Research On Distribution Pattern And Environmental Relevance Of Phytoplankton Functional Groups In Lalin Rivier

River ecosystem is one of the very important types of water ecosystem,which is a carrier of material flow and energy cycle,with functions of hydrophobic flooding,water supply,sand transport,navigation,agricultural irrigation,power generation,landscape recreation.Phytoplankton are primary producers of river ecosystems and are able to respond rapidly to changes in their surroundings,so they are often used in ecological health assessments.Lalin River is located in the north temperate monsoon climate zone,with a wide watershed area and abundant arable land resources in the middle and lower reaches of the watershed,which is a typical river polluted by agriculture.Therefore,it is of great ecological importance to study the structural characteristics of the phytoplankton community in Lalin River.In this study,phytoplankton samples were collected and fixed at 23 sampling points in Lalin River in spring,summer,autumn of 2021 and summer of 2022.Based on optical microscope and high-throughput sequencing technology,the composition of picophytoplankton and phytoplankton community and its coupling relationship with environmental factors were analyzed.The temporal and spatial changes ofαdiversity and functional diversity of phytoplankton in Lalin River were analyzed,and the succession characteristics of dominant phytoplankton species were discussed.The phytoplankton were divided into functional groups,morphofunctional groups and morphology-based functional groups.The succession of dominant functional groups and their responses to water environmental factors were explored by RDA,and the functional group method suitable for comprehensive monitoring of water ecological environment in Lalin River was analyzed.In addition,this study also conducted a comprehensive evaluation of the water quality of the Lalin River based on the integrated nutrient status index and the functional group Qr index,with a view to providing basic data and theoretical basis for the ecological protection and sustainable development of large rivers polluted by agriculture in the north.The main research results are as follows:（1）In spring,summer and autumn of 2021 and summer of 2022,182 taxonomic units of phytoplankton were identified,belonging to 7 phyla,10 classes,20 orders,35families and 80 genera.The average abundance of phytoplankton is 2.56×10⁶ind./L and0.52×10⁶ind./L;The average biomass is 3.06 mg/L and 0.64 mg/L;The abundance and biomass showed a pattern of woodland>plowland in autumn and plowland>woodland area in spring and summer,and the community structure was diatom-green algae type.A total of 46 taxonomic units of phytoplankton dominant species were identified,Nitzschia palea,Nitzschia acicularis,Asterionella formosa,Ulnaria acus,Aulacoseira pusilla,Cyclotella meneghiniana,and Ankistrodesmus angustus were the dominant species in all four seasons.（2）The results of high-throughput sequencing show that the picophytoplankton and phytoplankton community in Lalin River exhibit obvious seasonal succession law,with Bacillariophyta,Chrysoplyta and Cryptophyta favoring low temperature in spring to Cyanophyta which liked high temperature in summer,and finally they were replaced by Bacillariophyta and Dinophyta in autumn.（3）Water Temperature,Pondus Hydrogenii,Specific Conductivity,Dissolved Oxygen,Turbidity,Total Nitrogen,Total Phosphate,Chemical Oxygen Demand and Five-Day Biochemical had obvious spatial and temporal in Lalin River in 2021 and summer of 2022.The results of the combined nutritional status index showed:in the spring and autumn of 2021,the water of Lalin River is in the middle trophic type.in the summer of 2021 and 2022,the water of Lalin River was eutrophic and slightly eutrophic.The water in plowland is in a state of mild eutrophication,while the water in woodland is in a state of moderate eutrophication.（4）The mean phytoplankton Margalef diversity index,Shannon-Wiener diversity index,Simpson diversity index,Pielou evenness index,Diversity threshold,Functional richness index,Functional evenness index,Functional dispersion index,Functional dissimilarity index,Functional redundancy index had obvious spatial and temporal in Lalin River in 2021 and summer of 2022.（5）A total of 24 functional groups;23 morphofunctional groups;and 7morphology-based functional groups were classified in Lalin River in 2021 and summer of 2022.The spatial distribution of phytoplankton functional groups and C-S-R strategy are significantly different,and the difference of temporal distribution is not significant.Qr index analysis results show that:the water has a certain tolerance and the water quality is relatively good in Lalin River in 2021,the water condition is in a medium state,and the water condition is good in Lalin River in summer of 2022.（6）The major environmental factors affecting the succession and distribution of functional groups,morphofunctional groups and morphology-based functional groups in the Lalin River system in 2021 and summer of 2022 are WT,p H,DO,TN,Sp Cond.and COD_Mn.The functional taxa of were used as a research method to analyze the spatial and temporal distribution characteristics of phytoplankton functional taxa in large rivers in northern China,and the future changes of water environment could be predicted.