Analysis Of The Spatiotemporal Distribution Of Chromophoric Dissolved Organic Matter (CDOM) And Its Composition And Sources In Daihai Lake

Chromophoric Dissolved Organic Matter（CDOM）is the main component of dissolved organic matter in water.The concentration and spatial distribution of CDOM play an important role in the global carbon cycle.In this paper,CDOM were studied in the surface,bottom layers of a typical closed lake in Inner Mongolia-Daihai and five rivers entering the lake through 5 field works in summer and autumn.The study monitored the general water quality.The UV-visible absorption spectroscopy and fluorescence excitation-emission matrix spectra combined with parallel factors analysis（EEMs-PARAFAC）technology were adopted to analyze the spatiotemporal distribution,the spectral characteristics of absorption and fluorescence,the composition and source of CDOM and the influence factor.The main results are summarized as follows:（1）There is similar monthly variation of water quality parameters between surface and bottom layer in lake.The concentration of Suspended Particular Matter（SPM）was lower in the center area of lake（4.0 mg/L<SPM<12.9 mg/L）,and higher near the edge of the shore（4.5<SPM<45.1 mg/L）due to runoff input and resuspension of sediments.In the vertical space,the concentration of SPM in bottom layer was slightly higher than that in surface layer.The spatial difference of other water quality parameters in lake area was not significant,and the difference between the surface bottom layer was also weak.The range of Chl-a is 3.5～15.6μg/L.In summer,the Chl-a in surface layer was higher than that in bottom layer,but this phenomenon was opposite in autumn.Water quality parameters of different rivers showed greatly spatiotemporal difference due to their own hydrogeological conditions and the complexity of the flowing environment.（2）The CDOM absorption coefficient spectrum curves of Daihai Lake and rivers showed an exponential decline with the increase of wavelength in the ultraviolet and visible light bands.The CDOM absorption coefficient of rivers was higher than lake.The CDOM between surface and bottom layers presented same spatial and monthly variation.CDOM concentration was August>October>June>July>September in lake,and it was higher in the west of lake,but lower in the north.The spectral curves of different rivers were significantly different,and CDOM concentration in rivers was August>October>July>September.CDOM concentration was higher in the western rivers,but it was lower in the northern and eastern rivers.The growth,metabolism,and degradation of phytoplankton led to higher CDOM concentrations in August and October.（3）This paper analyzed the source and composition of CDOM by calculating the absorption parameters.The difference of absorption parameters between the surface and bottom layers was small,and there was also small difference in the horizontal spatial distribution.Tha range of absorption parameters in lake was as follows:S_275-295>0.011 nm^-1,S_R>1,1.63 L·（mg·m）^-1<SUVA₂₅₄<2.07 L·（mg·m）^-1,which indicated that the less input of vascular plants,and the CDOM is more produced internally.The larger M value in lake indicated that the relative molecular of CDOM was smaller,and it contained more aromatic and aliphatic functional groups.CDOM absorption parameters of rivers were opposite to those of lakes,and the range was 0.015 nm^-1<S_275-295<0.049 nm^-1,S_R<1,the value of SUVA₂₅₄ was larger,and the value of M was smaller.CDOM absorption parameters indicated that the relative molecular of CDOM was larger,and the water had a higher degree of humification in rivers which was more closer to exogenous water.The correlation analysis between CDOM concentration and water quality parameters in lake was established to analyze the influences factors of CDOM.It was found that the CDOM showed significantly positive correlation with Chl-a and DO,but it was significantly negatively correlated with SPM and eutrophication indicators such as TN and TP.These water quality parameters can affect CDOM by intervening in the growth,metabolism and death degradation process of phytoplankton.（4）The fluorescent components were identified for the lake areas and rivers water samples using EEMs-PARAFAC model.The fluorescent components of CDOM in surface layer included three humic-like components C1（E_x=245/305 nm,E_m=385 nm）,C2（E_x=265/350 nm,E_m=445 nm）,C3（E_x=275/420 nm,E_m=515 nm）and one protein-like component C4（E_x=235/285 nm,E_m=335 nm）.The fluorescent components of CDOM in bottom layer included three humic-like components C1（E_x=245/305 nm,E_m=385 nm）,C2（E_x=260/345 nm,E_m=440 nm）,C3（E_x=275/415 nm,E_m=515 nm）and one protein-like component C4（E_x=235/285 nm,E_m=335 nm）.Rivers included four humic-like components C1（E_x=245/350 nm,E_m=460 nm）,C2（E_x=240/330 nm,E_m=400 nm）,C3（E_x=235 nm,E_m=450 nm）,C4（E_x=285/420 nm,E_m=515 nm）and one protein-like component C5（E_x=225/280 nm,E_m=335 nm）.The contribution rate of the self-generating components in the lake area was greater,while the external input from the river contributes more.（5）The results of the temporal and spatial distribution of CDOM fluorescence parameters including FI,BIX,HIX andβ:αshowed that CDOM in lake were mostly produced internally,while CDOM in rivers were mostly external input.The relationship between fluorescence components,fluorescence parameters and water quality parameters in lake were established by RDA analysis.The results showed that the fluorescence components and fluorescence parameters of CDOM in lake were mainly affected by Chl-a,DO and p H.It further indicated that most CDOM fluorescence components in lake were self-generating sources and were less affected by external sources.