Distribution Patterns And Driving Forces Of Plankton Communities Along Elevational Gradients In The Yangtze River Source Region

Few information about biodiversity patterns in high-elevation aquatic systems has been gathered,even though these patterns are important for understanding the impacts of human activities.The source region of the Yangtze River,a typical alpine zone with vulnerable and sensitive ecosystems,was selected.Plankton,which plays a critical functional role as freshwater producers,serves as an important ecological indicator of aquatic systems.Here,systematic surveys on the water environment and phytoplankton were carried out to examine how the phytoplankton community varies with elevation in 10 typical rivers of the Yangtze River source region in March and October 2018.A total of 58 phytoplankton species belonging to 4 phyla and 28 genus were identified,among which Bacillariophyta(79.3%of the total species number),Chlorophyta(10.4%),Cyanophyta(8.6%)were dominant.The variations of phytoplankton density were 6.06×10⁴-39.90×10⁴ cells·L^-1,the average of phytoplankton biomass was 0.59 mg·L^-1 during the investigation.The synergistic effects of human activities and basic environmental factors(terrestrial productivity and slope gradient)were the drivers affecting the phytoplankton community in the Yangtze River source region.Human activities were the major drivers affecting the phytoplankton community along the elevational gradient.As the elevation decreasing,the disturbances to aquatic ecosystems caused by human activities intensified in the study area.The phytoplankton species identified were divided into 13 functional groups.Representative functional groups(relative biomass greater than 5%of the total species biomass)were C,D,LO,MP,P,TB,X1,Y,among which D,LO,MP and P,were found in each section and accounted for 91.8%of the total biomass.Redundancy analysis showed that phytoplankton functional groups were mainly affected by TN,TP,NO₂--N,EC,WT,Sal and DO in the Yangtze River source region.Combined with basic physical and chemical indexes,nutrient indexes and phytoplankton diversity indexes,water quality in the Yangtze River source region was generally non-polluted or slightly polluted.A total of 44 species of zooplankton were identified,among which Protozoa(34.09%of the total species number),Copepods(27.27%)were dominant.The average of zooplankton density was 174.05 ind·L^-1,the average of phytoplankton biomass was 0.36 mg·L^-1 during the investigation.The results of canonical correspondence analysis showed that the distribution pattern of zooplankton was affected by pH,TP and NO₂--N in the Yangtze River source region.According to the dominant zooplankton species,the rivers in the Yangtze River source region are in oligotrophic state.In order to mature the theoretical basis for study of aquatic plankton in plateau regions,the typical main stream and tributary reaches of the source region of the Yangtze River were selected as the study subjects,which is supposed to be the theoretical basis and technical support for the ecosystem health of the source region of the Yangtze River.