in Lake Dianshan was not obvious.3 The chlorophyll-a content of phytoplankton was on average 20.51 mg/m3, and ranged from high to low in the following season order:spring, summer, autumn, winer. There were double peaks in a year, and the peak in summer was almost the same as that in the spring. Before the algal bloom, the chlorophyll-a content in east and north areas were higher than that in the southwest and west. However after the algal bloom, the northwest, west and middle areas had higher chlorophyll-a content. The trend of temporal variation on chlorophyll-a content was almost the same at different depth, but the range was different. Generally, there was no significant difference between the phytoplankton communities at different depth.4 During the investigation, most of the time, Lake Dianshan was at light eutrophication level, except for the November 2008 and March2009 which were at the middle level, in most months of the year. Spatially, the comprehensive nutrition index in the north part was higher than the others while the southwest and north areas were lower, which showed that the nutrition state in the north was more serious. Combined with the phytoplankton bio-diversity indices at different sites, the areas with higher nutrients and organism had higher bio-diversity indices in Lake Dianshan.5 The SPSS software was used to analysis the correlation between phytoplankton and environmental factors, and the results were as follows:the main factors that impact the distribution and variation of phytoplankton in Lake Dianshan were water temperature, transparency, pH, suspended substance, dissolved oxygen, ammonia nitrogen and total nitrogen. However, the total phosphorus, BOD5, CODMn, species number and density of zooplankton had smaller impact on phytoplankton.6 The results of significance difference analyses on phytoplankton community between interior and exterior sites of the project were as follows:no other significant difference of phytoplankton parameters was showed between the interior and exterior except for the species number and Margalef index at site 15, phytoplankton density at site 5 and Shonnon-Weaver and Pielou indices at site 7. The temporal variation trend of phytoplankton density was the same as that of chlorophyll-a content interior and exterior the restoration projects. Before August 2009, the phytoplankton density and chlorophyll-a content in the project were lower than the outside by about 22.62% and 31.89% respectively. On the contrary, after the algal bloom, the two parameters above for within the project were 3.94 and 2.78 times higher than that of the outside, respectively. The above phenomenon showed that the ecological restoration projects were easily affected by algal bloom, and stricter precautionary measure should be taken during the algal bloom.