Spectral Characteristics Of Dissolved Organic Matter In Urban Rivers

In order to reveal the temporal and spatial variation of dissolved organic matter(DOM)in urban rivers and explore its potential sources,this paper selected Huangpu River and its main tributaries in Shanghai as research objects.Through 6 batches of field sampling and laboratory experiments from 2018 to 2019,and based on the study of the hydrochemical characteristics and dissolved organic carbon(DOC)in the study area,a comprehensive study was conducted on the spatiotemporal characteristics,compositions and sources of DOM by the uses of ultraviolet-visible absorption spectrum and excitation-emission matrix spectroscopy.The result showed that:(1)The annual mean DOC of the main and tributaries of Huangpu River was4.53±0.68 mg·L-1 and 4.42±1.10 mg·L-1,respectively.The difference of DOC content between basins was small,and the middle reaches had higher DOC content,while the downstream and estuary sections had significantly lower DOC content due to the dilution effect of the Yangtze river.The seasonal variation of DOC content in the main and tributaries was synchronous,showing the highest in summer,followed by autumn and winter,and lowest in spring.The correlation analysis showed that DOC was closely related to the primary production process of the river and had the same source or change trend as the nitrogen and phosphorus nutrients;DOC was significantly negatively related to dissolved oxygen,mainly related to the growth and consumption of phytoplankton and the degradation process of DOM.Compared with other waters,the content of DOC in the Huangpu River was significantly higher than that of the Yangtze River,the Yellow River,and the Pearl River,and was equivalent to that of Taihu Lake and Dianshan Lake.(2)Absorption spectrum of chromophoric dissolved organic matter(CDOM)decreased from ultraviolet to visible.The annual mean values of CDOM absorption coefficients a(355)in the main streams and tributaries were 6.79±3.80 m-1 and6.92±4.43 m-1,respectively,indicating a relatively high CDOM concentration.Inspatial and temporal distribution,the main and tributary a(355)at the estuary was significantly higher than that in other basins,while the river source was the lowest.The concentration of CDOM in August was higher than in other months,while it was lower in May and December.The absorption coefficient ratio E2/E3 and the spectral slope S275-295 indicated that the proportion of fulvic acid in CDOM was relatively large,and the molecular weight of CDOM in estuary and downstream was obviously high.The specific absorption coefficient SUVA254 indicated that the aromaticity of CDOM continuously increased from source to the estuary.The molecular weight,humification and aromaticity of CDOM in August and March were higher throughout the year.Correlation analysis showed that the change in CDOM concentration was one of the causes of DOC fluctuations.The CDOM concentration in spring and summer was significantly related to chlorophyll a,indicating that the release of algae and the decay of higher plants in rivers in spring and summer made important contributions to CDOM.(3)Three fluorescence components were identified by PARAFAC model,including one tryptophan-like component,one tyrosine-like component and one humic-like component,namely C1(Ex/Em=220/350 nm),C2(Ex/Em=270/320 nm)and C3(Ex/Em=245,310/430 nm),of which the contribution of protein-like components was dominant.The protein like fluorescence in the middle reaches was higher than that in other watersheds,with significant autologous source characteristics,and it decreased significantly after entering downstream,while the humic-like fluorescence spatial difference was small.The high values of protein-like fluorescence were concentrated in October,December and May,while the humus-like fluorescence was concentrated in June and August,indicating that the humification characteristics of FDOM were significantly higher in summer.Fluorescence index(FI),biogenic index(BIX)and humification index(HIX)showed that the FDOM in Huangpu River and its main tributaries was mainly derived from endogenous sources.Correlation analysis showed that tryptophan-like and tyrosine-like component were co-originated,and the humus-like component was only well correlated with protein-like in May,June and August.The humus-like component to some extent controlled the variation of CDOM concentration and had more aromatic structures,while protein-like components existed in the water environment in the form of small molecular weights.