Experimental Study On Ecological Flow Velocity At Ayu's Fattening Ground

The Nanxi River,a northern tributary in the lower reaches of the Oujiang River,is located in Yongjia County of Wenzhou City,Zhejiang Province.It is one of the major habitats of Ayu.During recent decades,due to anthropogenic activities,the habitat of Ayu has been destroyed,resulting in a drastic reduction in the Ayu population.In this thesis,the ecological survey on the Ayu's habitat in the Nanxi River is carried out;the relationship between the flow velocity and the shedding of benthic algae is studied in terms of habitat function,which will provide the reference for the protection of Ayu and the reservoir operation.The main research contents and conclusions of this thesis are as follows:1)Investigation and evaluation on the water quality of Ayu's habitat in the Nanxi River.The water quality of Ayu's habitat in the Nanxi River was studied by field investigation and analyzed with the comprehensive pollution index method.The results showed that the water quality in the spawning grounds during the spawning period and in the fattening grounds during the fattening period were relatively good,providing favorable water quality environment for spawning and fattening the Ayu.The water quality in the midstream spawning grounds and in the downstream tidal river reaches during the anadromous migration period were relatively bad,affecting the migration of Ayu from the lower river reaches to the fattening grounds.In addition,TN is a pollutant beyond the standard in the fattening grounds,spawning grounds,and downstream tidal river reaches at various times.2)Investigation on benthic algae in the Ayu's habitat in the Nanxi River.Diatoms are the major food sources of Ayu in the fattening and spawning periods,the growth condition of which can represent the Ayu's food environment during these periods.The results showed that diatoms are more dominant among algae in the Nanxi River;the densities of benthic algae at various sampling sites during the Ayu's fattening and spawning periods were quite different;the density of benthic algae at the sampling sites of the fattening ground during the fattening period was larger than the sampling sites of the spawning ground;the density of benthic algae at the sampling site of spawning ground during the spawning period was larger than the sampling sites of the fattening ground.The structure and temporal and spatial distribution of the benthic algae community in the Nanxi River are suitable for spawning and fattening Ayu.3)Methods for studying the ecological flow velocity in the Ayu's fattening ground.The aging and decay of benthic algae in rivers directly affect the growth of new benthic algae,and further,affect the fattening and growth of Ayu.Therefore,the methods for studying the impact of flow on the shedding of aging benthic algae on natural substrate through the experiments are proposed.A circulating water tank was produced,and pebbles with benthic algae were placed in the experimental section to analyze the relationship among flow velocity and its durations,and the benthic algae removal rate,so as to determine the impact of water flow on the shedding of benthic algae.In this method,the concentration of chlorophyll A in the test water samples was used as the benthic algae removal rate at various flow velocities and durations,and the species of the benthic algae in the test water samples were identified.4)Relationship among the flow velocity and its duration and the benthic algae removal rate.The experimental study was carried out to study the changes of benthic algae removal rate over time at flow velocities of 0.8 m/s,1.1 m/s,1.4 m/s,1.7 m/s,and 2.0 m/s.When the flushing time was the same,the removal rates of total benthic algae,blue and green algae and diatoms increased with the increase in the flow velocity.The removal rates of blue and green algae were significantly higher than the removal rates of diatoms,indicating that the diatoms have more erosion resistance.For the same flow velocity,the removal rate of total benthic algae and the removal rate of blue and green algae showed a slight increase with the flushing time,which was not significant.However,the removal rate of diatoms showed a tendency to decrease slightly with the flushing time.The increase in temperature of the test water and the death of diatoms were the main reasons.