Study On Eutrophication Characteristics And Interfacial Exchange Of Urban Shallow Lake

Xianghu Lake is located in the southwest corner of Nanchang City,Jiangxi Province.It is one of the important inner lakes in Nanchang City,and is also a typical shallow-water landscape lake.With the development of the surrounding area of the target lake in recent years,the eutrophication trend of the water body of Xianghu Lake has become increasingly obvious.Nitrogen and phosphorus are the main nutrient elements that lead to lake eutrophication.This article analyzes the eutrophication characteristics of the lake by studying the occurrence and spatial distribution of nitrogen and phosphorus in the water and sediments of Xianghu Lake.The pollution evaluation of the overlying water of the lake was carried out by the single factor evaluation method and the nutrient status index method,and the pollution status of the lake sediments was evaluated by the organic index method and the single factor index method.At the same time,by discussing the correlation between total nitrogen,total phosphorus and organic matter in the sediments,the influencing factors of the overlying water and sediment nutrients in Xianghu Lake were analyzed and compared,and the source of nutrients in Xianghu Lake was studied by combining the carbon to nitrogen ratio in the sediments.In addition,by studying the changes in the release kinetic characteristics and release potential of Xianghu sediments before and after the removal of organic matter,the release characteristics of nitrogen and phosphorus in Xianghu sediments to overlying water and the influence of organic matter on the release process are analyzed.The development of this research can provide a theoretical basis for the eutrophication control of urban shallow lakes.The main findings are:（1）The total nitrogen content in the water body of Xianghu Lake is between0.86mg·L^-1～2.65mg·L-1,the ammonia nitrogen content is between 0.15mg·L^-1～1.65mg·L^-1,and the nitrate nitrogen is Between 0.46mg·L^-1～0.92mg·L^-1.The spatial distribution characteristics of total nitrogen and ammonia nitrogen are basically similar,showing the highest content in the northwest and a trend of gradually decreasing toward the southeast;while the content of nitrate nitrogen is slightly higher in the wetland flower park,the overall difference in other sites is not significant.The total phosphorus content ranges from 0.12mg·L^-1to 0.23 mg·L^-1,and the highest content is located at the southern entrance of the lake.According to the single factor evaluation results,the total nitrogen in most of the waters of Xianghu Lake,the ammonia nitrogen in some waters,and the total phosphorus in the whole waters exceed the water quality standards of Class III.The evaluation result of the comprehensive pollution index method shows that the area around Xianghu Maya Paradise is at a serious pollution level,and the rest of the area is in a state of heavy pollution.The comprehensive nutritional status evaluation index shows that,except for the area near the Xianghu Tunnel,which is in a mesotrophic state,the rest of the area is in a mildly eutrophication state.（2）The total nitrogen content in the sediments of Xianghu Lake ranges from 1.27mg·g^-1to 3.32 mg·g^-1,with an average of 2.23 mg·g^-1.The content of ammonia nitrogen and nitrate nitrogen is low,the content of ammonia nitrogen is between 0.16～0.30mg·g^-1,the average is 0.23mg·g^-1,and the content of nitrate nitrogen is between 0.05mg·g-1～0.26mg·g^-1The average is 0.17mg·g^-1.Organic nitrogen is the main component,accounting for 71%to84%of the total nitrogen content,with an average content of 1.92 mg·g-1.The total phosphorus content in the sediments of Xianghu Lake is 0.36 mg·g^-1～1.66mg·g^-1,with an average of 0.89 mg·g^-1;the calcium and phosphorus content accounts for 9%～22%of the total phosphorus,with an average proportion of 15%;iron Aluminum phosphorus content accounts for 48%to 74%of total phosphorus,with an average proportion of 64%;the proportion of organic phosphorus in total phosphorus ranges from 12%to 32%,with an average proportion of 20%.According to the evaluation results of the comprehensive organic index method and the single factor index method,the sediments in the Maya Paradise area,the Wanshou Palace area,the Xianghu Wetland Park area,and the southern lake entrance area are in a state of organic pollution.At the same time,the total sediments in these areas are in a state of organic pollution.The phosphorus eutrophication status is also heavy pollution.（3）The distribution trend of nutrients in the overlying water and sediments of Xianghu Lake is similar.There is an exchange phenomenon between the sediments and nutrients in the overlying water in some areas,but the pollution of the overlying water in Xianghu Lake is mainly caused by external pollution.According to the person correlation analysis,the content of organic matter in the sediment was significantly positively correlated with the content of total nitrogen and total phosphorus（p<0.05）,indicating that the sources of total nitrogen and total phosphorus were basically similar,mainly aquatic plant residues.The C/N ratio of Xianghu Lake is between 4.97 and 16.8,and the C/N difference in each area is large,indicating that there are certain differences in the sources of organic matter in the sediments.（4）The release trend of ammonia nitrogen and soluble active phosphorus in the sediments is similar,the release rate increases rapidly and then gradually slows down,and finally the release amount reaches a certain maximum value and then stabilizes.The release potential is proportional to the release amount,and the release potential is also high in areas with a large release balance.The release amount and release rate of ammonia nitrogen in the sediment are proportional to the total nitrogen content in the sediment,and the release of soluble active phosphorus is related to the ecological environment of the area.The area where higher aquatic plants are dominant plants makes Xianghu Lake through biological action.The content of exchangeable phosphorus such as soluble active phosphorus in the sediment is reduced,and the phosphorus released to the overlying water is reduced.The content of organic matter in the surface sediments is positively correlated with the release of ammonia nitrogen（p<0.01）.The content of organic matter in Xianghu Lake can reflect the amount of ammonia released to a certain extent,while the phosphorus content released from the overlying water in Xianghu Lake sediments is affected by organic matter Limited,the release amount is mainly affected by iron,aluminum,and phosphorus.