Characteristics Of Phytoplankton Community And Water Quality Assessment In Changhu Lake

Changhu Lake,the third largest freshwater lake in Hubei Province,possesses different functions,such as flood discharge,irrigation,water supply et al.It is an important wetland resource for ecological balance in the middle reach of the Yangtze River,as well as a Germplasm Resources Protection Area for Cluters.Changhu Lake consists of four parts:Miaohu,Haizihu,Mahongtai,Yuanxinhu from the west to the east,in which,Miaohu was divided as still water by South–North Water Transfer Project,thus this area was discarded in our research.Anthropogenic activities,e.g.discharge of domestic wastewater,industrial wastewater and agricultural wastewater in Haizihu,Mahongtai works as shipping channel,and high density of fish farming in Yuanxinhu,together have severely impacted water quality and ecosystem balance in Changhu Lake.As main primary production in aquatic ecosystem,phytoplankton plays a vital role in the cycling of energy and materials,and its community dynamic change directly impact and indicate the state of the ecosystem.At the same time,using phytoplankton as bio-monitor and bio-indicator for water quality is widely accepted.Therefore,the investigation of phytoplankton community structure in Changhu Lake can provide solid data and advices to sustainable use and management of water resources in the lake.We conducted four field sampling in Changhu Lake in January(Winter),April(Spring),July(Summer)and November(Autumn)of 2015 separately,both phytoplankton and related10 phsiochemical factors were sampled/measured at the same time.Both Principal Component Analysis(PCA)and Analysis of Variance(ANOVA)were applied to detect the characteristics of spatiotemporal distribution of biotic and abiotic data.Diversity of phytoplankton community was evaluated by Shannon-wiener Index,Pielou Index and Margalef Index,and water quality was assessed by phytoplankton diversity indices as well as Charlson Comorbidity Index(TLI).Dominant species were selected by dominance index,their niche breadth and niche overlap were calculated afterwards.Pearson Correlation,Multiple Stepwise Regression and Redundancy Analysis were used to find the driving environmental factors for phytoplankton community distribution in Changhu Lake.Our results showed that:(1)Spatiotemporal Distribution of Physiochemical Factors: Physiochemical factors in Changhu Lake showed significant variance between seasons and areas,several factors in Summer were higher than that in other seasons,e.g.water temperature,water depth,dissolved oxygen et al.,which were the reflects of subtropical monsoon climate,high water temperature,high water depth and high dissolved oxygen correspond to high temperature,high rainfall and high photosynthetic intensity in summer respectively.Different parts of Changhu Lake showed different features: discharge of wastewater in Haizihu made totalphosphorus here higher than that in the rest two areas,high disturbance in Mahongtai made it featured with high suspend solid mater and low water transparency,high distribution of macrophytes in Yuanxinhu indirectly made dissolved oxygen higher there.(2)Water Quality: Carlson Index showed significant variance between seasons with the average of winter lower than that of other seasons,and significant variance between areas was not observed.Total nitrogen and total phosphorus were both higher than the value for eutrophic state,a conclusion that Changhu Lake was in eutrophic state and it deteriorated in summer could be drawn from the results of Carlson Index and phytoplankton diversity indices,which indicated consistency when compared to results of previous studies.Fish farming significantly affected water quality in Changhu Lake,and artificial floating island could be a main method for the restoration of the Lake.(3)Characteristics of Phytoplankton Community: A total of 120 species were identified,most of which are species in Chlorophyta,followed by species in Cyanophyta and Diatophyta,and abundance of species in Cyanophyta increased a lot in summer.Eleven dominant species were selected,and 4 species of them are blue-green algae.C.comta,C.vulgaris and Oscillatoria amphibia are dominant species in four seasons,and Oscillatoria amphibia became the main dominant specie in summer.Phytoplankton abundance in summer was much higher than that in other seasons,and significant variance between areas only observed in summer and autumn.Abundance of dominant species comprised most of the total,in most sites in four seasons,they accounted for more than80%.There were insignificant variance of Shannon-wiener index,Pielou Index and Margalef Index between 3 areas,and only Pielou Index showed a significant variance between seasons(the average of Pielou Index in summer was 0.39±0.06,which is lower than that in any other seasons).In summary,we could conclude that phytoplankton species diversity was low and stability of ecosystem was weak in Changhu Lake.The results of niche analysis indicated that species possessed more even distribution could gain a wider niche breadth,and dominant species in autumn presented intense competition between each other.Based on the results of Pearson Correlation analysis,Multiple Stepwise Regression and Redundancy Analysis,we found that water temperature,the ration of N:P and pH were the driving factors for phytoplankton community in Changhu Lake.Currently,despite researches about Changhu Lake cover a wide range,e.g.plankton community,fish community,macrophyte community et al.,a long-time,continuous study is still missing.We made a preliminary exploration of the relationship between phytoplankton community and its driving factors in Changhu Lake,which may service as valuable data for the sustainable protection and usage of the water resources.Phytoplankton-related researches could be important reference for monitoring the water quality especially whenfish farmingis banned in the lake,furthermore,trait-based approaches are welcome to be employed to gain a deeper understanding on how phytoplankton community responses to environmental gradients.And once more data is available,effects of fish farming on phytoplankton are expected to be quantified.