Spatio-temporal Characteristics And The Key Driving Factors Of Diatom Community In An Arsenic-contaminated Lake In Yunnan

Lakes provide important resources and environmental foundation for economic development.Yunnan Province is home to one of the five major lake areas in China.With the intensification of human activities and the sustained development of social economy,some lakes in Yunnan Province are under increasing pressure of environmental pollution,such as eutrophication and heavy metal pollution.This process can lead to a loss of biodiversity and a decline of ecosystem health,resulting in the significant decrease in ecosystem service.Yangzong Lake,one of the nine largest lakes in Yunnan Province,provides important ecosystem services such as water supply,aquaculture and tourism destination for catchment development.In recent years,the increased input of pollutant loading derived from the catchment has led to accelerating deterioration of water quality,especially the arsenic?As?spilling incident in Yangzong Lake in late 2008.This has seriously threatened human health,ecosystem health and the sustainable development of the catchment.To date,ecological assessment in Yangzong Lake has mainly focused on water quality and phytoplankton through discontinuous monitoring data,lacking the identification of seasonal and interannual fluctuations of lak-water environment and the evaluation of spatio-temporal characteristics in bioindicators.Diatoms are important primary producers in freshwater ecosystems and can influence the structure of lake food webs and ecosystem functioning.Diatoms are characterized by a short life cycle and rich in diversity.Diatom community structure is sensitive to the environmental characteristics of water bodies,and has become an important biological indicator in water quality monitoring and ecological evaluation.In this study,we combine monthly and seasonal limnological surveys to monitor the water chemistry indicators and community structure of living diatoms for the surface waters of Yangzong Lake during 2015²017.The water chemistry indicators include total phosphorus?TP?,total nitrogen?TN?,arsenic?As?,calcium?Ca?,magnesium?Mg?,sodium?Na?,potassium?K?,chloride?Cl?,sulfate?SO₄?,iron?Fe?,Manganese?Mn?,lead?Pb?,cadmium?Cd?,water temperature?WT?,chlorophyll a?Chl-a?,pH and conductivity?Cond?.Through combining principal component analysis?PCA?,redundancy analysis?RDA?,variance decomposition analysis,Pearson analysis,weighted average quantitative model with other statistical methods,we firstly identify the spatio-temporal patterns in the variation of water chemistry,particularly the change in lake-water arsenic?As?concentrations.The spatial and temporal characteristics of diatom community changes and its influencing factors were further discriminated.The results can provide an important scientific basis for ecological restoration and environmental protection of this arsenic-contaminated lake.The major research results derived from this thesis are followed.?1?The lake-water chemical properties are characterized by strong seasonal fluctuation,while the spatial variation is moderate.The concentration of As in the surface water changes from 7¹29?g/L?average value is 50.71?g/L?,and the maximum value appears in June 2016,and then continues to decline and drops to around 20?g/L in December 2017.The TP and TN of water were changed between18.78⁵7.51?g/L and 0.45¹.83 mg/L,with average values of 38.42?g/L and0.86 mg/L,respectively.The ratio of nitrogen to phosphorus was greater than 16:1,indicating that the water body was in overall a phosphorus-restricted lake.TP and TN are high in winter and spring,and low in summer and autumn.Some indicators?such as pH and WT?are high in summer and low in winter.The concentration of Chl-a?average value 13.89mg/L?was high in spring and autumn,and the average biomass of diatom was 0.32mg/L.The change trend of diatom biomass was consistent with Chl-a.The vertical water temperature and the dynamic change of the thermocline indicate that Yangzong Lake is a single-mixed lake.The water body is obviously stratified from mid-May to mid-October.The thickness of the thermocline is up to 3.76 m and the depth is up to 15.8 m in October.For the rest of the months,the vertical difference of the water body temperature is small,and the lake water is either mixed or weakly stratified.?2?A total of 22 species of 12 genera of diatoms were identified in Yangzong Lake,and most of them were planktonic species.The main dominant species are Achnanthes minutissima,Fragilaria nanana,Fragilaria ulna,Cyclotella pseudostelligera,Cyclotella comta,Stephanodiscus hantzschii,Cyclotella meneghiniana,Aulacoseira granulate,the distribution of diatom community has obvious seasonal changes and spatial differences.During the period of December to April,mainly large diatom species were dominated,such as F.ulna,C.comta,S.hantzschii and C.meneghiniana.Silicon-enriched A.granulata appeared only showed on April;Relatively small diatom species such as A.minutisssima,F.nanana and C.pseudostelligera are predominantly dominant from April to October.The species richness of diatoms varied between 4 and 15 with an average value at 9,showing a higher value in the winter time and a lower one in the summer.?3?Multivariate statistical analyses showed that diatom communities were affected by multiple environmental stressors and their interaction.The main influencing factors include WT,nutrients?TP,TN?,As and chemical ions?pH,K,Na,SO₄,Ca,Cond?,which accounted for 12.9%,6.1%,6.0%and 23.4%of the total variance in diatom assemblages,respectively.The effect of water temperature was mainly derived from the associated changes in thermal stratification and lake mixing.In terms of seasonal variation,the key environmental factors vary among different seasons.In spring,Ca,Na and WT explain 33.0%,21.4%and 13.1%of diatom community changes,respectively.In summer,Cond and K account for 14.1%and9.3%,respectively.During the fall time,As and WT explain 35.3%and 29.0%,respectively,while As,WT and TP explain 45.1%,39.9%and 27.1%,respectively,in winter.The results of statistical analysis showed that As and Ca were the key environmental factors that drive the changes of diatom species richness,explaining23.2%and 8.2%of the total variation in species richness.Furthermore,the species richness of diatoms displayed a unimodal response characteristics along the gradients of As and Ca.Na and K are key environmental factors in driving the change of diatom biomass,which accounted for 19.7%and 6.8%of the diatom biomass change,both showing a significant positive correlation with diatom biomass.Further,the response characteristics of the dominant species of Yangzong Lake to environmental gradients were identified,including A.minutisssima,F.nanana,F.ulna,C.pseudostelligera,C.comta,S.hantzschii,C.meneghiniana,and A.granulata.It has high TP and As tolerance values and can be used as sensitive species indicating eutrophication or higher As concentration water bodies.In summary,the limnological properties and diatom community structure display strong seasonal and interannual variations during 2015²017 in Yangzhong Lake.Furthermore,the change of lake-water arsenic concentration is obviously related to the seasonal fluctuation of thermal stratification that is linked with the As release from bottom sediments,showing a clear trend of seasonal fluctuation.Statistical analyses showed that the diatom community structure was significantly affected by the level of arsenic pollution in the waters,with a loss of diatom species richness.Therefore,control on the pollutant input from watershed and reduction in lake-water arsenic concentration are important prerequisites to improve environmental quality and ecosystem health in Yangzong Lake.