Phytoplankton Community Composition And Dynamics In Lake Zhelin And Their Effect On Cyanobacterial Blooms Of Lake Poyang

Lake Poyang is the largest freshwater lake in China with numerous inner lakes and outer water sources.Lake Zhelin is the largest reservoir along Lake Poyang watershed.The flowing water of Lake Zhelin into Lake Poyang leads to the impact on Lake Poyang to some extent.Previous studies mainly focused on the aquatic environment and ecosystem in the main lake area,and less research has been done on phytoplankton and their ecological effects of outer waters along the Lake Poyang.Meanwile,cyanobacterial blooms,dominated by Microcystis and Dolichospermum,frequently occur both in Lake Poyang and Lake Zhelin.Therefore,it is important to study characteristics of these two bloom forming genera and phytoplankton in both Zelin Lake and Lake Poyang,for exploring the ecological association between these two lakes.We investigated the phytoplankton community structure,migration,distribution,and molecularly detected and characterized the diversity and toxicity potential of cyanobacteria blooms in Lake Zhelin and the entrance area of Lake Poyang in 2019.The results are as follows:1.In 2019,Lake Zhelin was in a mesotrophic state,and the inlet of Lake Poyang was in a eutrophic state.The main group of phytoplankton in Lake Zhelin by cell density was cyanobacteria,and the dominant genera were Pseudoanabaena and Leptolyngbya.Dolichospermum spp.and Microcystis spp.were found throughout the lake.The average cell density of phytoplankton in four seasons varied from 8.38×10~5to 2.47×10~7 cells/L.The main groups of phytoplankton by biomass were Dinophyta and Bacillariohyta.The average biomass variation of phytoplankton in four seasons in2019 ranged from 0.66 to 7.72 mg/L.The main phytoplankton at entrance sites of Lake Poyang by cell density in summer 2019 were Cyanobacteria and Bacillariohyta.The dominant genera of cyanobacteria were Leptolyngbya.and Merimopedia,and each sampling site had Dolichospermum and Microcystis,and the cell density range was 5.22×10~5~3.76×10~7 cells/L.The main phytoplankton groups by biomass at the entrance of Lake Poyang were Chlorophyta and Bacillariohyta.The dominant species of cyanobacteria was Dolichospermum sp..Biomass variation ranged from 2.02 to25.77 mg/L.The phytoplankton community structure and the cell density and biomass in both Lake Zhelin and Lake Poyang changed during the process of water transport.2.Based on the Real-time PCR,gene copies of cyanobacteria 16S rRNA gene,Microcystis 16S rRNA gene,and toxic Microcystis in mcyE gene in Lake Zhelin showed a spatial-temporal difference.The abundance of toxin-producing Microcystis populations increased from the west to the east in the lake.The distribution characteristics of the population abundance of Microcystis and Toxin-producing Microcystis at the entrance of Lake Poyang in the upstream were higher than those in the downstream.Water transport and flow resulted in lower abundance of toxin-producing Microcystis and total Microcystis populations at the inlet of Lake Poyang,compared to the higher ones at tributary of River Xiuhe.Nutrients and temperature were shown as the main factors promoting production of microcystins.3.Based on Multilocus Sequence Typing(MLST),twenty strains of Microcystis isolated from the Lake Zhelin and Lake Poyang were used to examine their genetic diversity.It was found that the twenty strains all presented new sequence types(STs)and higher genetic diversity.The phylogenetic tree indicated that toxic and non-toxic Microcystis strains were separated away,and the sequence types of Microcystis at the entrance of Lake Poyang and Lake Zhelin are different,implicating that the source of Microcystis strain at the entrance of Lake Poyang may not come from Lake Zhelin.4.Twenty-four strains of Dolichospermum isolated from Lake Zhelin,were morphologically and molecularly characterized for exploring the biological properties of Dolichospermum species.They were classified into four species as D.planctonicum,D.affine,D.circinale and D.spiroides.Phylogenetic analysis based on the 16S rRNA gene sequences showed that all Dolichospermum strains here were highly similar to the strains isolated from Japan and other regions,but only D.affine was well grouped into one cluster.Based on the secondary structures of 16S-23S ITS,all the Dolichospermum strains showed to have the same structures in D1-D1'region.In Box-B and V3 regions,the structures of D.planctonicum and D.affine were different from those of D.circinale and D.spiroides,indicating the significant difference between two morphological types(coiled and straight type).Based on the DNA molecular detection,the 24 strains of Dolichospermum were not cyanotoxin producers,but four strains of D.planctonicum and one strain of D.circinale showed to contain geosmin synthase gene.Such results not only presented the basic scientific information on biodiversity of bloom forming cyanobacteria and their ecological risk in Lake Zhelin,but also provided the basis on aquatic ecosystematic studies and protection along Lake Poyang watershed.