Research On Seasonal Thermal Stratification Characteristics Of Source Water Reservoir And Algae Control

In recent years,the acceleration of eutrophication process in reservoir water body has led to periodic outbreak of harmful cyanobacteria,which has seriously affected the water ecological environment and water supply safety.In order to find out the thermal stratification characteristics of a water source reservoir in Shenzhen,the seasonal succession law of algae and the control mechanism of water-lifting aerator system on algae,a vertical monitoring section was set up in the deepest part of the main reservoir area.From May,2020 to April,2021,the algae and its growth environment factors at this monitoring section were monitored,and the algae composition,seasonal succession law and change law of growth environment influencing factors were studied,and the relationship between seasonal succession of algae and growth environment factors was found out.From July to October,2020,monitoring sections were set up in the old reservoir area and the new reservoir area,respectively,and the in-situ control effect of water pumping aeration enhanced biological water quality improvement system on nutrients and algae in the reservoir before and after operation was studied.The main results and conclusions of this paper are as follows:(1)The seasonal variation of environmental factors of algae growth is obvious,and the rainfall is mainly concentrated from April to September.The wind speed reaches the maximum in the mixing period(October 2020 ～ January 2021);During the stratification period(May-July,2020,February-April,2021),the vertical distribution of water temperature showed the characteristics of "high surface layer,low middle layer and low bottom layer".Influenced by water diversion,the water temperature in the middle layer gradually increased from August to September,and the vertical distribution of water temperature showed the characteristics of "high surface layer,high middle layer and low bottom layer".During the mixing period(2020 The depth of photic zone does not change much throughout the year,and the depth of mixed layer is affected by the temperature gradient,showing a smaller stratification period and a longer mixing period,which gradually increases after July 2020 and reaches the maximum after entering the mixing period.The concentration of dissolved total phosphorus is obviously affected by endogenous release and the incoming water of Dongjiang River after rainfall,which fluctuates greatly in the stratification period and tends to be stable in the mixing period.Nitrate concentration is mainly influenced by the water from Dongjiang River after the rain,which is higher in stratification period,fluctuates greatly,and tends to be stable in mixing period.(2)The seasonal succession law of algae is obvious,and 118 species of phytoplankton belonging to 71 genera,8 phyla were detected throughout the year.The relative abundance of functional group M composed of Microcystis sp.,functional group J composed of Scenedesum sp.,Tetra?dron sp.,etc.and functional group N composed of Cosmarium sp.was relatively high in May-September 2020 and April 2021,which was dominant.The relative abundance of functional group D,which is composed of Synedra acus,and functional group X1,which is composed of Cglorella sp.and Ankistrodesmus sp.,is relatively high from October 2020 to March next year,and it is dominant.The functional group P composed of Melosira granulata was dominant in August ～ October and December.The functional group B,composed of Cyclotella sp.,exists stably in water all year round.(3)After the operation of the water-lifting aerator system.1)through vertical mixing and bottom oxygenation,the anaerobic environment of the bottom layer of the reservoir was improved,and the release of iron,manganese and other pollutants from sediments to the overlying water body was inhibited;2)Water-lifting aerator system reduce the nitrogen and phosphorus nutrients in the input reservoir to a certain extent.During the operation period,the in-situ reduction of the total nitrogen system in the incoming water is 7.4 t and 7.3 t,respectively,accounting for 18% and 20% of the input in the current month,and the in-situ reduction of the total phosphorus system in the incoming water is 0.69 t and 0.82 t,respectively,accounting for 46% and 58% of the input in the current month;3)The algae density in the surface layer of water body rapidly decreased,and the algae density in the vertical direction of water body tended to be the same.In the action area,the dominant algae species Microcystis sp.and Cylindrospermopsis Komarek were replaced by Pseudanabaena sp.,while the dominant algae species Achnanthes exigua in diatom class were replaced by Melosira granulata and Synedra ulna,while dominant algae species Cosmarium laeve Rabenh and Scenedesum sp.appeared in chlorophyta.The results show that the water quality improvement system of water lifting aeration bioaugmentation can effectively control the intensification of eutrophication process in water body,adjust the algae population structure and effectively inhibit the growth and reproduction of cyanobacteria producing algal toxins.