Spatiotemporal Characteristics Of Riverine Chromophoric Dissolved Organic Matter(CDOM) Optical Characteristics In Liaohe River

Chromophoric dissolved organic matter(CDOM), which is an important part of dissolved organic matter(DOM), is considered to be the largest reservoir of dissolved organic carbon(DOC) in the aquatic environment. CDOM is greatly influenced by environmental factors such as land use; as a result it can provide important information of human activities acting on the surrounding landscape changes and optical properties. Therefore, this paper, focusing on CDOM spata-temporal characteristics, sources and composition of the rivers in Liaohe River Basin as well as the influences on CDOM of the surrounding environment elements, investigates the optical properties of CDOM in this particular region of the Liaohe River in order to provide support data for overall understanding of inland water bodies better and to provide an important reference for the development and improvement of bio-optical model. A comprehensive study of CDOM optical properties in the Liaohe River based on field sampling and laboratory analysis in 2012 and 2013 is carried out, and the main results are shown in the following aspects.1. Spatiotemporal characteristics of the Liaohe River CDOM absorption.Great variation was obtained between different sampling points. Great spatial differences of CDOM absorption are shown among the East and West Liaohe River watershed and the main stem of Liaohe River watershed. Obviously, greatly temporal characteristics of the Liaohe River CDOM absorption are acquired based on the comparison between January, April, May and September in 2013. CDOM absorption spectrum of ice in January is significantly lower than absorption spectrum of water samples in other months, while CDOM absorption spectrum of water in January are significantly higher than that in other months, indicating that the CDOM absorption in Liaohe River watershed showed obvious temporal characteristics. CDOM absorption contribution rate is low compared with other constituents in the water samples of Liaohe River based on the analysis of the absorption contribution rate of the optically active constituents of water. Meanwhile, obvious variations of CDOM absorption contribution are exhibited in different seasons, which are indicating seasonal changes of CDOM absorption contribution rate. In addition, strong correlations between CDOM and DOC are obtained both in spatial and temporal scales. The determination coefficient(R2) between CDOM and DOC is 0.74 for the spatial analysis. Moreover, R2 between the two parameters in the entire three watershed(the East and West Liaohe River watershed and the main stem of Liaohe River watershed) are higher than 0.5 while great variations of correlation are obtained in different months. The correlations between CDOM and DOC are strong both for the water and ice samples in January and water samples in May but weak correlations in April and September(R2 < 0.5).2. Spatiotemporal characteristics of the Liaohe River CDOM fluorescenceCDOM fluorescence characteristics also showed strong spatial and temporal characteristics in the Liaohe River. The CDOM fluorescence intensities and fluorescence peaks of sampling points vary with the sampling locations and showed great differences while correlations between CDOM fluorescence and absorption show great varieties in the spatial scale. The R2 between CDOM fluorescence and absorption in the three watersheds(the East and West Liaohe River watershed and the main stem of Liaohe River watershed) are 0.86, 0.92 and 0.77 respectively. Moreover, different fluorescence intensities are obtained in different months showing great temporal characteristics. CDOM fluorescence intensities of water samples in January is lower than other months while water samples in January exhibit the highest fluorescence intensity among all the months. Changes trends of fluorescence intensity in September are closer to January, while varying pattern of fluorescence intensity in April is not consistent with the other months. Meanwhile, better results are obtained between CDOM fluorescence and absorption in September(R2 = 0.98,P < 0.001) and May(R2 = 0.95,P < 0.001), following by water samples in January(R2 = 0.75,P < 0.001) and the correlation in April(R2 = 0.12,P < 0.01) is weaker than other month.3. Significant spatial and temporal characteristics for the composition and sources of CDOM.Little differences of the molecular weight of CDOM in different watersheds(the East and West Liaohe River watershed and the main stem of Liaohe River watershed) are identified based on the absorption slope(S) and SUVA254 both in the spatial and temporal aspects. Molecular weight of the composition of CDOM in the West Liaohe River watershed is little lower than that in the other watersheds while molecular weight of CDOM in the East Liaohe River watershed is little higher than the other two for the spatial analysis. Moreover, CDOM molecular weight in January and September is more likely to fulvic acid with low molecular weight while it is inclining to humic acid with high molecular weight in April and May. In addition, great variations of CDOM fluorescence peak are found in different watersheds. All the samples in the three watershed exhibit strong humic-like fluorescence peaks(Peak A and C) while samples in the main stem of Liaohe River watershed also show strong protein-like fluorescence peaks(Peak T280 and T230). In different months, samples exhibit different fluorescence peaks. Strong protein-like peaks(Peak T280 and T230) are obtained in January and May while protein-like peaks(Peak B and T230) and humic-like peaks(peak A and C) are obtained in April and strong humic-like peaks are identified in September. Samples collected near the Liaohe River estuary exhibit strong protein-like peaks in each month.4. Anaysis the factors affecting CDOM and DOC of Liaohe River and determine main factors.Environmental factors are analyzed in two scales; one of them is the three watersheds obtained from Songliao water resources commission of Ministry of water resources and the other one is obtained based on the SWAT model. In the three main watersheds, different water quality parameters bring different influences. Significant positive correlation is obtained between CDOM/DOC and total alkalinity in the main stem Liaohe River watershed while correlations between Chla, TSM and CDOM/DOC are negative. Meanwhile, significant positive correlations are obtained between TN, TP and CDOM/DOC in the West Liaohe River watershed. As the East Liaohe River watershed concerned, there are no correlations between water quality parameters and CDOM/DOC. While in the small watersheds extracted based on SWAT there is only a strong positive correlation between CDOM and total alkalinity. Land use types have great effects on DOC and CDOM. DOC concentration is the lowest in the paddy oriented fields subwatershed while the highest DOC concentration is found in the grass subwatershed. CDOM concentration in cropland oriented subwatershed is the highest in the different land use subwatershed, following the forest oriented subwatershed. Meanwhile, CDOM composition in the subwatershed dominated by wetland and cropland is more likely to fulvic acid compared with the composition of CDOM inclining to humic acid in the grass oriented subwatershed based on the analysis of CDOM absorption and fluorescence characteristic parameters. In addition, humic-like peaks are the highest in forest oriented subwatershed, following by cropland subwatershed which owns the highest protein-like fluorescence peaks. At the same time, different watersheds dominated by different soil type exhibit different DOC and CDOM concentrations. The highest DOC and CDOM concentrations are found in the subwatershed with chernozem and aeolian sandy soil are the main soil type compared with the lower DOC concentrations in the subwatersheds dominated by brown, paddy soil and brown/cinnamon soil and lower CDOM concentration in the subwatershed dominated by brown/cinnamon soil. Moreover, the highest fluorescence intensity and humic-like fluorescence peaks are obtained in the subwatershed with the main soil type chernozem and aeolian sandy soil while the subwatershed dominated by grass exhibit the highest protein-like peaks. The other environmental factors such as terrain data, meteorological data, and socio-economic data also have important impacts on the DOC and CDOM. Negative correlations are obtained between slope, aspect, the average annual temperature, annual rainfall, per unit area of population as well as GDP and DOC(CDOM) whiles no correlation between sunshine duration and DOC or CDOM. Analysis between response variables(DOC concentration, CDOM and Fn) and environmental factors based on RDA show that Chla concentration, slope, cinnamon, chernozem, paddy filed soil and bare land are the main factors having great influences on the response variables. Moreover, TN, slope, temperature, chernozem and paddy filed soil are the main environmental factor influenced on the CDOM fluorescence characteristics based on RDA. In addition, Chla, TN, TSM, rainfall, temperature, chernozem and bare land are the main factors acted on the absorption contribution rate of the optically active constituents of water samples.