The Nitrogen And Phosphorus Forms Distribution Characteristics And Ecological Risk Assessment In The Sediment-Water Interface Of Daye Lake

At present,the environmental management of urban lakes is facing an important problem for eutrophication of water bodies,and the root cause of lake eutrophication is the excessive input of nitrogen,phosphorus and other nutrients.In order to control the process of water eutrophication and mitigate the harm caused by eutrophication,this paper regarded Daye Lake in Huang Shi City as the research zone,and carried out the study of nitrogen and phosphorus forms at the sediment-water interface of Daye Lake by means of field investigation,experimental analysis and data analysis,and analyzed its distribution characteristics,and finally evaluates the pollution status of the sediment.In order to provide data support and scientific basis for the prevention and treatment of nitrogen and phosphorus pollution and eutrophication of water bodies in Daye Lake of the same type.The main conclusions are as follows:（1）Forms and change characteristics of nitrogen and phosphorus in the overlying water and interstitial water:water quality evaluation showed that total nitrogen（TN）and total phosphorus（TP）are the main influencing factors about water quality in the overlying water,so that we need to analyze the forms of N and P.The results showed that nitrate nitrogen（NO₃^--N）is the main form of N,and particulate phosphorus（PP）is the main form of P.The analysis of nitrogen and phosphorus concentrations in the overlying water showed that there were seasonal differences in the concentrations of all forms of nitrogen and phosphorus,but the seasonal patterns of nitrogen and phosphorus were not different,in which the contents of TN and NO₃^--N were higher in winter than in summer,while the contents of TP and its form were higher in summer than in winter.The analysis of nitrogen and phosphorus fugitive forms in interstitial water found that the main form of N in summer and winter was ammonia nitrogen（NH₃-N）,while the main form of P had seasonal differences,with orthophosphate（DIP）dominating in summer and organic phosphorus（DOP）in winter.Investigation on the migration and release of nitrogen and phosphorus in the interstitial water and the overlying water:The concentrations of TN and DTP（total dissolved phosphorus）in the interstitial water were 4.54 mg/L and 0.26 mg/L,which were 1.5 times and 4 times of the corresponding overlying water,so it can be seen that the TN and DTP in the interstitial water were higher than those in the overlying water."In order to further determine the source of nitrogen and phosphorus,it is necessary to investigate the distribution pattern and concentration of nitrogen and phosphorus in the sediment.（2）Sediment nitrogen fugacity and distribution:the annual average content of TN in sediment is 3491.2 mg/kg,which is in the upper reaches of domestic lakes;while the annual average content of TTN（transformable nitrogen）is lower,761.0 mg/kg.In terms of nitrogen fugacity,the four forms of TTN in winter are ranked as strong oxidant leached nitrogen（SOEF-N）>weak acid leached nitrogen（WAEF-N）>strong alkali leached nitrogen（SAEF-N）>ion exchange nitrogen（IEF-N）,and the forms of TTN were all dominated by SOEF-N.The spatial distribution of SOEF-N varied greatly in different seasons,with the highest spatial content of SOEF-N（919.0 mg/kg）in summer at the sampling site near Lake Leshan（site 11）and the highest value（779.7 mg/kg）in winter near the Daye Lake Bridge（site 1）.The sediment phosphorus fugacity and distribution:the annual average sediment TP content was 622.9 mg/kg,which was in the middle level of all lakes in China,while the annual average BAP（bioavailable phosphorus）content was in the middle level,253.6 mg/kg.from the viewpoint of phosphorus morphology,the four inorganic phosphorus forms accounted for the percentage of TP in summer in the following order:HCl-P>Na OH-P>BD-P>NH₄Cl-P,and the inorganic phosphorus in the sediment was dominated by HCl-P and Na OH-P.The spatial distribution patterns of HCl-P,Na OH-P and BAP were similar in summer,with the highest values occurring near the Wuli Lake fishery in the east side zone of the lake（site 19）.Correlation of sediment nitrogen and phosphorus forms:WAEF-N and SAEF-N were significantly and positively correlated with TP,NH₄Cl-P,BD-P and Na OH-P（P<0.01）,and there was no significant correlation with TN and SOEF-N,indicating that WAEF-N and SAEF-N were homologous with each form of phosphorus.Further analysis of TTN and BAP revealed that TTN had a strong linear relationship with SOEF-N,and BAP had an extremely strong linear relationship with Na OH-P.It was hypothesized that the main source of TTN was SOEF-N,and the main source of BAP was Na OH-P.Ecological risk evaluation of sediment nitrogen and phosphorus:The nitrogen and phosphorus pollution status of Daye Lake sediment was evaluated by the integrated pollution index method,and it was found that its integrated pollution index reached an average of 2.9throughout the year,which is heavy pollution.In view of the fact that bioavailable nitrogen and phosphorus are available nutrients for algae and phytoplankton in water bodies and have a greater impact on water body eutrophication,the bioeffective index method was used to study the pollution of bioavailable nitrogen and phosphorus in the sediments of Daye Lake.The results showed that the sediments of Daye Lake were mainly polluted by phosphorus,and its risk level was low and belonged to light pollution.In summary,this paper analyzed the nitrogen and phosphorus distribution at the sediment-water interface of Daye Lake,and obtained the content and spatial distribution characteristics of each form of nitrogen and phosphorus in each medium,and finally evaluated the pollution degree of Daye Lake sediment using two evaluation methods,providing a theoretical basis for the prevention and control of endogenous pollution and water eutrophication in Daye Lake,as well as a reference for the water ecology restoration of medium-sized shallow lakes in China.