The Hydrodynamic Process And Its Ecological Effect In The Wujiang River-reservoir System

The sequence discontinuous sections formed by cascade dam interception have unique hydrodynamic characteristics.Phytoplankton is the main primary producer in water bodies and an important indicator of environmental change.Changes in hydrodynamic conditions?mainly flow velocity?influence phytoplankton growth.The cascade reservoir formed by dam construction is a typical sequence of discontinuous river sections,and an ideal place to study the effects of changes in hydrodynamic conditions on the growth of phytoplankton due to continuous changes in flow rate and phytoplankton community structure at different stages of evolution.In this study,the effects of hydrodynamic conditions on the growth of phytoplankton in the Wujiang cascade reservoir system are studied in combination with the hydrodynamic process and the environmental factors.The main conclusions are listed as the followings:1)Haimakong is the transition area from the river to the reservoir area of the Wujiangdu Reservoir.The distance from each monitoring point to the dam?s,km?is set as the abscissa and the flow velocity?v,m/s?is the ordinate.The longitudinal change of flow velocity in January satisfied the relationship:v=3×10^-5e^0.115s,R²=0.81.The longitudinal variation of velocity in April satisfied the relationship:v=6×10^-4e^0.065s,R²=0.41.The longitudinal variation of velocity in July satisfied the relationship:v=1×10^-5e^0.130s,R²=0.9.The longitudinal change of flow velocity in October satisfied the relationship:v=1×10^-7e^0.169s,R²=0.9.2)From the river to the reservoir,the reservoir water body has seasonal temperature stratification,the evaporation of the surface water of the reservoir is strengthened,resulting in the hydrogen and oxygen isotope signal of the surface water in the reservoir area is higher than that of the inflowing river.As a characterization,hydrogen and oxygen isotope factor can quantify the cumulative effect of cascade damming caused by reservoir retention time and average water depth variation.3)There is a certain impact of the change of flow velocity on the environmental factors.The water temperature decreases,the pH value decreases,the dissolved oxygen content decreases,and the total dissolved solids content increases with the flow velocity larger.The variation of cell abundance of phytoplankton with the flow velocity.The cell abundance increases first and then decreases as the flow rate increases.The relationship between flow velocity and cell abundance satisfies the Lorentz model?R²=0.69,p<0.01?,the optimum flow velocity of phytoplankton growth in the Wujiang River Basin is 0.047m/s.At different flow velocity the diversity of phytoplankton was also affected.As the flow velocity increased,the Shannon index increased first and then decreased.When the flow velocity was 0.898m/s,the Shannon index reached the maximum.At 0⁰.05m/s,cyanobacteria accounted for the largest proportion,reaching 55%,while diatoms accounted for 29%.As the flow velocity increases,the proportion of cyanobacteria continues to decrease,and the proportion of diatoms continues to rise.At 0.5-1.0 m/s,the proportion of diatoms has risen to 81%,and the proportion of other algae species lower than 10%.In this paper,we study the mathematical relationship model between flow rate and phytoplankton,predict the critical flow rate conditions of algae growth,and provide verification basis for mathematical relationship model.To provide a scientific basis for the development of correct and effective eutrophication prevention measures,technical measures for reservoir water environment safety and comprehensive development and utilization of water resources.