Study On The Microbial Community In Sediment Of Backwater Area In Pengxi River And Its Correlation With River Internal Nutrient Release And Algal Growth

After its impounding in 2003,water environment of the Three Gorges Reservoir(TGR)has been significantly changed due to dramatically increased water level,lower flow velocity.Eutrophication has become increasingly prominent.Owing to longer retention time and accumulation of nutrient factors,the algal blooms has outbreaked particularly serious in the tributary backwater area.Eutrophication is resulted from excessive nutrients in water and there are two resoureces of nutrients: external source,includind the non-point source or point source pollution from river bank and the internal nutrient release from river bed.A lot of research has been done on external source in TGR,and very less has been on the internal nutrient source.Sediment performs as a source of endogenous nutrients.Microbes plays an important role in the process of nutrient cycling between water and sediments and also the growth of algae,formation of algal bloom.Therefore,a field long-term fixed-point monitoring was carried out from January 2016 to January 2017 in Gaoyang Lake of the Pengxi River,the largest tributary on the northern bank of the TGR.The seasonal variation characteristics of algal community structure,physicochemical properties of overlying water and sediments,and biological enzyme activities in sediments at different periods of algal blooms were analyzed.The correlation between microbial populations in sediments with internal nutrient cycling and algal blooms was preliminary discussed.This dissertation provided reference scientific basis for pollution control of eutrophication and algal bloom prevention in the tributary backwater area of Three Gorges Reservoir.Major works and conclusions are as follows.(1)The composition and succession of phytoplankton community in the backwater area of the Pengxi River were studied from March 2016 to February 2017.The results show that a total of 7 phyla,117 species(including varieties)of phytoplankton community in water samples were identified.The seasonal variation of algal community was significant(p<0.05).When algal bloom happened in spring,the algal community consisted of 5 phyla,32 species,and the main dominant algae wasChroococcus minor,Oscillatoria acuminate,Lyngbya limnelica,and Chlorella vulgaris.However,phytoplankton community increased to 7 phyla,82 species in autumn.Furthermore,the most dominant algae gradually alternated to Synedra sp.andChroomonas acuta.(2)During the monitoring period,the seasonal variation of algal biomass(Bio M)and Chla in Pengxi Backwater Area were consistent,while the trend of algal cell density(CellD)was slightly different.Chlorophyta was detected at each sampling point,but the cell density wasin small proportion.Bacillariophyta mainly appeared in autumn and winter.Numberof species of Cyanophyta and Cryptophyta were slightly less than Chlorophyta but their cell density was higher than the latter,the maximum values were 48.89×106 cell/L,1.61×106 cell/L,respectively,and became the dominant algae.The seasonal succession of the phyla was: Cyanophyta,Dinophyta?Chlorophta?Bacillariophyta,Cryptophyta.(3)During different stages of algae blooms from January 2016 to July 2016,variation of environmental factors of overlying water and sediment in the backwater area of Pengxi River were monitored.The pH value of overlying water increased significantly along with the blooms,and so were the dissolved oxygen(DO),total nitrogen(TN),total phosphorus(TP),dissolved total nitrogen(DTN),dissolved total phosphorus(DTP),nitrate nitrogen(NO3--N)and orthophosphate(PO43--P)before the bloom and the early stage of algal bloom.Dissolved phosphorus was the main form of phosphorus,and nitrification is one of the sources of nitrate nitrogen content in overlying water.On the contrary,the contents of total nitrogen,total phosphorus and nitrate in sediments decreased significantly during the formation of water bloom.Moreover,the contents of TN,TP and NO3--N in sediments were 401.9~515.8,5062.5~5675.7 and 58.5~89.2 times the corresponding nutrients in overlying water,respectively,which reflecting the important role of sediment as an internalsource of water nutrients.Nutrients were released from sediment to water before algal bloom.(4)In the different periods of algal bloom,phosphoruswas the limited element.Inorganic phosphorus(IP)was the major form of phosphorus in sediment,withconcentration ranged from 0.40 to 0.54 g/kg,accounting for about 66.7%~95.2% of total phosphorus in sediments.The concentration of different forms of Pfollowed the listed order: NaOH-P> HCl-P> BD-P> NH4Cl-P,leading NaOH-P to the main IP in sediments.HCl-P,NaOH-P content decreased before algal bloom,but increasedlater along with the development of algal bloom,while BD-P content continued to decline during March to May.Contents of TP,NaOH-P and NH4Cl-P in sediments were most significantly related with the nutritional factors of overlying water(p<0.01),which proved that sediments play an important role in maintaining water eutrophication.(5)Compared with January,the content of microbial PLFAs in sediments showed a tendency of increasing before the blooming(76.0% and 151.8%,respectively),and significantly reduced with the sustain of algal bloom.In addition,the abundance of individual bacteria in sediments was significantly increased,which resulted in significant changes in the composition and relative proportions of bacterial groups,and decreased diversity and uniformity of bacterial communities(respectively decreased by 49.6% and 24.0% in May).Alkaline phosphatase activity(APA)in sediments is significantly increased(p<0.05)in the formation period of bloom,and its activity is inhibited with the continuous outbreak of water bloom.(6)The results of the Redundancy Analysis(RDA)show that the community of phytoplankton was characterized by Cyanophyta and Chlorophyta during the early stage of water bloom,after the bloom.Chlorophta and Bacillariophyta were the dominant groups.Microbial PLFAs and TP in sediments were closed related.The content of APA,PLFAs and TP were the dominant factorsrelated withalgal bloom in the Backwater area of Pengxi River.Furthermore,the content of NH4Cl-P and BD-P had mainly affectedalgal growth in the early stage,and post stage of algal bloom,respectively.