Correlation Between Spatial And Temporaldistribution Pattern Of Phytoplanktoncommunity Structure And Nutrients In Qiandaolake

Phytoplankton,as the primary producer in the lake and reservoir ecosystem,the change of its community structure was often closely related to the change of water environment conditions,especially nutrients.The phytoplankton community structure in deep-water lakes and reservoirs has significant spatial distribution differences and typical vertical differentiation characteristics,which was one of the factors to consider its water quality and ecosystem functions.Therefore,it was necessary to explore the spatial and vertical characteristics of phytoplankton community structure change and its main influencing factors in deep-water lakes and reservoirs.In this study,Qiandao Lake,a super-large deep-water reservoir,was taken as the research object,and the spatial grid monitoring and vertical stratified collection of phytoplankton and hydrochemical indexes in Qiandao Lake were carried out from 2020 to 2021.By comprehensively understanding the temporal and spatial differences of phytoplankton community composition,abundance and diversity in Qiandao Lake,the relationship between phytoplankton community composition,abundance and diversity and environmental factors was discussed,and the role of nutrients in driving the change of phytoplankton community structure was revealed.Combined with physical and chemical indexes and phytoplankton community characteristics,the Water environmental quality of Qiandao Lake was comprehensively evaluated,in order to provide theoretical reference for the regulation of Qiandao Lake ecosystem and the protection of water environment.The main research results were as follows:1)There were 8 phyla and 196 species of phytoplankton in Qiandao Lake.The most species of phytoplankton were chlorophyta,followed by diatom.The spatial abundance of phytoplankton ranged from 2.54×10~6to 2.89×10~7cells·L^-1in spring and from 1.39×10~6to 2.68×10~7cells·L^-1in autumn.Four dominant species(genera)of phytoplankton were identified in the whole lake,which were Achnanthes sp.,Cyclotella sp.,Pseudoanabaena limnetica and Pseudanabaena minima.In autumn,Achnanthes sp.and Cyclotella sp.were dominant species(genera),while in spring,Achnanthes sp.,Cyclotella sp.and Pseudanabaena minima were dominant species(genera).2)In Qiandao Lake,Margalef richness index ranged from 0.51 to 2.18,Shannon-Wiener diversity index ranged from 0.25 to 2.56,Pielou evenness index ranges from 0.06 to 0.56 and Simpson dominance index ranged from 0.07 to 0.89.In spring and autumn,the diversity of phytoplankton in the river area and the central area of Qiandao Lake was the highest,while the diversity index of lakes in the city was relatively low.The corresponding nutrient concentration showed the same rule,that is,it gradually decreases from west to east,and the nutrient concentration in the lake in the city was higher.3)The spatial change of phytoplankton community structure in Qiandao Lake was mainly influenced by water temperature(WT),organic suspended matter(OSS),nitrogen nutrients,dissolved oxygen(DO)and turbidity(Tur).The greater the abundance of phytoplankton in autumn,the lower the concentration of phosphate(PO₄-P),while the diversity index of phytoplankton increases with the increase of nitrate nitrogen(NO₃-N).The abundance and diversity index of phytoplankton in spring were mainly influenced by nitrogen nutrients,which were negatively correlated with the abundance of phytoplankton and positively correlated with the diversity index.4)There were 8 phyla and 195 species of phytoplankton in Qiandao Lake vertically.The most species of phytoplankton are chlorophyta,followed by diatom.The abundance of phytoplankton varied from 1.42×10~4to 9.68×10~6cells·L^-1in spring and from 3.38×10~3to 2.83×10~6cells·L^-1in autumn.In spring,the dominant species in the surface layer and subsurface layer were Cryptomonas sp.and Pseudanabaena sp.,the middle layer was Cryptophyta,and the abundance of Cyclotella in the bottom layer was significantly higher than that of other algae species.In autumn,the dominant species in the surface layer were Anabaena sp.and Aphanizomenon sp.,the dominant species in the sub-surface layer and middle layer were Leptolyngbya sp.and Anabaena sp.,and the only dominant genus was Leptolyngbya sp.in the bottom layer.5)The vertical Margalef richness index of Qiandao Lake ranged from 0.29 to 1.68,Shannon-Wiener diversity index ranged from 0.96 to 2.13,Pielou evenness index ranged from 0.27 to 0.51 and Simpson dominance index ranged from 0.44 to 0.84.The vertical diversity index of phytoplankton decreased with the increase of water depth in spring,but increased in the bottom layer,and decreased gradually with the increase of water depth in autumn.Nutrients show the same law,and the concentration decreases with the increase of water depth,among which the concentration of nutrients in the bottom layer was higher in autumn.6)The main influencing factors of vertical distribution characteristics of phytoplankton community in Qiandao Lake were WT,TN and NH₄-N.The main environmental factors affecting the vertical distribution of phytoplankton in spring were WT and TN,while the vertical distribution of phytoplankton in autumn was mainly affected by WT and NH₄-N.The abundance of phytoplankton in spring and diversity index of phytoplankton in autumn were positively correlated with PO₄-P significantly.7)In this study,various evaluation methods were used to evaluate the water quality of Qiandao Lake.Based on the evaluation results of surface water environmental quality standards,Qiandao Lake is in Class II and III water bodies.The evaluation results based on the modified trophic state index(TSI_M)showed that Qiandao Lake was in the middle trophic state.Evaluation of phytoplankton pollution indicator species indicated that Qiandao Lake is in the middle trophic state;The proportion of cyanobacteria in water bloom showed that the proportion of cyanobacteria in water bloom was higher in Qiandao Lake in spring,but higher in river area in autumn.Comprehensive diatom index(CDI)evaluation results showed that most lake areas in Qiandao Lake were in a state of mild eutrophication in spring and moderate eutrophication in autumn.The results of comprehensive evaluation methods showed that Qiandao Lake was in a middle trophic state on the whole.It was suggested that the water environment of Qiandao Lake should be monitored in spring,and necessary measures should be taken to reduce the eutrophication level and prevent the occurrence of water bloom.