Investigation of Phytoplankton Community Structure and Water Quality in Sanjiao Lake

The community structure of phytoplankton in Sanjiao Lake was investigated and then used in the assessment of water quality.Phytoplankton diversity is widely used as a water quality indicator and the results of this study provide a basis for managing eutrophication and support ecological restoration efforts.The shallow lakes in the Hanyang district of Wuhan are suffering from water quality deterioration and eutrophication.The Six Lakes Connection Projecthas been implemented to restore water quality and the project includes Sanjiao Lake.The social function of lakes in the project is changing from primarily aquacultural to ecological as restoration efforts improve the quality of these stressed urban waters.Sanjiao Lake is a typical small city lake with an area of 2.45 km2,average depth of 0.88 m and maximum depth of 1.86 m.Samples were collected at ten sites in July(summer) and October(autumn) of 2011 and January(winter) and April(spring),2012.Water transparency,p H,conductivity,total nitrogen(TN) and total phosphorus(TP) were measured in July and October.Phytoplankton samples for qualitative analysis were collected with 25 μm plankton nets and fixed in 4% formaldehyde.1 L samples for quantitative analysis were collected 0.5 m below the surface and concentrated to 50 ml after settling 48 h.Phytoplankton was identified using a microscope.A total of 94 species,belonging to 51 genera,25 families and 8 phyla,were collected.The phytoplankton community was dominated by Chlorophyta,accounting for 59.6% of all species collected,followed by Bacillariophyta(12.8%),Cyanophyta(11.7%) and Euglenophyta(8.5%).The average cell density of phytoplankton was(102.32 ± 59.89) × 106 cells / L and dominated by Cyanophyta(86.69%),followed by Chlorophyta(7.77%) and Bacillariophyta(4.91%);abundance of other phyla were low(0.63%).Cell density of phytoplankton displayed clear seasonal variation,densities are the hightst in summer,decline during autumn and are relatively low in the winter and spring.Cyanophyta dominated the phytoplankton community during summer and autumn and the community shifted toward Bacillariophyta and Chlorophyta during winter and spring.The average Margalef index,Shannon-Weaver index and Pielou index of phytoplankton community were 2.75,1.78 and 0.46,respectively.The Shannon-Weaver and Pielou indices were higher in winter and spring,and the lowest in autumn.When cell density is high(summer,autumn),the phytoplankton community is dominated by Cyanophyta,an indication of eutrophic conditions.During winter and into the spring,cell density is lower and species richness higher.The overall assessment,based on the structure of the phytoplankton community and the values of the water quality parameters,indicates that Sanjiao Lake is mesotrophic-eutrophic.