The Enviromental Effects Caused By Decomposition Of Potamogeton Crispus In Dongping Lake

Potamogeton crispus（P.crispus）,as the dominant species in Lake Dongping,will decay and decomposition in Late May or Early June.The decay and decomposition of P.crispus will change the physicochemical properties of water body and lead to the deterioration of water quality.Hence,by using a combination of the field observation and laboratory simulation and culture methods,the project studiedthe water quality during the growth of P.crispus,and explored the decay law and nutrient element release law during the decomposition of P.crispus.The impact of the decomposition of P.crispus on the water quality and physicochemical properties of sediments was clarified.Based on the change of greenhouse gas emission during the decomposition of P.crispus,the response mechanism of sediments was preliminarily analyzed.The results showed that:（1）The averagevalue of dry biomass and wetbiomass are 302.05 g·m^-2and 2.63 kg·m^-2.The distribution trend of P.crispusis was higher in lake center and lower in north of the lake.Dissolved oxygen concentrations were higher than the limit value of the class standard of national environmental quality for surface water（GB 3838-2002）,but NH₄⁺,TN and TP concentrations in 6%,76%and 42%of the samples exceeded the standard value respectively.The increase of nitrogen content in the lake can promote the growth of the P.crispus,and the growth of P.crispus favored the increase of dissolved oxygen and removal of phosphorus in the lake water.（2）The double exponential model could accurately describe the mass loss of P.crispus.The fast and slow decomposition rate were 0.081 d^-11 and 0.001 d^-11 respectively,about73.7%of the biomass（dry weight）was lost after 365 days.C and H in the P.crispus had a similar change,slow decline after small increase trend.But N had an opposite change.C,N and H were released to the overlying water during the decomposition of P.crispus,and the release rates of C and H were higher than N.（3）The concentrations of DO decreased dramatically and the concentrations of TP increased greatly,the concentrations of nitrogen had no significant change,the nitrate concentrations in the sediments also increased,and other indicators in the sediments had no significant difference.The concentration and fluxes of CH₄ and N₂O increased at the later stage of the decomposition.The water body mainly acted as the source of CO₂,CH₄ and correlation analysis shows temperature was the main factor to control the emission of greenhouse gases.Hence,before the temperature rises,P.crispus should harvested to reduce impact on the environment.（4）The results of incubation experiment showed that the pH and DO concentrations in the high biomass group decreased significantly,while the nitrogen and phosphorus concentrations were on the opposite.The content of organic matter and nitrate in the sediments increased while the ammonium content decreased.The concentrations,saturation ratios and fluxes of greenhouse gases in the high biomass group were higher than those in the low biomass group and the blank group.The water body in the high biomass group mainly acted as the source of CO₂ and CH₄ but sink of N₂O in the atmosphere.The indicators of water and sediments,the concentrations of greenhouse gases in the low biomass group and the blank group had no significant change,but in the low biomass group the concentrations of CO₂ and N₂O decreased,the concentrations of CH₄had no significant change.Hence the moderate P.crispus growth is proptious to reduce greenhouse gas production.The above research results systematically explained the environmental effects of the decomposition of P.crispus across the macrophyte-sediment-water-air interface,and preliminarily revealed the response mechanism of sediments,which provides important scientific basis for the prevention of eutrophication of Dongping Lake and the safety guarantee of water quality for the water transfer projects in this area.