Analysis Of Influencing Factors Of Cascade Dams On The Dynamic Changes Of Phytoplankton In Plateau Rivers

Phytoplankton,as the evaluation basis of water eutrophication and habitat change,the dynamic change of community structure can directly and effectively indicate the water quality and provide theoretical basis for water habitat management in the basin.Because of its special plateau karst landform,Guizhou has many rivers and lakes,rich water resources,relatively concentrated water level drop and mostly mountainous terrain,which is suitable for the development of water conservancy and hydropower projects.Therefore,many rivers are built with continuous large-scale cascade dams for water storage and power generation.In order to explore the influence of cascade damming in Beipanjiang River basin on the dynamic changes of phytoplankton in its water habitat,sampling surveys were conducted at Guangzhao Reservoir,Mamaya Reservoir,Dongqing Reservoir and several river sections in October 2020,April2021,July 2021,December 2021,May 2022 and July 2022,respectively.The results showed that:1.A total of 70 species（genera）of phytoplankton were identified in Beipanjiang River Basin,62 species（genera）of phytoplankton in 7 phyla in river section and 66 species（genera）of phytoplankton in reservoir area;The rivers are dominated by chlorophyta and diatom,and there are relatively many species of chlorophyta,cyanophyta and diatom in the reservoir area.Spirogyra,Navicula,Microcystis menenii and Cryptophyta are the dominant algae species in three periods,and Microcystis menenii is the dominant algae species in river section and reservoir area,and the dominance is relatively high.The average biomass of phytoplankton in the three periods is in the order of high water period>normal water period>low water period.The biomass of phytoplankton in reservoir area is significantly higher than that in river section,and the contribution rate of diatom biomass is relatively high.2.There are 24 functional groups in Beipanjiang River Basin,which are B,D,E,F,G,H1,J,K,LM,Lo,M,MP,N,P,S1,SN,T,TC,W1,W2,X1,X2,X3,Y,among which 22 are river sections.During the investigation,there were 14 dominant functional groups,and there were11 dominant functional groups in the river section,namely B,D,F,J,Lo,MP,N,P,S1,X1 and Y;There are 13 dominant functional groups in the reservoir section,which are B,D,E,F,G,J,Lo,M,MP,P,S1,X1 and Y respectively.The main influencing factors of phytoplankton functional groups in river reach are chemical factors such as N and P,and physical factors such as WT and pH are the main influencing factors in reservoir area.3.According to the difference of niche breadth,the dominant species of phytoplankton in Beipanjiang River Basin can be roughly divided into three categories,namely,Cyclotella meninii,Phaeophyta and Cryptophyta,and Anabaena pseudonata,Chlorella,Straight-chain algae,Cladosporium crispum,Phaeophyta,Phaeophyta ovata and Phaeophyta as narrow niche populations.According to the eco-ecological response rate,the phytoplankton community in dry season and wet season is in the decline stage,while the phytoplankton community is in the development state in normal season,and the development（decline）phenomenon in reservoir area is more obvious than that in river section.4.The existence of cascade dams has changed the physical and chemical factors of river water quality.From the river section to the reservoir area,the average concentration of TN,TP,DIN,PO₄^3-P,COD_Mnand DO factors showed a downward trend,and the average decline range in different hydrological periods was in the order of wet season>normal season>dry season.WT,pH and EC values have increased,and the increasing range is in the order of wet season>dry season>normal season.Some chemical factors in the downstream of the river section are lower than those in the upstream area,and the physical and chemical factors in the downstream of the reservoir area are generally higher than those in the river section.Cascade damming has formed two habitats,reservoir and river,in which the reservoir area habitat is relatively more suitable for the growth and reproduction of phytoplankton,and the number and diversity of phytoplankton species in the downstream area are higher than those in the upstream area.5.According to the results of Redundancy Analysis（RDA）of phytoplankton functional groups and environmental factors,it can be seen that in different hydrological periods,the main influencing factors in dry season and normal season are N and P nutrients,while in wet season,the main influencing factors are WT and pH,and functional groups L0 and Y make great contributions to chlorophyll.From the point of view of two different habitats,the environmental factors affecting the succession of phytoplankton functional groups in rivers are mainly chemical factors such as N and P,and the environmental factors affecting the succession of phytoplankton functional groups in reservoirs are mainly physical factors such as WT and pH.