Effects Of Water Velocity On Succession And Growth Of Phytoplankton Communities In The Main Canal Of The South-to-North Water Diversion Project

The main channel of the middle route of the South-to-North Water Diversion Project is a unique concrete hardening channel,which plays an important role in optimizing the allocation of water resources.Since its operation in 2014,biomass of phytoplankton community in water body has increased rapid in a specific period of time.The dominant position of Diatoms such as Needle sp.,Cyclotella sp.,and Fragile sp.was obvious,and cell density was greater than 10~7cells/L,which exposed the central trunk canal to the potential risk of phytoplankton blooms;the main reason minght be the influence of water velocity.In order to understand the effects of water velocity conditions on the succession and growth of phytoplankton communities in the canals of the middle route of the South-to-North Water Diversion Project,this study investigated the community succession and growth of the dominant diatom species Cyclotella meniscus under different water velocity conditions through in situ experiments and laboratory simulations.The main findings obtained were the follows:1.Field simulation experiment was carried out under different water velocity of the raw water in the main canal of the middle line of the South-to-North Water Diversion;four groups of water velocity(0 m/s,0.3 m/s,0.5 m/s and 0.7 m/s)were set up to investigate the effect of water velocity on the change of phytoplankton community in the main channel of the middle route of the South-to-North Water Diversion Project.The results showed that:phytoplankton species of the raw water treated at different water velocity changed significantly,cyanobacteria disappeared,diatoms became the dominant group,and the relative abundance of diatoms and green algae was the highest at the water velocity of 0.3?0.5 m/s;Shannon Weiner diversity index and evenness index were highest at 0.3 m/s(runway pool simulation device)and 0.5 m/s(pyramid simulation device),respectively.The results showed that water velocity had significant effect on phytoplankton community.2.Indoor microcosm simulation experiment further verified the effect of water velocity on algal growth.Based on water velocity during the operation of the central canal,five water velocity conditions(0 m/s,0.3 m/s,0.5 m/s,0.7 m/s,1.0 m/s)were set up to measure the growth,nutrient uptake and photosystem ?(PS?)activity of the dominant species Cyclotella meniscus.The results showed that water velocity showed an obvious"low-promoting and high-inhibiting"effect on the growth of Cyclotella meniscus.A moderate water velocity disturbance could enhance the absorption of phosphorus by cells and increase the activity of alkaline phosphatase.The utilization rate of phosphorus showed decreasing trend.When water velocity was greater than 0.5 m/s,growth was inhibited,and the 1.0 m/s water velocity group had the best inhibition effect on growth.High water velocity stress significantly affected the activity of cells.The maximum photochemical efficiency(Fv/Fm)value of PS? in the 0.3 m/s group was the highest,and its photosynthetic activity was the highest.The 1.0 m/s group had the strongest inhibitory effect on the photosynthetic activity of PS?.This study confirmed the effect of water velocity on the succession and growth of phytoplankton communities.The research results provided support for the use of hydraulic scheduling technology to regulate phytoplankton population dynamics,prevent and control abnormal phytoplankton proliferation,and have good practical application guidance value.In the future,it is necessary to further study the influence mechanism of water velocity on the succession of phytoplankton communities,and to explore the specific action pathways and methods of water velocity,to clarify the water velocity thresholds of different phytoplankton growth,and to find possible technical means for hydrodynamic scheduling to prevent and control phytoplankton blooms.