Study On Phytoplankton Community Characteristics And Environmental Factors Of The Recharge Lakes And Reservoirs In The Central Line Project Of The South-to-north Water Diversion

Phytoplankton is the primary producer in the aquatic environment system and plays significant role in the material circulation and energy flow of the ecosystem.The community structure of phytoplankton can reflect the changing laws of the water environment and is an important indicator of the level of eutrophication of the water environment.The opening of the middle route of the South-to-North Water Diversion Project is of great implications for alleviating the shortage of water resources in central China and realizing the rational allocation of water resources.In this paper,the water quality and phytoplankton samples collected from the water sources and replenishment lakes in the three seasons(summer,autumn,and winter)of the Middle Route Project of the South-to-North Water Diversion Project in 2020 were collected.The eutrophication status of water bodies was evaluated by using the Comprehensive Pollution Index(TLI)to evaluate the water source areas and replenishment lakes and reservoirs of the middle route of the South-to-North Water Transfer Project.Meanwhile,the number of phytoplankton species,the temporal and spatial distribution of phytoplankton abundance,phytoplankton diversity index,and phytoplankton dominance were described.The similarities of phytoplankton communities in different seasons of the Central Line Project were compared.At the same time,the main environmental factors affecting the water source and various recharge lakes and reservoirs of the middle route of the South-to-North Water Transfer were identified through redundant analysis.The results of the study are as follows:(1)The comprehensive pollution index values of the water source and replenishment lakes of the South-to-North Water Transfer Project are as follows:Baiyang Lake>Jiangang Reservoir>Yahekou Reservoir>Baiguishan Reservoir>Danjiangkou Reservoir.Among them,Danjiangkou Reservoir has the best water quality and the lowest degree of eutrophication.(2)A total of 76 species of phytoplankton in 5 phyla were detected in the water source area and replenishment lakes and reservoirs of the middle route of the South-to-North Water Transfer Project.The distribution and proportion of the populations of each lake and reservoir are different,and all the lakes and reservoirs have the highest proportion of green algae species.Chroococcus minutus,Cyclotella comensis Grun.and Cryptomonas erosa are the dominant algae species in Danjiangkou Reservoir;Synedra sp.is the Yahekou Reservoir the dominant algae species of the lake.Pseudanabaena limnetica,Merismopedia elegans,Cyclotella koman,and Cryptomonas erosa are all dominant algae species in the Baiguishan Reservoir.Cylindrospermum raciborskill is the dominant algae species in Jiangang Reservoir.Cyclotella comensis Grun.,Synedra sp.and Cryptomonas erosa are all dominant algae species in Baiyang Lake.Cylindrospermum raciborskill is the dominant algae species in Miyun Reservoir.(3)The abundance of phytoplankton in the water source areas and replenishment lakes and reservoirs of the middle route of the South-to-North Water Transfer Project showed obvious seasonal changes.The abundance of phytoplankton was higher in summer and decreased obviously in winter.The range of phytoplankton abundance varied from7.14×10~4 to 1.79×10~7 cells/L,with an average of 4.22×10~6 cells/L,and the seasonal variation range of different lakes and reservoirs varied to varying degrees.From a spatial point of view,there is no significant difference in abundance of phytoplankton in the recharge lakes and reservoirs of the middle route of the South-to-North Water Transfer Project.The average total abundance of phytoplankton in descending order is:Jiangang Reservoir>Baiyangdian>Yahekou Reservoir>Baiguishan Reservoir>Danjiangkou Reservoir.(4)A total of 24 functional groups were identified in the water source area and its supply lakes and reservoirs of the middle route of the South-to-North Water Transfer Project,namely A,B,D,E,F,G,H1,J,L_O,M,MP,N,P,TC,S1,S_N,W1,W2,and Y functional groups,the main advantageous function groups are B,D,P and L_O function groups.Among them,the functional group B represents the species of Cyclotella comensis Grun.,the functional group D represents the species of Synedra sp.and Nitzschia amphibia Grun.,and the functional group L_O represents the species of Chroococcus minutus,Chroococcus minus,Peridinium zonatum Playf.and Ceratium.The P functional group represents the algae species Closterium cynthia and Melosira granulata.(5)After the opening of the middle route of the South-to-North Water Transfer Project,the proportion of cyanobacteria in the Yahekou Reservoir increased.The species of phytoplankton increased significantly after Baiguishan Reservoir was recharged by Danjiangkou Reservoir for the first time.The species and abundance of diatoms increased,and Microcystis aeruginosa disappeared.The dominant algae species in Jiangang Reservoir changed from Synedra sp.,Dinobryon,Ceratium hirundinell and Mougeotia in summer to Cylindrospermum sp.,and the abundance of phytoplankton is also significantly higher than before receiving water.The phytoplankton species in Baiyang Lake are more polluted than before the water was refilled,and the abundance of phytoplankton decreased.Miyun Reservoir species decreased,the abundance of cyanobacteria decreased,and the abundance of diatoms increased.(6)Redundancy analysis shows that nitrate,nitrous nitrogen and total nitrogen are the environmental factors that mainly affect the dominant functional groups of phytoplankton in Danjiangkou Reservoir.Total nitrogen,total phosphorus and nitrous nitrogen are the main environmental factors that affect the dominant phytoplankton in Yahekou Reservoir Environmental factors of functional groups.Water temperature and chlorophyll are the environmental factors that mainly affect the dominant functional groups of phytoplankton in Baiguishan Reservoir.Total nitrogen and ammonia nitrogen are the environmental factors that mainly affect the dominant functional groups of phytoplankton in Jiangang Reservoir.Nitrous nitrogen,total phosphorus water temperature and chlorophyll a are the environmental factors that mainly affect the dominant functional groups of phytoplankton in Baiyangdian Lake.Water temperature and nitrogen nutrients are the environmental factors that mainly affect the dominant functional groups of phytoplankton in Miyun Reservoir.This paper reveals the seasonal succession of phytoplankton community structure in the replenishment lakes and reservoirs of the middle route of the South-to-North Water Transfer Project,which is of great significance to water ecology and water resources protection.It also provides important background data for evaluating the impact of large-scale water transfer projects on the lake and reservoir water ecology.