Research On The Relationship Between Phytoplankton And Environmental Factors Based On Structure Equation Model

Human-driven eutrophication has caused great changes of phytoplankton community in many China lakes.As a result,the functioning of these lake ecosystems is threatened.In this research,field investigations and sampling were carried out in June 2011 and 2012 in Lake Qixinghu by setting 16 sampling sites and separately in April,July,September and November of 2012 in Lake Nansihu by establishing 12 sampling sites.We can see the temporal and spatial variations of community composition,density and biomasses of phytoplankton in the two lake.And we also acquired the data of water environmental factors.Besides,we evaluated the water quality of lake Qixinghu by using abundance,biomass and biodiversity of phytoplankton.After that,through the Structural Equation Modeling(SEM),we showed the correlations between phytoplankton and environmental factors of the two lake.We can know the key influencing factors of phytoplankton community in the two lakes which could be favor to make measures to improve water quality protect.The main results of the present research are shown as follow:(1)In lake Qixinghu,mean water temperature(WT)was 24.4?,mean Secchi disk depth(SD)was 45.17 cm,mean pH was 8.11 and the average concentration of dissolved oxygen(DO)was 7.25 mg/L.The nutrient concentrations in the lake were too high so that the overall lake was in eutrophic state.Mean total nitrogen(TN)concentration was 6.82 mg/L,average values of ammonia nitrogen(NH4+-N)was 2.27mg/L and the mean concentrations of total phosphorus(TP)was 2.27 mg/L.(2)In the study of Lake Qixinghu,there were 7 phyla,106 species among which the majority were 44 kinds of Chlorophyta,followed by Bacillariophyta and Cyanophyta.Besidess,there was little difference with the number of species in two years.In the study of Lake Nansihu,there were 135 species among which the majority phyla was Chlorophyta,followed by Bacillariophyta and Cyanophyta.Besidess,there was huge difference with the number of species during the four seasons among which the majority of species was in summer,followed by spring and fall.And the number of species in winter was the least of the year.(3)In the investigation,the mean abundance of phytoplankton was 7.30×106cells/L and the average biomass of phytoplankton was 14.44 mg/L.Both of them had significant difference between 2011 and 2012.We can see that the mean abundance of phytoplankton was 1.41×108 ce11s/L and the average biomass of phytoplankton was 23.69 mg/L in 2011 and the mean abundance of phytoplankton was 4.69×106 cells/L and the average biomass of phytoplankton was 5.18 mg/L in 2012.The Shannon-Weaver index and Pielou index of Lake Qixinghu are 2.29 and 0.67 separately with huge difference for both of them in two years.Besides,the water quality of lake Qixinghu was in eutrophic state,and the water quality in 2012 is slightly better than in 2011.In lake Nansihu,the mean abundance of phytoplankton was 2.58×106cells/L and the average biomass of phytoplankton was 1.67 mg/L.Both of them had significant difference in four seasons.The abundance of chlorophyta and biomass accounted for the maximum in autumn which become the major phyla of all phytoplankton and the chlorophyll abundance and biomass reached the bottom in winter.(4)Results of SEM showed that WT,DO and pH are the key factors influencing the density of phytoplankton in Lake Qixinghu.WT and DO and pH are the key factors influencing the density of Cyanophyta.TP and DO are the key factors influencing the density of Chlorophyta.TP and DO are the key factors influencing the density of Bacillariophyta.In Lake Nansihu,WT,SD,TN,TP and NH4+-N are the key factors influencing phytoplankton;WT and NH4+-N are the key factors influencing Cyanophyta;WT and SD are the key factors influencing Chlorophyta;TP,NH4+-N and zooplankton biomass are the key factors influencing the density of Bacillariophyta.Results in this study will be helpful in understanding the applicability of structural equation modeling in studying the correlation between phytoplankton and environmental factors,and could be favor to make measures to improve water quality protect and ecological security.