Horizontal And Seasonal Dynamics Of The Bacterioplankton Community Composition In The Large Shallow Chaohu Lake, China

Chaohu Lake,as one of the five largest freshwater lakes in China,is a typical,large, shallow and subtropical freshwater lake.In recent years,Chaohu Lake has been well known as one of the three most polluted freshwater lakes in China.In the ecosystems of lakes,the changes of the trophic material in quantity and kinds could result in the change of bactefioplankton diversity.The microbial diversity is always used as the criterion which could monitor and expect the healthy status of aquatic ecosystems.In this study,we analysed horizontal and seasonal dynamics of the bactefioplankton community composition in Chaohu Lake on the molecular level,and the relationship to the environmental parameters.The objective was to provide the basic information for ecological restoration of Chaohu Lake.In this study,samples were collected from autumn of 2006 to summer of 2007 at five sampling positions with different trophic status.The total bacterial population abundance in each sample was determined by direct microscopical counts,using the DAPI staining technique.Bacterioplankton community composition(BCC) was determined using a combination of PCR amplification of 16S rDNA gene and denaturing gradient gel electrophoresis(DGGE).Canonical correspondence analysis(CCA),a direct gradient analysis method,was carried out to investigate relationships between BCC and measured environmental variables including the physical and chemical parameters.The results indicated that,there was little variation in temperature at the five sampling positions in the same season,the seasonal temperature variation at the five sampling positions showed a typical trend,with the highest values in summer and the lowest values in winter.The pH ranged in 6.91- 8.84,and the lake was almost alkaline.The values for BOD₅,COD_Mn,TN,NH₃-N,and TP concentrations at Shikou were almost always significantly higher than those in the other four positions,except for TN concentration in summer at Shikou.Total numbers of bacterioplankton in Chaohu Lake were obtained by counting DAPI-stained cells.Cell counts ranged from 7.5×10⁴ to 6.75×10⁶ cells mL^-1 at the five positions during four seasons.The highest and the lowest values for the 20 samples were at position Donghuxin in summer and winter,respectively.Spatially,Maweihe showed the highest average abundance of bacterioplankton over the four seasons,and Zhonghan showed the lowest.The abundance and diversity of bacterioplankton communities at different sampling positions shared different seasonal patterns.The BCCs in 20 samples varied substantially in horizon and season,but the patterns of change in bacterioplankton communities showed that the seasonal difference might have a significant impact on structuring the BCCs in the lake.Canonical correspondence analysis(CCA) combining the DOGE patterns and physicochemical parameters indicated that the temperature and concentrations of BOD₅, NH₃-N,COD_Mn,TN,TP and DO significantly influenced the BCC.Our results indicate that the eutrophic status and season were the most influential factors on determining BCC in Chaohu Lake.Phylogenetic analysis indicated that some clusters in Cyanobacteria and Actinobacteria did not showed differences horizontal and seasonal scales,but the others did.However,Bacteroidetes and Chloroflexi varied in horizon and season.