Effects Of Reservoir Construction On Growth Characteristics Of Hydrilla Verticillata In Karst Area

Water resources engineering is necessary in karst areas because of the uneven distribution of water resources on spatial and/or temporal dimensions.But water conservancy projects are bound to affect the downstream water body.In order to probe the influence of water conservancy projects in karst areas,on-the-spot investigation and simulation experiment in laboratory were combined in this research.We performed an on-the-spot investigation on Guizhou Huaxi Reservoir Dam,a typical water conservancy project in karst area.Based on the investigation as reference,simulation experiments with Hydrilla verticillata were carried out in lab.The main results were as follows.（1）The downstream water body of Guizhou Huaxi Reservoir Dam was with the characteristics of karst water.Total N and P contents in the water was higher than those common river water.The major organic content in the bottom mud was the residues of submerged plants and alga.The organic pollution was level IV and showed ecology-toxicological effect.Hydrilla verticillata distributed widely and was of high ratio in the bottom mud organic content.（2）The simulated karst water environment promoted the growth of roots,stems and lateral branches of Hydrilla verticillata and the accumulation of biomass.Compared between simulated karst water environment and non-karst water environment in the same period,the length of stem of Hydrilla verticillata was 30.1%longer,and the maximum growth rate of stem length was 8.59 times higher;the fresh weight of Hydrilla verticillata increased by 43.5%,and the maximum growth rate was 2.54 times higher;the root length increased 130.0%,fresh root weight increased 127.7%and root weight ratio increased 65.1%;the length and weight of lateral branch increased by 114.3%and 35.9%,respectively.The average leaf area of Hydrilla verticillata in simulated karst water environment did not change much,but the total leaf area increased by 35.7%with the increase of apical leaf number.（3）The karst water environment promoted the ability of submerged plants to absorb nutrients from the water column.HCO₃^-decreased by 24.6%,Ca²⁺decreased by 24.2%compared with the initial data,the rate of phosphorus uptake by Hydrilla verticillata in the karst eutrophication group（TN=6 mg/L,TP=0.6 mg/L）was 0.122 mg/（g·d）,the rate of nitrogen uptake by the water column was 1.11 mg/（g·d）,the water body nitrogen and phosphorus nutrient removal efficiency is high can be promoted and applied to control the eutrophication of karst water bodies.Therefore,Hydrilla verticillata could be applied to control the eutrophication of karst water.（4）The change of hydrological conditions in the river after the construction of water conservancy facilities in the karst area has created a suitable environment for the growth of submerged plants and promoted the growth of Hydrilla verticillata,which in turn has promoted the absorption of nutrient salts in the karst water ecosystem by the submerged plant-based community.