Controlling Eutrophication And Phytoplankton Blooms Via Water-lifting Aerations In A Stratified Reservoir: A Case Study Of Lijiahe Reservoir

The canyon-shaped stratified reservoir,as an important type of water source,has suffered from a lot of water quality problems due to the change of river hydrodynamic conditions by damming including that:（1）the decrease in flow rate and material exchange rate results in the accumulation of water pollutants,which intensifies the reservoir eutrophication;and（2）the increase in water depth and water retention time induces the thermal stratification and weakens the intensity of water convection,which can induce the algal blooms.Artificial mixing and oxygenation technology has been proved to successfully solve the water quality problem of the reservoirs.However,how to achieve low-cost water quality improvement including water pollution control and its extension needs to be solved.Lijiahe Reservoir（LJHR）,as a typical canyon-shaped stratified reservoir,is the one of the water-supply sources in Xi’an City,which faces the eutrophication and seasonal algal blooms.Here,based on the in situ field data before and after the installation of water-lifting aerators（WLAs）in the LJHR,this study explored the mechanism of nitrogen and phosphorus pollution and seasonal algal blooms under natural processes,and further revealed the effect,period and mechanism of algal control via WLA as well as the effect and mechanism of nutritive and algal control via induced natural mixing.The main conclusions were as follows:（1）The main sources and succession features of nitrogen and phosphorus were revealed.The results showed that the main sources of nitrogen and phosphorus pollution in LJHR were surface runoff,followed by sediment release.The total pollution loads of nitrogen and phosphorus input were 207.698 t/a and 6.189 t/a,respectively,among which the contribution of surface runoff was more than 98.2%.Besides,the contribution ratio of the rain runoff to the surface runoff was over 82.5%.Rain runoff occurred mostly in August to mid-October,but its pollution effect lasted until March of the next year.Their different intensities presented the different inflow processes and effects on reservoir hydrodynamic status and water quality.Especially,rain runoff above moderate intensity（Q>50 m³/s）had a greater impact on water quality in the main reservoir with the pollution levels of total nitrogen（TN）and total phosphorus（TP）lifted by 0.6 times and 4.5 times higher than these before,respectively,and the proportion of particulate pollution increased to 56.0%and 69.3%,respectively.Sediment release,covering from May to December,resulted in the increase of ammonium（NH₄⁺-N）and TP by 4.0 times and 5.5 times higher than these before.The further results indicated that the values of TN,TP,and TN/TP ration of 2.90 mg/L,0.06 mg/L,and 78,respectively.LJHR was an eutrophic phosphorus-limited water with perennial nitrogen pollution and periodic phosphorus pollution,which showed an interannual increasing pollution trend and the higher pollution levels during the periods of strom runoff and stratification as well as at the bottom during stratification.（2）The start-up mechanism and precaution model of algal bloom in a stratified reservoir were expounded.The results showed that algal blooms occurred in mid-spring,early summer and early fall during stratification in LJHR,whose biomass and structure differed in spring and summer.Spring algal blooms dominated by Chlorophyta,Bacillariophyta,and Dinoflagellate with the characteristic of low-temperature,small,and high surface to volume ratio（S/V）,but summer and fall algal blooms dominated by Chlorophyta,Bacillariophyta,and Cyanobacteria with the characteristic of high-temperature,large,and low S/V.The change of the relationship between the euphotic depth(Z_eu)and mixing depth(Z_mix)induced by stratification was the external cause of algal blooms,while the survival strategies of algae with different physiological and morphological characteristics were the external cause of the differences in the biomass and structure of seasonal algal blooms.In an eutrophic stratified water,seasonal algal blooms could occur when water temperature（WT）,Z_mix,and light availability(Z_eu/Z_mix)in spring were>9.4℃,<10.9 m,and>0.24,respectively,and WT,Z_mix,Z_eu/Z_mix,and TP in summer and fall were>16.0℃,<11.6m,>0.16,and>0.011 mg/L,respectively.Besides,the neural network fitting was applied to the prediction of Chlorophyll-a,total cell density,and cell density of different algal phyla using input parameters including WT,light,p H,and wate transparency,whose prediction precision ranged from 76.2%to98.7%.（3）The mechanism and environmental thresholds of algal bloom control via WLAs were verified.The results showed that the WLAs achieved the successful control of seasonal algal blooms at the different classification levels.the WLAs resulted in the reduction of algal cell density of 93.6-98.6%in spring and 98.2-99.9%in summer after the WLA operation of 10 to 31 days,and the shift of algal structure to Chlorophyta and Bacillariophyta（i.e.,low-temperature,small,and high S/V algae）.The relationship between the reduction rate of algae and operating period of WLAs confirmed that the control rate of 50%needed only 2.4 days to 6.3 days,and the control rate of 95%needed about 14 days,whose control efficiency was related to the algal size.The mechanism of algal control via the WLAs is to increase the Zmix to reduce the surface algal cell density,and increase the Zmix to reduce surface WT and Zmix inhibiting the surface algal growth.Meanwhile,the difference in the reduction efficiency of algae with different physiological and morphological characteristics caused the algal structural shift.The hydraulic conditions for seasonal algal blooms controlled by the WLAs were that the Z_mixwere>14.5 m in spring and>16.9 m in summer,respectively.（4）The theoretical condition triggering an induced natural mixing and its effect on water quality were explored.The results showed that the conditions for deactivating the WLAs in autumn to trigger an induced natural mixing are air temperature<the mixed surface WT.Induced natural mixing could advance and extend the three-month period of naturally complete mixing,with the increase in initial mixed WT from 7.3℃ to 20.2℃,and maintained a well-oxygenated and complete mixing status for 5.5 months.Induce natural mixing continued to achieve high-efficiency control of water quality pollution after WLA deactivation,with a decreasing trend of nutrients and the removal rates of TN,nitrate（NO₃^--N）,NH₄⁺-N,and TP of 3.6-11.8%,6.4-10.8%,50.0-71.4%,and 50.0-71.4%,respectively,as well as the removal rates of total cell density and chlorophyll-a content of 95.1%and 85.9%,respectively,than these in September to October before the installation of the WLAs.This suggested that higher initial temperature and higher dissolved oxygen state during induce natural mixing could hence the aerobic microbial activity to improve nutritive pollution and inhibit the anaerobic release from sediments,and also,the complete mixing status could inhibit the algal growth.