Study On The Exchange Flux Of Nitrogen And Phosphorus And Influencing Factors At The Sediment-overlying Water In Daye Lake

With the rapid development of the economic construction of our country,a large amount of domestic sewage and production wastewater have been discharged into the lakes which cause the eutrophication of lakes to become more serious.Nutrient exchange between bottom sediment and water layer is an important link of nutrient cycling,which has important influence on the control of endogenous pollution in lakes.Daye Lake is the largest shallow lake in Huangshi City,and the nutrient status index of the lake was gradually increasing.Eutrophication has become a problem that can not be ignored in the water environment health of the Lake.By means of data collection,field investigation,laboratory simulation and analysis,the exchange fluxes of nitrogen and phosphorus between bottom sediment and water layer of Daye Lake were estimated,the release rules of exchange fluxes under re-suspension conditions were studied,simulation tests under different environmental factors were carried out,and the environmental factors affecting the release were analyzed.This study contributed to an in-depth understanding of the migration and release behavior of nitrogen and phosphorus between bottom sediment and water layer,and has important guiding significance for controlling the endogenous load of lakes and clarifying the contribution of eutrophication endogenous pollution.The main conclusions are as follows:（1）The simulation experiments under static conditions showed that there were some differences in the exchange fluxes between different points of Daye Lake in winter and summer.The exchange fluxes of total nitrogen（TN）,ammonia nitrogen（NH₄⁺-N）,total phosphorus（TP）and orthophosphate(PO₄^3--P)were larger in summer,and the decrease of temperature in winter was disadvantageous to the release of nutrients,so the exchange flux decreased.The potential contribution rates of interface exchange to nitrogen and phosphorus in water were estimated and found that the contribution rates of TN were-1.86～49.53%（winter）and 8.01～22.63%（summer）.The contribution rates of NH₄⁺-N were 4.24～80.02%（winter）and-1.66～-0.22%（winter）.The contribution rates of TP were-1.66～-0.22%（winter）and-6.32～11.36%（summer）.The contribution rate of PO₄^3--P were-9.71～-0.15%（winter）and 12.96～71.71%（summer）.According to the positive and negative contribution rates,TN and NH₄⁺-N played the role of"source"in winter,while TP and PO₄^3--P acted as"sink".In summer,TN,NH₄⁺-N,TP and PO₄^3--P all played the role of"source".（2）The resuspension flux experiment showed that nitrogen and phosphorus in the sediment were more easily released in a short time after disturbance.When it was 300 r/min,the highest release concentrations of TN,NH₄⁺-N,TP and PO₄^3--P within 12 hours were1.97mg/L,0.97 mg/L,0.038 mg/L and 0.035 mg/L respectively.With the increase of disturbance time,the concentration between sediment and overlying water gradually reached equilibrium.In the range of perturbation experiments（0～300 r/min）,the resuspension fluxes of TN,NH₄⁺-N,TP,and PO₄^3--P had a significant linear correlation with the disturbance rate,and R²were 0.9605,0.7766,0.9084 and 0.8822 respectively.The above results showed that the intensity and duration of external disturbance were important conditions affecting the release of nitrogen and phosphorus.（3）The experiment on the effect of environmental factors on the release of nitrogen and phosphorus showed that the temperature,dissolved oxygen and p H of overlying water in Daye Lake were significantly correlated with nitrogen and phosphorus release in sediments.1)At 30°C,the concentrations of TN,NH₄⁺-N,TP and PO₄^3--P were significantly higher than those at 10°C,and the exchange fluxes were 2.24 times,8.35 times,1.78 times and 1.72 times of those at 10°C respectively.2)Under aerobic condition,the concentrations of TN,NH₄⁺-N,TP and PO₄^3--P were lower than those under anaerobic condition,and the exchange flux increased with the decrease of dissolved oxygen concentration.It may be that anaerobic is beneficial to the transformation and decomposition of nitrogen and phosphorus,which promotes the release of nitrogen and phosphorus in sediments.3)With the increase of p H,the concentration of TN and NH₄⁺-N decreased gradually,while the release of TP and PO₄^3--P showed a"U"curve,that is,the exchange flux was the lowest in neutral condition,larger in both acidic and alkaline conditions,and slightly higher in alkaline condition than in acidic condition.This is due to the change of p H,which leads to changes in the adsorption of nitrogen and phosphorus by H⁺and OH^-.4)The optimal fitting functions of different nutrients and environmental aspects were different:the optimal fitting curve of TN was a power function,the optimal fitting curve of NH₄⁺-N and PO₄^3--P was a linear function,while the optimal fitting curve of TP was a log function.The best fitting curves of TN,NH₄⁺-N and PO₄^3--P with dissolved oxygen in sediments were logarithmic functions,and the best fitting curves of TP was a power functions.In the fitting with p H,the best fitting curve of TN was a power function,the best fitting curve of NH₄⁺-N was an exponential function,and the best fitting curve of TP and PO₄^3--P was a quadratic function.