Study On The Picoeukaryotic Plankton Diversity In The Main And Tributaries Of The Three Gorges Reservoir

Since the impoundment of the Three Gorges Reservoir,the hydrological situation of the main stream and the tributaries in the reservoir area has changed,showing different degrees of eutrophication.The pattern of picoeukaryotic plankton biodiversity in the reservoir has changed.Picoeukaryotic plankton is an indispensable role for primary producers,consumers and decomposers in aquatic ecosystems.Most ultraplankton organisms cannot be purified and cultured,and it is difficult to study their biodiversity using classical analytical methods.The development of high-throughput sequencing technology provides an unprecedented depth and breadth for the analysis of microbial communities.In this paper,the main stream of the Three Gorges Reservoir and the tributaries of Shennong Creek and Xiangxi River were taken as research waters,and the picoeukaryotic plankton was taken as the research object,and the two-month sampling was carried out during 2015.High-throughput sequencing technology was used to characterize the small subunit ribosomal gene?SSU rRNA?V4 amplicon in environmental samples.The structure,succession and spatiotemporal distribution characteristics of the picoeukaryotic plankton community in the main stream of the Three Gorges Reservoir and its typical tributaries were studied.Based on the correlation analysis of environmental factors,the picoeukaryotic plankton groups in different waters was compared on time and spatial scale.Structural differences to obtain key environmental factors that influence changes in the picoeukaryotic plankton group.The research results were as follows:1.The spatiotemporal distribution of the main physical and chemical indicators were monitored in the main stream,the Shennong Creek and Xiangxi River tributaries in the Three Gorges Reservoir during 2015:the time difference of water temperature changes in the whole year was significant,and the surface water temperature of each monitoring point was synchronized in time;the spatiotemporal distributions of Chl a,Cond,ORP,PAR and NO₃-N were significantly different,pH and COD were relatively stable,TN,TP and turbidity changed significantly with time in the tributary of Xiangxi River,and the changes of TN and TP content showed the same trend;the NH₄-N has a significant spatial variation in the tributary.The Three Gorges Reservoir's main stream of the Three Gorges Reservoir and the two tributaries of Shennong Creek and Xiangxi River were eutrophication,suggested that the tributaries of the Xiangxi River and the main stream of the Three Gorges Reservoir were highly eutrophic.The eutrophication of the former was slightly higher than that of the latter.The tributary of Shennong Creek was the medium-rich eutrophic water body.2.The spatiotemporal distribution characteristics of the picoeukaryotic plankton community structure were analyzed from the levels of phylum and classes.The results showed that:During the survey period from February to December in 2015,a total of24 phylum,47 class and 104 genera of picoeukaryotic plankton were detected in the surface waters of the main stream in the Three Gorges Reservoir,Shennongxi and Xiangxi River.At the phylum level?relative abundance greater than 0.5%?,community composition had similar distribution pattern in space but differed significantly in time.The composition of the picoeukaryotic plankton community of main stream and tributary were mainly Cryptophyta,Chlorophyta and Diatom,and the temporal succession of the main groups of picoeukaryotic plankton was as followed:from Cryptophyta,Bacillariophyta,Ciliophora to Cryptophyta,Chlorophyta,Streptophyta and then to Cryptophyta,Bacillariophyta,Ciliophora.At the class level?the top 15 abundance rankings?,the relative proportions above 5%were:Chlorophyceae,Ventrifilosa,Myxogastrea,Chrysophyceae,Coscinodiscophyceae.Chlorophyceae was absolutely dominant,and the main groups of the tributary of the Xiangxi River were more abundant.3.Alpha diversity and its correlation with environmental factors showed that,in terms of time,there was a significant difference in the community diversity index except for the ACE index between December and February?P<0.05?.There was no significant difference between other indexes;there was no significant difference in the spatial diversity of the four diversity indices.Shannon index was negatively correlated with pH and dissolved oxygen,Richness index,Chao1 index and ACE index were negatively correlated with dissolved oxygen,Simpson index was positively correlated with pH and dissolved oxygen.4.The spatiotemporal variation of the picoeukaryotic plankton community in the main stream and two tributaries of the Three Gorges Reservoir showed that the distribution of the picoeukaryotic plankton community was different in time scale,and the samples in adjacent months have certain similarity in community composition,but no significant difference in spatial scale.5.Redundancy analysis?RDA?was used to study the correlation between the community structure and environmental parameters of the main stream and the Shennong Creek and Xiangxi River in Three Gorges Reservoir on a spatial scale.The results showed that the main groups of picoeukaryotic plankton and their environmental regulation factors were not exactly the same.The driving factor of the main group distrib ution of picoeukaryotic plankton in the main stream of the in Three Gorges Reservoir was NO₃-N.The environmental regulators of the main group of picoeukaryotic plankton in Shennong Creek were COD and WT,the environmental factors that had a significant impact on the main groups of picoeukaryotic plankton in Xiangxi River tributary were WT,Chl a,and SD.Comprehensive analysis of the spatiotemporal distribution of community structure suggested that WT was a key factor affecting the community structure of picoeukaryotic plankton.