Cyanophyta Community Correlation With Environmental Factors And Water Quality Assessment In Dianshan Lake

Lake Dianshan is one of the drinking water conservation areas in Shanghai with other important functions in ecology and water transportation, etc. Over the last decade, water quality keeps declining, with its comprehensive standards mainly at class II in the past decreasing to IV～V class currently. High frequency of algal blooms in the summer has seriously threatened the water safety. In order to understand the cause of the bloom based on cyanobacteria and also their spatial and temporal distribution in water and the influencing factors, an investigation was carried out from May 2009 to April 2010. In this period,10 sampling sites were selected and the samples were run once a month (the investigation was interrupted by heavy rain and wind in February 2009). Phytoplankton, especially cyanobacteria taxa and physical and chemical indicators of water were comprehensively studied. The main results are as follow:1 During investigation, water temperature varied from 4.1～31.1℃, pH values range from 6.99 to 7.63. Mean of transparency was 46.8 cm, average dissolved oxygen content was 8.38mg/L, both of which were highest in winter and lowest in summer. Trends in nutrient (nitrogen and phosphorus) and BOD5 were as follows:NH4-N, ranged from 0.14～4.87mg/L and TN, ranged from 1.25～10.20mg/L, both of whose seasonal order from high to low were winter, spring, autumn, summer; TP, changed from 0.11～0.54mg/L, was highest in winter and lowest in summer. The range of BOD5 was from 5.10 to 7.00 mg/L, and the seasonal order from high to low was autumn, winter, spring, summer. Concentrations of nitrogen and phosphorus were higher in the north and west area than that in the east.2 A total of 8 phylums,123 genera,398 species of phytoplankton were identified (with varietas and metatype). Of which Chlorophyta were 58 genera,40.95% of total species, Bacillariophyta accounted for 26.38% with 29 genera, Cyanophyta were 15 genera and accounted for 12.06%, Euglenophyta accounted for 12.81%, were 29 genera. The remaining four phylums all had fewer species composition, quantity stability, accounting for a total of 7.79%.Number of phytoplankton species was highest in autumn, following with summer, autumn and winter. Chlorophyta was one of the most important composition groups of phytoplankton species in Dianshan Lake. The combination of Chlorophyta, Bacillariophyta, Cyanophyta and Euglenophyta affected the overall changes in phytoplankton species.The average of phytoplankton Shannon-Weaver diversity index H', Margalef index D and the Pielou index J were respectively 1.05,3.99 and 0.59. The seasonal of Shannon-Weaver index H'from high to low was autumn, winter, spring, summer and Margalef index ws the same. Meanwhile Pielou index J was in order of autumn, winter, spring and summer. Shannon-Weaver index and Pielou index were more consistent as they were both higher in autumn and winter than that in spring and summer while Margalef index was different from the former two. 3 Cyanophyta had 48 species,15 genera. Of which Microcystin was the highest proportion, representing 68.58% of the total number of cyanobacteria, followed by Oscillatoria algae that accounted for 22.03%; from the composition of types, Oscillatoria algae had more than 10 species, followed by Microcystin that are 6 species, and the remaining generas had 4 species or less.The seasonal variation of species number of Cyanophyta from high to low was summer, spring, autumn, winter and it was similar with seasonal variation of phytoplankton. The species number was highest in north region, followed by east, center and west, while lowest in south.Cyanobacterial cell density was highest in summer. Summer and spring had significantly higher density than autumn and winter. The annual average density was 1.60×106 cells·L-1. Density appeared two peaks in May and September,2009, and the maximum density was almost the 72 times the minimum. There was significant difference between each month. As the dominant species of algae in Dianshan Lake were only Oscillatoria tenuis and Microcystis aeruginosa and to whose presence there were obvious time and space limit.The Cyanophyta didn't exist as dominant species in the whole lake except in May and September,2009.4 The result of Canonical Correspondence Analysis (CCA) showed that the main environmental factors affecting Cyanobacterial population changed as sensons. In spring those were water temperature, total phosphorus, and turbidity conditions; in summer, they were transparency, pH, and phosphorus concentrations; in autumn the greatest environmental factors included the concentration of total phosphorus, dissolved oxygen and transparency; while in winter dissolved oxygen, total nitrogen concentration and water temperature were the main. The contaction between Cyanobacterial community succession with the environmental factors were as follows:cyanobacterial cell density had most significant correlation with temperature in spring and winter. In summer it was mainly affected by phosphorus and nitrogen. In autumn it mainly restricted by transparency of water.5 Evaluation results showed the water in Dianshan Lake was Mesotrophication-Eutrophication. The degree of eutrophication changed with seasonal variation. It was highest in summer, then spring, autumn and winter the lowest. North area had higher eutrophication degree than other regions. Water quality evaluation results showed the current water quality was basicly in the IV～V class. North and west of the inlet water the quality was worse than others. As one of Shanghai water supplies, Lake Dianshan's water quality needs for more attention and urgent improvement which will ensure that partial loss of function of the lake not to appear.