Spatial And Temporal Distribution And Release Characteristics Of Dissolved Organic Nitrogen In Highland Lakes

The eutrophication of lakes is a serious ecological problem in lake management.As the largest natural plateau fresh water lake in Guizhou,Weining Caohai is a typical representative of Karst Plateau Lakes in China.However,with the development of human beings,its aquatic environment is deteriorating.Dissolved Organic Nitrogen(DON),as the most active component of sediment nitrogen,is an important nitrogen pool of nitrogen cycle in aquatic ecosystem and a nutrient source of aquatic organisms.Therefore,it is of great significance to study DON in lake sediments of Karst Plateau.In order to reveal the temporal and spatial distribution,source and release characteristics of DON in the surface sediments of Caohai,Guizhou Province,a typical karst plateau lake.The components and spatial-temporal distribution of DON in Caohai surface sediments were analyzed by UV-Vis and 3DEEM combined with PARAFAC model;the source analysis of fluorescence components from 3DEEM-PARAFAC model were analyzed by combining 3DEEM-PARAFAC and Principal Component Analysis(PCA);the release rate of DON and its release characteristics are studied by release simulation experiments,The main results are as follows:(1)The content of DON in Caohai surface sediments ranged from 0.77 mg/kg to91.17 mg/kg in wet season,with an average of 28.57 mg/kg;it ranged from 2.24 mg/kg to 146.17 mg/kg in dry season,with an average of 67.79 mg/kg.During the wet season,the DON content of Caohai surface sediments was higher in the southwest and east coastal inlet,northwest outlet and the center of the lake,and lower in the south and north coastal areas,while in the dry season,the average DON content in the southeast was higher than that in the northwest.The average DON content in the dry season(67.79 mg/kg)was higher than that in the wet season(28.57 mg/kg).Compared with other lakes in Yunnan Guizhou Plateau,the DON content in Caohai surface sediments is at a higher level.(2)3DEEM-PARAFAC model identified four fluorescent components from the DON of Caohai surface sediments in wet and dry seasons,which were C1(tyrosine like protein substance),C3(tryptophan like protein substance),C2 and C4(terrestrial humus like substance).The ratio of C1 and C3 to the total fluorescence intensity is almost equal to that of C2 and C4 in wet and dry seasons,which indicates that the terrigenous and endogenous effects of DON in Caohai surface sediments are almost the same.In wet and dry seasons,C1 and C3 are mainly distributed in the southwest,the center of the lake and the north of the center of the lake,C2 and C4 are mainly distributed in the southwest and East,indicating that the sediments near the river entrance are obviously affected by terrigenous input,and the analysis results obtained by 3DEEM-PARAFAC-pca are consistent with the above analysis.(3)The simulation results of DON release from Caohai surface sediments show that the release process of DON reaches the equilibrium state in 60 min,the main release period of DON is 0-15 min,and the maximum release rate occurs in 0-5 min;the results of two spectral indices of UV Vis and 3DEEM changed with the release of DON showed that DON with weak aromaticity,low degree of molecular polymerization and humification,obvious autotrophic characteristics and low relative molecular weight was more likely to be released into the water when it was disturbed;based on the3DEEM-PARAFAC model,it was found that endogenous small molecules such as tryptophan like and tyrosine like proteins were dominant in the 0-15 min period,which indicated that the control of endogenous small molecules such as tryptophan like and tyrosine like proteins should be considered in order to reduce the risk of DON release from Caohai surface sediments.