Phytoplankton Community Variations In Danku Of Danjiangkou Reservoir Under Water Transfer And Driving Factors Analysis

The Middle Route of the South-to-North Water Diversion Project has effectively relieved the severe water shortage in the northern of China.The water quality of Danjiangkou Reservoir as the water source,relates to the drinking water safety of 120 million people along the route.The reservoir bay is a sensitive ecology region,and its water quality directly influences water transfer safety.This research was conducted in Danjiangkou Reservoir from July 2018 to July 2020.According to the flow direction,a total of 18 sampling points were set up from the inlet to the water transfer outlet in Danjiangkou Reservoir,including three types of disturbing intensity,that is,intensive disturbance,mild disturbance and light disturbance.The variations and driving factors of the phytoplankton community in bays in Danku under different intensity of human disturbance were studied.One-way ANOVA,Pearson correlation,CCA(RDA)ordination were used to analyze the tempo-spatial changes of the hydrology,water quality and phytoplankton communities.This aims of this study was to provide theoretical support for the ecological environmental protection and scientific management of water quality in Danjiangkou Reservoir,and the dataset for the reservoir ecological regulation in China.The main results are as follows:1.The water level in Danjiangkou Reservoir varied significantly across the seasons,with the highest water level in autumn and the lowest in spring.The flow and velocity of water diversion port were the largest in spring and the smallest in winter.Dissolved oxygen(DO),total nitrogen(TN),total phosphorus(TP),ammonia nitrogen and chemical oxygen demand(COD)at each sample point all reached Class I water quality standard.The permanganate index exceeded 4 mg/L,complying the Class II.The DO contents in the Heijizui,Kuxin and Qushou were higher than those in reservoir bays.The bays were more disturbed by human activities,and the water quality was worse.The Chl.a content in Huzhai bay was the highest,where the water body was strongly disturbed and nutrients were more accumulated benefit or algae growth.Therefore,we must strengthen the management and control of the water body in reservoir bays.2.The phytoplankton community in Danjiangkou Reservoir was composed of104 species in 71 genera and 7 phyla.It had the highest abundance,the most species numbers and the most abundant dominant species in summer,and the least number and the lowest abundance in spring.The phytoplankton community was composed of diatoms-chlorophyta in spring,chlorophyta-diatoms in summer,cyanobacteria in autumn,and cyanophyte-chlorophyta-diatoms in winter.In spring diatoms became the absolute dominant species.In summer the dominant species changed to diatoms,chlorophyta and cyanophyta.In autumn dominant species were cyanophyta,chlorophyta and diatoms,and in winter only diatoms were the dominant species.Both of the Shannon-Wiener diversity index and Margalef abundance index decreased first and then increased in Heijizui,Kuxin and Qushou.The Shannon-Wiener index was highest in summer and winter,when the communities were more stable than those in autumn.The phytoplankton community was more stable in summer and winter,and the phytoplankton community was less stable in autumn.Pearson correlation and CCA indicated that water level,flow velocity,water temperature,pH,water depth,TP,ORP,and,Cond were the key environmental factors that affected phytoplankton community structure.Dominant species played important roles in maintaining the stability of the phytoplankton community and ecosystem.3.The phytoplankton community in Reservoir bay was composed of 99 species in 66 genera and 7 phyla.The highest abundance and the most numbers of dominant species appeared in summer,and the least species numbers and the lowest abundance appeared in spring.Diatoms-chlorophyta was dominant in spring,cyanobacteria-chlorophyta-diatoms in summer,cyanophyte in autumn,and chlorophyta-diatoms-cyanophyte in winter,which were different from the types in another sites.In spring,diatoms were the absolute dominant group,followed by green algae.In summer,the cyanophyta,diatoms,and chlorophyta became the main dominant groups.In autumn,the dominant species mainly were cyanophyta,chlorophyta,and diatoms.In winter diatoms and chlorophyta were dominant.Dominant species played key roles in the phytoplankton community,and the over-reproduction of certain dominant species would affect the stability of the whole phytoplankton community.Pearson correlation and RDA showed that water level,water temperature,DO,ORP,TP,Cond,and pH were the key driving factors that affected the phytoplankton community in bays.Among the three bays,the Shannon-Wiener diversity index in Quandian with light disturbance was the highest,followed by Songgang with mild disturbance,and Huzhai with intensive disturbance.The disturbance intensity affected the phytoplankton community structure.The intensive disturbance reduced the species diversity,and the light disturbance had little effect.The Shannon-Wiener diversity index was the highest in spring and decreased significantly in autumn.It showed that the individual distribution in summer was more even and the community structure was more stable.However,the diversity index decreased in autumn,which made the community structure and ecosystem structure simpler and reduced the productivity and stability of the ecosystem.