Study On The Spatial And Temporal Pattern And Driving Mechanism Of Phytoplankton Communities In The Qinghai Lake Basin

High-altitude ecosystems are one of the most important hotspots for global biodiversity conservation.These ecosystems can sensitively and rapidly respond to environmental changes,and even slight disturbances can significantly affect their physicochemical properties and biological communities.The Qinghai Lake basin is an important water body of the Qinghai-Tibet Plateau,playing an important role in maintaining the biodiversity resource in this plateau.However,this basin has a simple and fragile environment that is highly vulnerable to disturbances.With the influence of global warming and increasing human activities,Qinghai Lake basin has been facing a series of ecological problems.As the most important primary producer,phytoplankton are critical in the nutrient cycle and energy flow in aquatic ecosystems.However,little is known about the phytoplankton assemblages in the Qinghai Lake basin and the current studies only focus on the taxonomic composition.Therefore,there is a urgent need to conduct phytoplankton researches at spatial and temporal scales and different diversity dimensions.This study systematically investigated the spatiotemporal patterns and driving mechanisms of phytoplankton communities in the main lake area and four major tributaries（Buha River,Haergai River,Quanji River,and Shaliu River）of Qinghai Lake during the spring,summer,and autumn of 2020.The main results are as follows:1.A total of 65 phytoplankton species belonging to 39 genera,23 families,and 6 phyla were identified in the Qinghai Lake.Phytoplankton communities showed significant differences at the spatial（four lake regions）and temporal（three seasons）scales.The abundance of phytoplankton was significantly higher in deep zone than other zones,and the species richness was higher in the inlet zone.Among the three seasons,the abundance and biomass of phytoplankton in summer and autumn were nearly 10 times higher than those in spring,and the dominant groups changed from Bacillariophyta in spring to Cyanophyta in summer and autumn.Salinity,water temperature and TP were the key environmental factors affecting the phytoplankton communities in the Lake Qinghai.However,spatial factors showed insignificant influence on the phytoplankton communities.Compared with the historical surveys,this study indicated that the main dominant groups of phytoplankton community in Lake Qinghai gradually changed from Bacillariophyta to Cyanophyta.2.A total of 76 phytoplankton species belonging to 48 genera,24 families,and 7 phyla were identified in the four tributaries to the Qinghai Lake.There were some differences in the community structure among different rivers.The species richness was highest in Quanji River,whereas the species richness,abundance and biomass were the lowest in Buha River.The relative abundance of Bacillariophyta increased from summer to autumn（78.6%to 87.7%）,while the relative abundance of Cyanophyta decreased significantly（20.6%to 3.8%）through time.NH₄⁺-N,p H,water temperature,altitude and TP were the main environmental drivers affecting phytoplankton community structure in the inlet tributaries,while spatial factors had less influence on phytoplankton community structure.The phytoplankton community structure of the inlet tributaries was characterized by higher species richness,lower abundance and higher dominance of Bacillariophyta compared to the main lake area at the same time.3.The taxonomic,function and phylogenetic alpha diversity showed significant seasonal and regional differences.The taxonomic diversity indices（Shannon,Margalef,Pielou and Simpson）,as well asΔandΛ⁺in the tributaries were significantly higher than those in the main lake,while the functional diversity indices（FEve,FDis and FDiv）,Δ^*andΔ⁺were lower than those in the main lake.Among different seasons,the three facets of alpha diversity in the main lake area were all higher in spring than in summer and autumn.In the tributaries,only taxonomic diversity（Shannon diversity and Margalef indices）and functional evenness（FEve）showed seasonal differences,with higher taxonomic diversity but lower functional evenness in summer.Environmental filtering plays a dominant role in shaping the diversity of planktonic communities in the Qinghai Lake basin,and functional diversity can better respond more reliably to environmental factors than taxonomic and phylogenetic alpha diversity.4.The three dimensions of phytoplankton beta diversity and their components（turnover and nestedness components）differed in spatial and temporal scales.Taxonomic diversity was mainly composed of turnover components（63.3%～81.3%）,while functional and phylogenetic diversity were mainly composed of nestedness components（55.9%～91.4%）.The functional beta diversity of the inlet tributaries was significantly higher than that in the main lake area.Among different seasons,the functional beta diversity of the main lake was higher in spring,while the phylogenetic beta diversity was highest in summer.In the tributaries,however,the three dimensions of beta diversity were all higher in autumn.The harsh habitat conditions in high-altitude regions are the main factor influencing changes in beta diversity,especially in functional dimension.In addition,environmental filtering explains a high variation of the turnover components in all three dimensions,while dispersal limitation has a significant effect on three-dimension nestedness components.