Dynamics And Influencing Factors Of Phytoplankton And Water Quality In Reservoirs Using As Drinking Water Source

Reservoirs are the centralized drinking water source of many large and medium cities,the safety of water quality is related to urban economic development and social stability.The high occurrence of harmful algal blooms,odorous compounds,excessive nitrogen and phosphorus and other water environmental problems in China's water source reservoirs,especially the occurrence of algal blooms is characterized by uncertainty and contingency,which brings challenges to the security of urban water supply.Understanding the changing characteristics and influencing factors of algae and related water quality indexes in reservoirs can provide scientific and technological support for water source security.Annual high frequency monitoring,summer bloom sensitive season survey and sediment trap observation was used in Xin'anjiang reservoir?lake qiandaohu?,the largest reservoir water source in east China,and 17reservoir water sources in Jiangsu province to describe the characteristics of annual changes of algae community structure in reservoir water source reservoirs,reveal the main factors affecting the concentration of odorous compounds in reservoirs in summer,and discuss the influence of hydrological processes such as rainstorm inflow on the precipitation of suspended solids in reservoirs and its water quality.The main findings are as follows:1.In the lake area of Xin'anjiang reservoir,a large water source reservoir,the community succession of algae was obvious.Diatoms appeared at their peak from April to June,while Cyanophyta and Chlorophyta appeared in July to August,Cryptophyta biomass was lower,and the peak appeared in October to November when other algae were less.Chlorophyta are distributed in the surface layer of the water,and Cyanophyta and Diatoms are mainly distributed in the subsurface layer?about 5?7 m?.The biomass of algae was significantly correlated with temperature?P<0.01?,besides,the optimum growth temperature of Cyanophyta was28?29?and that of Diatoms was 25?.We also found that,continuous extreme rainfall events in the short term might lead to the increase of total algae biomass in the middle?10-20 m?water body,the increase of Cyanophyta in the surface water body and the decrease of Chlorophyta,which indicates that Cyanophyta have better adaptability to high turbidity and low transparency water body than Chlorophyta.The accuracy of BBE was found to have a good correlation with the traditional method in the determination of total algae biomass?R²=0.57?,however,the accuracy of each divisions is poor,which is closely related to the limitation of the BBE algae determination method.2.The rainstorm events have obvious influence on the water quality of reservoir.At Xin'anjiang reservoir,the turbidity data of buoy has significant correlation with rainfall,inflow and SS,particularly for SS?P<0.01?,especially turbidity and SS ?R²=0.86?.There have obvious spatial difference of suspended solids between spring and summer when rainfall season?river area>transition area>lake area?.However,there was little difference of SS concentration between autumn and winter.There is a spatial trend of river area>transition area>lake area(with the rate of 27.82,4.34,and 0.26 g??m²?d?^-1,respectively),and time presents spring and summer>autumn and winter.The sedimentation flux of the whole lake is 2.57×10⁶ t combined with the investigation of the 4-season suspended solids at 60 points in whole lake,and the settlement flux in spring and summer is higher than that in autumn and winter.The contents of particulate nitrogen?PN?in JK,XJS,and DB were 6812,15886,and21986 mg·kg^-1,and the particulate phosphorus?PP?contents were 2545,3269,and 3077 mg·kg^-1,respectively.Statistical analysis shows that,there is a good exponential relationship between moderate rainfall and turbidity growth rate in the river area of the reservoir?R²=0.81?.Moreover,the continuous heavy rainfall affected turbidity in river area,but has little effect on the transition area.The concentration of SS has a good exponential decay with the distance from the river to the dam?R²=0.84?,especially in spring and summer.Researches shows that,the average annual deposition rate in Xin'anjiang Reservoir is 0.07%,it is lower than other large reservoirs in our country,however,there are certain risks in front of the dam because the nutrient sediment is high.3.The risk of odorous compounds is common in water source reservoirs at hilly and mountainous areas.The results show that,2-methylisoborneol?MIB?exceeded the standard in some water layers of one-third of the reservoir's,which average concentration is?13.7±20.7?ng?L^-1.Geosmin?GSM?was also detected in several reservoirs,but the concentrations were not exceeded the drinking water quality standard(maximum concentration was 4.6 ng?L^-1).With respect to the relationships between odorous compounds and environmental conditions,the significant correlation?P<0.05?between MIB concentration and eutrophication indicators,including chlorophyll-a,Secchi depth,suspended solids,comprehensive nutrition state index?TLI?,particularly for chlorophyll-a and TLI?P<0.01?.Above research description,we propose the following suggestions.?1?.The algal high frequency automatic monitoring technology can effectively monitor the succession law of algal community in reservoir and has a good application prospect.?2?.Managers should pay attention to the impact of extreme rainfall events on reservoirs and strengthen water and soil conservation in the basin.?3?.The potential nutrient internal release risk in the downstream are before the dam should be considered although the sedimentation of suspended solids is small.The results of this study can provide scientific support for the water quality safety assurance of the water source reservoirs.?4?.The nutrients reduction,improvement of vegetation coverage of the reservoir basin,reasonable fishery are considered as effective strategy to avoid the risk of the odorous compounds in reservoir.