Microplankton Community Structure And Water Quality Characteristics In The Mainstream Of Laoguanhe River

The Laoguanhe River is not only the first tributary of Danjiang River around Danjiangkou reservoir,but also the nearest tributary to the South-to-North Water Diversion Project.Therefore,the water quality safety of Laoguanhe River plays an extremely important role in ensuring the water quality safety of diversion project.The community composition of microplankton can affect the function and stability of the aquatic ecosystem.In this study,four ecological sites including Yanghe(YH),Xixia north(XX),Dangziling(DZL)and Zhangying(ZY)in the mainstream of Laoguanhe River were selected.The YH with human light disturbance locates in the mountainous area.The river from YH to XX flows through the towns,belonging to mild disturbance.The river from XX to DZL flows through Xixia County,belonging to serious disturbance.The river from DZL to ZY flows through Xichuan County,belonging to mild disturbance.High throughput sequencing of planktonic bacteria(16S r RNA)and eukaryotes(18S r RNA)were performed before the flood season(May)and after the flood season(October)in 2018 and 2019,respectively.The diversity of bacteria and eukaryotes in the surface water of Laoguanhe River mainstream was analyzed by high-throughput sequencing of Illumina Mi Seq.The distribution characteristics and function of microplankton were studied in order to explore their relationships between the ecological compositions and environmental factors in Danjiangkou reservoir.CCA and Spearman correlation were used to explore the relationship between their community composition and the physico-chemical factors of water quality.It provides a reference for the sustainable development and utilization of water resources,and the formulation of environmental protection policies in laoguanhe River Basin.The main results are as follows:1.In May and October of 2018 and 2019,the water quality showed that the contents of total nitrogen(TN)in Laoguanhe River were less than the national class III of water standard during the two years,and those of YH were the lowest.The contents of total phosphorus(TP)and ammonia nitrogen were both at the class I and class II,the chemical oxygen demand(COD)in DZL and XX reached the class III,and the dissolved oxygen(DO)in DZL reached the class III.By comparing these indices in previous years,we found that the water quality had improved in recent years.2.A total of 40 phyla and 599 genera mainly including Proteobacteria,Bacteroidetes,Actinobacteria,Cyanobacteria,Firmicutes,Verrucomicrobia,Armatimonadetes and Chloroflexi showed the diversity of community composition in the mainstream of Laoguanhe River.From YH to ZY,the ? diversity of bacterioplankton decreased at the beginning and then increased.The ? Diversity indicated significant differences among different sites before flood season except for XX,and the other sites had similar bacterial community composition.After the flood season,there was no significant difference of bacterioplankton community among different sites.The CCA between bacterioplankton community and environmental factors showed that,DO,COD,ORP,Chla and Cond significantly affected the bacterioplankton distribution.Spearman correlation showed that COD,nitrate nitrogen,TN and TP were the main factors affecting the distribution of bacterial community.Molecular ecological correlation network analysis indicated that Chla,DO,ORP and TP had significant effects on the community composition of bacterioplankton.Dominant species played a key role in the community.The synergistic effect between rare and non-rare taxa might play a central role in maintaining the stability and ecological function of bacterioplankton community.3.Bacterioplankton PICRUSt prediction showed that 25 gene functional families were identified in Laoguanhe River,reflecting the function diversity.The differences of the functional gene KO before the flood were larger than those after the flood,which implied that the land use increased the input of nitrogen,and excessive nitrogen would cause eutrophication of water body and change the nitrogen metabolism process of bacterioplankton.4.Planktonic eukaryotes were composed of 11 phyla in the mainstream of Laoguanhe River,mainly including Cryptomonadales,Kathablepharidae,Ciliophora,p1-31 and Ascomycota.The ? diversity indexes first decreased and then increased from YH to ZY.NMDS showed differences of species composition at different sites in the same year or at the same sites in different years.In 2018,DZL and ZY had the most similar community compositions,which were largely different from those of YH and XX.However,in 2019,the community compositions of YH,XX and ZY were the most similar except those of DZL.Due to the strong human disturbance in DZL,the species compositions were more similar in two years.CCA ordination showed that temperature,p H,TP,DO,COD and Cond were all important factors affecting the distribution of eukaryotes.Spearman correlation indicated that COD,nitrate nitrogen,TN and TP were important factors affecting eukaryotic diversity index.Molecular ecological correlation network analysis showed that ORP,TN,TP and nitrate nitrogen had significant effects on the community composition of planktonic eukaryotes,and rare species played a key role in the species community.