Research On The Nitrogen And Phosphorus Composition And Sedimentary Characteristics Of The Water And Sediments Of Dianchi Lake

Dianchi Lake is the largest fresh lake in Yunnan-Guizhou Plateau,and eutrophication has become the biggest ecological and environmental problem.In recent years,with the development of economic and population in Dianchi Lake basin,the eutrophication of Dianchi Lake is very prominent and the outbreak of algal blooms is common.Excessive input of nitrogen and phosphorus in water is the main cause of eutrophication and also the most important limiting factor of algae growth.As the "source" and "sink" of nitrogen and phosphorus in lakes,sediments play an important role in the eutrophication and algae growth of Dianchi Lake,which is one of the keys to maintain the eutrophication of Dianchi Lake.In this paper,Dianchi Lake water body and sediments are taken as research objects.Water and sediment samples are collected and analyzed in laboratory.The composition of nitrogen and phosphorus in water and sediment of Dianchi Lake is studied,as well as their effects on eutrophication.The main findings are as follows:(1)Twenty-eight water samples were collected around Dianchi Lake.The content of nitrogen and phosphorus and other physical and chemical parameters including pH and DO were measured.The effects of nitrogen and phosphorus composition on eutrophication were analyzed.The concentrations of nitrogen and phosphorus in Dianchi Lake were high and the spatial variation was significant.The concentrations of TN and TP varied from 1.14 to 5.54 and from 0.19 to 0.7 mg/L,respectively,and decreased from the north to the south of Dianchi Lake.Nitrogen and phosphorus in water bodies are mainly in the form of particulate PN and PP,and are closely related to algae.The ratio of N/P in water decreased from the north to the south of Dianchi Lake,while the phosphorus in the north of Dianchi Lake was mainly restricted to algae.The nitrogen in the south mainly showed the nitrogen limitation to algae.The low N/P ratio and alkaline pH of the water body in the southern part of Dianchi Lake are suitable for the growth of cyanobacteria.With the southward movement of the algal bloom area,the risk of cyanobacteria blooms in southern Dianchi Lake will be further increased.(2)Seven sedimentary column samples from Dianchi Lake were collected to determine the forms of nitrogen and phosphorus in the sediments.The 210 Pb dating method was used to analyze the ages.The contents of nitrogen and phosphorus in sediments of Dianchi Lake are very high.The average contents of TN and TP are 1266.9-3242.7 and 1127.7-3526.1 mg/kg,respectively.TON is the main form of nitrogen and IP is the main form of phosphorus.The spatial distribution of nitrogen and phosphorus in sediments is not uniform,and the overall trend is that the southern part is larger than the northern part and larger than the middle part.With the economic development of Kunming city and the enhancement of industrial and agricultural production and human activities,the nitrogen and phosphorus content of sediments in Dianchi Lake has been increasing year by year since the 1970s.According to the quality standards of the sediment of EPA,sediments in Dianchi Lake have been in the serious nitrogen and phosphorus contamination.According to the assessment method of organic nitrogen pollution index,sediments of Dianchi Lake began to be in the level of organic nitrogen pollution from the 1960s to the 1970s,and the degree of pollution increased year by year.The correlation analysis between different forms of nitrogen and phosphorus in sediments and water shows that sediments in Dianchi Lake are mainly playing roles of 'nitrogen sinks' and 'phosphorus sinks' for overlying water bodies.Nitrogen and phosphorus pollution in water bodies will promote nitrogen and phosphorus pollution in sediments.(3)The deposition rate of sediments in Dianchi Lake was calculated by CRS model,and the nitrogen and phosphorus reserves of sediments from 1900 to 2014 were further estimated.Sediment deposition rate in the Dianchi Lake increased slowly before the 1990s.After that,due to the rapid economic development and rapid population growth,the sedimentation rate increased rapidly and the ensuing nitrogen and phosphorus deposition gradually accelerated,TN maximum deposition rate increased to 1.29 mg/cm2/a by 2014.Sediment is a huge reservoir of nitrogen and phosphorus for water body.From 1900 to 2014,the sedimentation reserves of TN and TP in Dianchi Lake were respectively 88562 and 81621 t,and the reserves in the middle of Dianchi Lake were the largest,followed by the south and the north is the smallest.(4)On the basis of estimating the buried reserves of nitrogen and phosphorus in sediment,the reserves of bioavailable nitrogen and phosphorus in the sediment were calculated.The reserves of bioavailable nitrogen were about 5847 t and the reserves of bioavailable phosphorus were about 50693 t.Bioavailable nitrogen reserves are the largest in the central part of the lake,followed by the southern part of the lake and lowest in the northern part.The bioavailable phosphorus reserves are the lowest in the southern part of the lake,followed by the northern part and the middle part with the highest reserves.The overall water environment in Dianchi Lake is alkaline,which is favorable for the release of bioavailable phosphorus in sediments.Although the reserves of available nitrogen and phosphorus in the northern part of Dianchi Lake are relatively low,the eutrophication in the northern part is more serious and bioturbated.The lower the oxygen content of the water,making the northern part of the sediment to be the most risky area for nitrogen and phosphorus release.