| Booting high-quality development of South-to-North Water Diversion Follow-up Project requires us to strengthen water pollution prevention and ecological environmental protection.Hydraulic regulation changes the hydrodynamic conditions,water quality conditions and water ecological conditions through water redistribution.As an important component of food web and a key evaluation indicator of the water ecosystem,zooplankton are of practical significance for exploring the ecological benefits of hydraulic regulation and clarifying the water environment conditions during water diversion.However,the feedback mechanism between zooplankton growth and the changing environment remains unclear.And the water ecological simulation based on zooplankton model needs to be further studied.In this study,Nansi Lake,one of the regulating lakes of the East Route of the South-to-North Water Diversion Project(SNWDP),was selected as the study area.We aims to investigate the impact of hydraulic regulation on ecological processes,especially on zooplankton,by applying the method of monitoring data sorting analysis,model calculation,etc.Firstly,based on the redundancy analysis method and the observed data of zooplankton,phytoplankton and conventional water quality indexes,we analyzed the species diversity of zooplankton and its correlation with water environment.Secondly,we applied an aquatic ecological model PCLake to simulate the zooplankton,Chl a,DO,nitrogen and phosphorus concentrations in the Downstream Nansi Lake during 2018 to 2019.The controlling equation of zooplankton model and the sensitivity of zooplankton parameters were used to calibrate and verify our model.Finally,model predictions were made for different hydrological year,different inflow and different water quality scenarios,which were intended to provide theoretical basis for the scheduling and operation management of Nansi Lake.The observation data demonstrated a seasonal pattern in zooplankton community of Nansi Lake,with the dominant species being branchiostomes(59%)in summer and autumn,and copepods(87%)in winter and spring.The redundancy analysis indicated that clear-water diversion intensified the direct intervention of nutrients and dissolved oxygen on the growth of zooplankton such as Copepoda and Cladocera.The modeled results showed that water diversion shortened the hydraulic residence time and reduced the nutrient level,triggering a shorten of the zooplankton growth cycle by 8-13 days and a reduction of the average dry weight level by 0.15-0.49 mg/L through the upstream control effect.When the water quality of the diverted river changed from Class III to Class I,the control of zooplankton biomass accumulation by water quality improvement began to show at 180 d ～ 210 d after water diversion.The hysteresis of zooplankton response to water quality was attributed to the fact that water renewal requires a certain period.The nitrogen and phosphorus loading to the Nansi Lake from the upstream rivers promoted zooplankton proliferation,and the promotion effect covered both the transfer and non-transfer periods.Especially in dry years,zooplankton biomass increased by 17.8%with the increase of nutrient concentrations(from Class III to Class V).In addition,it is crucial to pay more attention to the ecological problems caused by the operation of water diversion in special years,such as harmful algal blooms in the summer of wet years and the conservation of zooplankton diversity during the water transfer period in dry years. |
