Optical Absorption Characteristics, Spatial Distribution, and Source Analysis of Colored Dissolved Organic Matter in Wetland Water around Poyang Lake

Colored dissolved organic matter (CDOM) is an important part of aquatic ecosystems and plays a key role in the biogeochemical cycle. In this study, CDOM absorption spectrum curves and water quality parameters from 30 sampling sites in the wetlands of Poyang Lake, Jiangxi Province, China, were collected in October 2016. The optical absorption characteristics and spatial distribution of CDOM, the correlation between the absorption coefficient of CDOM at a wavelength of 355 nm (ag(355)), and the concentration of dissolved organic carbon (DOC) were analyzed. Spectral characteristic parameters—namely, E2/E3 (the ratio of the CDOM absorption coefficient at a wavelength of 250 nm to the CDOM absorption coefficient at a wavelength of 365 nm), SUVA254 (the ratio of the CDOM absorption coefficient at a wavelength of 254 nm to the DOC concentration), and spectral slopes—were used to infer the composition and sources of CDOM in the Poyang Lake wetlands. The results showed the following: (1) the CDOM absorption spectrum of water of the Poyang Lake wetlands presented significant spatial variation, showing a trend of south > west > north > east; (2) there was a strong linear correlation between the CDOM absorption coefficient and the DOC concentration in the water of the Poyang Lake wetlands (ag(355) = 1.075DOC–0.659 (r2 = 0.723, p < 0.001, n = 30)); (3) the analysis of the spectral characteristic parameters E2/E3, SUVA254, and spectral slopes showed that the CDOM in the Poyang Lake wetlands has relatively high aromaticity and molecular weight, which were shown to be mainly affected by terrestrial inputs. The results showed that the molecular weight and aromaticity of CDOM were higher in the south of the lake than in other parts.


Introduction
Colored dissolved organic matter (CDOM) is an assortment of organic matter composed of humic acid, fulvic acid, lipids, and polycyclic aromatic hydrocarbons dissolved in water [1]. This matter is an important part of the dissolved organic matter pool and dissolved organic carbon (DOC), and it plays a key role in the water environment, aquatic ecosystems, and the global carbon cycle [2,3]. The concentration and composition of CDOM can change the underwater light field, thereby affecting primary productivity [4,5]. Additionally, CDOM strongly absorbs ultraviolet light and can thereby protect aquatic ecosystems from harmful ultraviolet radiation [5,6]. The spectral absorption characteristics of CDOM are closely related to the concentration of DOC. The analysis of the absorption characteristics of CDOM helps us understand its source and the carbon-cycling process in aquatic ecosystems.
Due to the important influence of the ultraviolet-visible absorption characteristics of CDOM on water bio-optics, much research has been conducted on the optical characteristics, spatial and temporal distribution, and sources of CDOM in different regions. For example, such research has been extensively conducted in lake and wetland water [7][8][9][10][11]. Song et al. [7] studied the optical properties of CDOM in major lakes in China and found that the CDOM absorption is higher in saline lakes than in freshwater lakes. Zhang et al. [8] studied the seasonal changes and spatial distribution of optical properties of CDOM in Lake Taihu and found that these changes are related to river input and the degree of algae blooms. In recent years, the optical properties, spatial and temporal distribution, and sources of CDOM in wetland water have attracted research attention. Qi et al. [12] found that Phragmites australis and Suaeda salsa are important sources of CDOM in wetlands in the Yellow River Delta. Clark et al. [13] reported that the absorption coefficient of CDOM in a constructed wetland in Southern California decreased between the inlet and outlet of the wetland, and they additionally showed that wetland plants (pampas and reeds) produced CDOM during the decomposition process. Griffin et al. [14] found that rivers flowing through wetlands have higher CDOM concentrations. Maie et al. [15] conducted quantitative and qualitative research on the DOC of senescent plants in oligotrophic wetlands and found that polyphenols extracted from senescent plants are important sources of CDOM in the wetland environment. However, due to different sources, the characteristics of CDOM are also different in different aquatic ecosystems [16].
Poyang Lake, Jiangxi Province, China, is a seasonal lake with high dynamic changes in water level (Max of Hukou gauging station: 22.59 m on 31 July 1998, Min of Hukou gauging station: 5.9 m on 6 February 1963) [16]. Affected by the subtropical monsoon climate, the seasonal variations of water levels present a unique annual changing landscape of lake water and wetlands [17]. Poyang Lake is a unique wetland ecosystem with complex climatic and hydrological conditions [17,18]. Wetland ecosystems are transitional ecosystems between terrestrial and aquatic ecosystems. The wetlands of Poyang Lake not only have important social and economic functions but also play a very important role in maintaining the health of the ecosystem of the lake. However, the Poyang Lake wetlands are affected by human activities such as industrial and agricultural production and the lives of surrounding residents, which has caused its water quality to continuously decline and its degree of eutrophication to intensify [19,20]. CDOM, one of the important parameters that affects the composition of wetland water, can be used as an indicator of the pollution and eutrophication status of wetland water environments. Therefore, in this study, CDOM absorption spectrum curves and water quality parameters were collected at 30 sampling sites in the Poyang Lake wetlands in October 2016. The optical absorption characteristics and spatial distribution of CDOM, and the correlation between CDOM and DOC were analyzed. Spectral characteristic parameters were used to infer the composition and sources of CDOM in the Poyang Lake wetlands. The results of this study are expected to provide basic data and a scientific basis for the protection of the water ecological environment of the Poyang Lake wetlands.

Study Area and Sampling Site
Poyang Lake, the largest freshwater lake in China [21], is located in the north of Jiangxi Province on the south bank of the Yangtze River [22] (Figure 1). Poyang Lake receives water from five major rivers (Jiangxi, Ganjiang, Fuhe, Xinjiang, Raohe, and Xiushui) and exchanges water with the Yangtze River. Poyang Lake has a subtropical monsoon climate with an average annual precipitation of 1680 mm [23]. The seasonal variations in water level of Poyang Lake present a changing landscape of wetlands and lake water throughout a year [24]. The lake is divided into many disconnected parts separated by the exposed floodplains with an inundation area less than 1000 km 2 during the dry season between October and March. During the wet season between April and September, almost all of the disconnected parts of the lake are connected to form an inundated area of >3000 km 2 [25]. Additionally, Poyang Lake is one of the world's most famous wetlands, hosting millions of migratory birds, including about 98% of the world's population of Siberian cranes [26], which winter in the lake area [27]. The northern and northeastern parts of the Poyang Lake area are mostly hilly and mountainous, while the western and southern parts are mostly estuary delta plains. Historically, the estuary delta of Poyang Lake underwent frequent reclamation activities, which have resulted in the widespread construction of arable land and repeated human activities.
Water 2021, 13, x FOR PEER REVIEW 3 of 15 lake water throughout a year [24]. The lake is divided into many disconnected parts separated by the exposed floodplains with an inundation area less than 1000 km 2 during the dry season between October and March. During the wet season between April and September, almost all of the disconnected parts of the lake are connected to form an inundated area of 3000 km 2 [25]. Additionally, Poyang Lake is one of the world's most famous wetlands, hosting millions of migratory birds, including about 98% of the world's population of Siberian cranes [26], which winter in the lake area [27]. The northern and northeastern parts of the Poyang Lake area are mostly hilly and mountainous, while the western and southern parts are mostly estuary delta plains. Historically, the estuary delta of Poyang Lake underwent frequent reclamation activities, which have resulted in the widespread construction of arable land and repeated human activities. From September to October each year, the water level of Poyang Lake gradually decreases, and the wetland landscape appears; therefore, the sampling time selected in From September to October each year, the water level of Poyang Lake gradually decreases, and the wetland landscape appears; therefore, the sampling time selected in this study (October 2016) corresponds to the exposure time of the Poyang Lake wetland landscape. The number of sampling sites (30) was selected considering factors such as the representativeness and spatial balance of sampling sites [28] (Figure 1). To better analyze the spatial variation of CDOM in different areas of the Poyang Lake wetlands, Water 2021, 13, 274 4 of 14 the 30 sampling points were divided into four groups-nine samples in the east, eight samples in the south, eight samples in the west, and five samples in the north-as shown in Figure 1.
At each sampling site, surface water was collected from depths between 0 and 50 cm in a 2500 mL cleaned plastic bottle. All water samples were placed in a cooler equipped with ice packs and were brought back to the laboratory on the same day for the determination of absorption coefficients and the concentrations of water constituents. The positions of the sample points were determined using a handheld GPS (GPS map 62sc, Garmin, Olathe, KS, USA). At each sampling site, a portable water quality monitor (YSI 6600, Yellow Springs Instruments, Yellow Springs, OH, USA) was used to monitor water temperature, pH, turbidity, electrical conductivity, and total dissolved solids. The transparency of the wetland water was measured at each sampling point using a Secchi disk, and water depth was simultaneously measured with a depth sounder (SM-5, Speedtech Instruments, Great Falls, VA, USA).

Water Quality Analysis
Water samples for DOC measurement were filtered under low vacuum through pre-combusted (pre-fired at 450 • C for four hours) glass fiber filters (GF/F, Whatman, Buckinghamshire, United Kingdom) (0.7 µm) and stored in 60 mL opaque glass bottles at 4 • C in the dark until analysis [29]. DOC concentrations were determined by a hightemperature catalytic oxidation method using a total organic carbon analyzer (TOC L-CPN, Shimadzu, Kyoto, Japan). Chlorophyll a (Chl-a), collected on Whatman GF/F filters (0.7 µm) and extracted with hot 90% ethanol [30], was measured using a UV-vis spectrophotometer (UV-2600PC, Shimadzu, Kyoto, Japan).

CDOM Absorption Analysis
The water samples were filtered using a Whatman GF/F filter (0.7 µm) membrane before being filtered again with a Millipore membrane with a pore size of 0.22 µm; then, they were placed in a refrigerator at 4 • C until analysis. The absorption coefficient of CDOM was measured in a 10 cm quartz cuvette between 240 and 750 nm (1 nm interval) using a Shimadzu UV-2600PC UV-vis with ultrapure water (from a Millipore Direct-Q 5 system) as a blank. The absorption coefficient of CDOM was obtained using the following Formulas (1) and (2): a' g (λ) = 2.303·D(λ)/r (1) a g (λ) = a' g (λ) − a' g (700)·λ/700 (2) where a' g (λ) and a g (λ) represent the uncorrected CDOM absorption coefficient and the modified CDOM absorption coefficient that has been corrected for scattering, both for wavelength λ; D(λ) refers to the optical density at wavelength λ; and r is the cuvette light path length in m (0.1 m in this study).
As the CDOM composition is difficult to measure directly due to its complex composition, in previous studies, a wavelength of 355 or 400 nm was generally selected to characterize the CDOM concentration [31,32]. In this study, a g (355) was used to characterize the CDOM content.
The ratio (E 2 /E 3 ) of the absorption coefficient of CDOM at a wavelength of 250 nm (a g (250)) and that at 365 nm (a g (365)) were used to characterize the molecular weight of CDOM [33]. The E 2 /E 3 was expressed as follows: The specific absorption coefficient (SUVA 254 ) of CDOM in the 254 nm band was used to characterize the light absorption capacity of CDOM per unit DOC concentration and was obtained by the following Formula (4) [34]: Water 2021, 13, 274

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The spectral slope of CDOM was calculated using a single exponential function according to the non-linear regression of Formula (5): where λ 0 is a reference wavelength at 440 nm, and S g refers to the fitted parameter for the exponential decay of a g (λ) with increasing accuracy. In this study, the spectral slopes (S 275-295 , S 350-400 , and S 400-600 ) between wavelengths of 275-295 nm, 350-400 nm and 400-600 nm were analyzed using the OriginPro 8.0 software (OriginLab Corp, Northampton, MA, USA). The spectral slope ratio S R was defined as the ratio of the spectral slope of the shorter wavelength range (275-295 nm) to that of the longer wavelength range (350-400 nm) and was calculated as the following [35]: The spectral characteristic parameters of E 2 /E 3 , SUVA 254 , and spectral slopes are important parameters to characterize the absorption spectrum of CDOM and are related to the molecular composition and sources of CDOM [36][37][38].

Data Analysis
A diagram showing the spatial distribution of CDOM was made using the ArcGIS 10.5 software (ESRI, Redlands, CA, USA). The OriginPro 8.0 software (OriginLab Corp, Northampton, MA, USA) was used for the fitting analysis and plot. The statistical analysis of the parametric data was performed with the SPSS version 19 software (IBM Corp, Chicago, IL, USA). The Kruskal-Wallis test was applied to check the significance of the differences in the parameters of different groups (eastern, southern, western, and northern groups). Linear regression analysis was used to check the relationship between the different parameters. The Pearson method was used to determine the correlation coefficient. For all tests, the results were considered to be insignificant when the p value was greater than 0.05, and they were significant when the p value was less than 0.05.

Water Quality Parameters of the Poyang Lake Wetlands
The water quality parameters of the Poyang Lake wetlands (including DOC concentration, CDOM absorption coefficient, pH, dissolved oxygen (DO), water temperature (WT), electrical conductivity (EC), chlorophyll a (Chl-a), turbidity (Tur), water transparency (SD), and total dissolved solids (TDS)) are presented in Table 1. The DOC concentration of the samples ranged from 1.79 to 7.22 mg/L, with a mean value of 3.85 ± 1.41 mg/L; the a g (355) values of the samples ranged from 0.91 to 7.97 m −1 , with a mean value of 3.48 ± 1.75 m −1 . A comparison among different regions showed that the mean value of a g (355) was the highest in the south of the lake (5.09 ± 1.68 m −1 ), with values ranging from 2.91 to 7.97 m −1 , and it was the lowest in the east of the lake (2.08 ± 0.65 m −1 ). The pH of the samples ranged from 7.15 to 8.04, with a mean value of 7.68 ± 0.24. The DO of the samples ranged from 4.75 to 10.5 mg/L, with a mean value of 8.61 ± 1.58 mg/L. The WT of the samples ranged from 14.64 to 28.85 • C, with a mean value of 19.84 ± 3.04 • C. The EC of the samples ranged from 4 to 341 µS/cm, with a mean value of 125.10 ± 73.51 µS/cm. The Chl-a concentration ranged from 2.2 to 28.8 µg/L, with a mean value of 10.94 ± 7.54 µg/L. It was found that the mean concentration of Chl-a was the highest in the western part of the lake (16.33 ± 9.31 µg/L), and the lowest value of mean Chl-a (8.24 ± 6.5 µg/L) was found in the east. The Tur of the samples ranged from 3.7 to 824.4 NTU (Nephelometric Turbidity Units), with a mean value of 95.24 ± 158.67 NTU. The SD of the samples ranged from 2 to 45 cm, with a mean value of 16.35 ± 11.44 cm. The TDS of the samples ranged from 0.003 to 0.249 g/L, with a mean value of 0.090 ± 0.051 g/L. Note: DOC-dissolved organic carbon; a g (355)-absorption coefficient of colored dissolved organic matter (CDOM) at 355 nm; DOdissolved oxygen; T-water temperature; EC-electrical conductivity; Chl-a-chlorophyll a; Tur-turbidity; SD-water transparency; TDS-total dissolved solid; n denotes the number of samples. Figure 2 shows the CDOM absorption spectra of the samples. The CDOM spectra curves decreased exponentially with increasing wavelength. The absorption of CDOM was particularly strong in the ultraviolet and blue range, whereas the absorption was almost zero in the red range. As shown in Figure 2, the absorption spectra are quite different in different parts of Poyang Lake, showing a trend of south > west > north > east. According to the CDOM absorption spectra, the CDOM absorption coefficients were calculated to obtain the characteristic indexes (Figure 3). The values of E2/E3 ranged  According to the CDOM absorption spectra, the CDOM absorption coefficients were calculated to obtain the characteristic indexes ( Figure 3). The values of E 2 /E 3 ranged from 4.17 to 8.55, with a mean of 6.01 ± 0.91; the SUVA 254 values ranged from 2.23 to 5.30 L mg −1 m −1 , with a mean of 4.22 ± 0.80 L mg −1 m −1 ; the S 275-295 values ranged from 0.0152 to 0.0194 nm −1 , with a mean of 0.0170 ± 0.0009 nm −1 ; and the S R values ranged from 0.940 to 1.108, with a mean of 0.994 ± 0.039. The results show that the average values of E 2 /E 3 and S R were lower in the west and south than in the east and north, while the average value of SUVA 254 was higher in the west and south than in the east and north.

Spatial Distribution Characteristics of DOC and CDOM in Wetland Water of Poyang Lake
The spatial distribution of the DOC and CDOM of the samples is shown in Figure 4. As can be seen in the figure, the low values of the DOC concentration and CDOM absorption coefficient were found in the east and north of Poyang Lake, while the high values were observed in the southwest of the lake. High values in the west of the lake were concentrated in the Poyang Lake Nature Reserve, which is centered on Wucheng Town, Yongxiu County, while high values in the south were concentrated in the Nanji Wetland Nature Reserve. Water 2021, 13, x FOR PEER REVIEW 9 of 15         Through the Kruskal-Wallis test of the significance of the difference, it was found that the CDOM absorption coefficients (a g (355)) in the east of the lake were significantly different from those in the south (p = 0.003) and the west (p = 0.028).

The Relationship between CDOM and DOC in Wetland Water of Poyang Lake
The DOC concentrations of the samples ranged from 1.79 to 7.22 mg/L, with a mean value of 3.85 ± 1.41 mg/L ( Table 1). The DOC concentrations were much lower than those reported for Lake Taihu (6.00-10.9 mg/L, average 8.17 ± 1.38 mg/L), which is a eutrophic lake in Jiangsu Province, China [4]. In the present study, the spatial distribution of the DOC concentration was found to be similar to that of the CDOM absorption coefficient a g (355) (Figure 4), with high values appearing in the southwest of the lake. The present study found a significant positive correlation between the CDOM absorption coefficient a g (355) and DOC concentration in wetland waters of Poyang Lake, and the correlation formula is a g (355) = 1.075DOC-0.659 (r 2 = 0.723, p < 0.001, n = 30) ( Figure 6). Through the Kruskal-Wallis test of the significance of the difference, it was found that the CDOM absorption coefficients (ag(355)) in the east of the lake were significantly different from those in the south (p = 0.003) and the west (p = 0.028).

The Relationship between CDOM and DOC in Wetland Water of Poyang Lake
The DOC concentrations of the samples ranged from 1.79 to 7.22 mg/L, with a mean value of 3.85 ± 1.41 mg/L ( Table 1). The DOC concentrations were much lower than those reported for Lake Taihu (6.00-10.9 mg/L, average 8.17 ± 1.38 mg/L), which is a eutrophic lake in Jiangsu Province, China [4]. In the present study, the spatial distribution of the DOC concentration was found to be similar to that of the CDOM absorption coefficient ag(355) (Figure 4), with high values appearing in the southwest of the lake. The present study found a significant positive correlation between the CDOM absorption coefficient ag(355) and DOC concentration in wetland waters of Poyang Lake, and the correlation formula is ag(355) = 1.075DOC-0.659 (r 2 = 0.723, p < 0.001, n = 30) ( Figure 6).

CDOM as a Proxy for DOC in Wetland Water around Poyang Lake
CDOM is an important component of DOC that is mainly composed of humic acid and fulvic acid and has strong absorption in the ultraviolet-visible band. Many studies have shown that the CDOM absorption coefficient is significantly positively correlated with DOC concentration [39]. Thus, the absorption coefficient of CDOM can track changes in DOC concentration [40]. For example, Del Vecchio and Blough [31] reported strong correlations (r 2 ranging from 0.501 to 0.904) between ag(355) and DOC for waters from the Mid-Atlantic Bight across different seasons. Additionally, a previous study observed a significant linear relationship (ag(355) = 0.01DOC-0.39 (r 2 = 0.59, p < 0.001)) between ag(355) and DOC in water from Chesapeake Bay [41]. Furthermore, another study found strong positive relationships between DOC and CDOM absorption at a range of wavelengths (254 nm, 350 nm, 440 nm) in the majority of rivers in 30 selected watersheds in the United States [35]. A significant relationship was also reported between

CDOM as a Proxy for DOC in Wetland Water around Poyang Lake
CDOM is an important component of DOC that is mainly composed of humic acid and fulvic acid and has strong absorption in the ultraviolet-visible band. Many studies have shown that the CDOM absorption coefficient is significantly positively correlated with DOC concentration [39]. Thus, the absorption coefficient of CDOM can track changes in DOC concentration [40]. For example, Del Vecchio and Blough [31] reported strong correlations (r 2 ranging from 0.501 to 0.904) between a g (355) and DOC for waters from the Mid-Atlantic Bight across different seasons. Additionally, a previous study observed a significant linear relationship (a g (355) = 0.01DOC-0.39 (r 2 = 0.59, p < 0.001)) between a g (355) and DOC in water from Chesapeake Bay [41]. Furthermore, another study found strong positive relationships between DOC and CDOM absorption at a range of wavelengths (254 nm, 350 nm, 440 nm) in the majority of rivers in 30 selected watersheds in the United States [35]. A significant relationship was also reported between DOC and CDOM (a g (355) = 1.14DOC-4.77 (r 2 = 0.77, p < 0.001) in Lake Taihu [4]. In this study, a significant positive correlation between the CDOM and DOC in wetland water around Poyang Lake had also been found. This indicates that CDOM can be used for the remote sensing-based estimation of DOC [42,43], thus improving the ability to estimate the input and output DOC flux from rivers to wetlands. Additionally, establishing the relationship between CDOM and DOC is useful for the optical remote-sensing-based inversion of the DOC concentration in large lakes such as Poyang Lake [44].

Sources of CDOM in Wetland Water around Poyang Lake
Peuravuori and Pihlaja [39] observed that the increase of E 2 /E 3 values indicated a decrease in the molecular weight and aromaticity of CDOM. For example, the E 2 /E 3 values of inland water were found to be 8.7 ± 1.67 in Taihu Lake [8], between 8.80 and 17.5 in Lake Superior [45], 8.02 ± 3.84 in terminal lakes of the Hulun Buir Plateau [32], and 7.80 ± 2.30 in rivers of the Hulun Buir Plateau [32]. In the present study, the E 2 /E 3 values in the Poyang Lake wetlands were found to range from 4.17 to 8.55, with an average value of 6.01 ± 0.91, which presented a case of lower E 2 /E 3 value and higher aromaticity and molecular weight of CDOM.
SUVA 254 can also be used as an effective index to evaluate the aromaticity and molecular weight of CDOM [34,46]. A previous study found that high SUVA 254 values indicated that the organic matter in aquatic systems was mainly derived from allochthonous sources, with significant inputs from vascular plants [35]. Conversely, low SUVA 254 values were found to indicate that the main source of organic matter was from algae and microorganisms [34]. In the present study, the value of SUVA 254 in the Poyang Lake wetlands was found to range from 2.23 to 5.30 L mg −1 m −1 , with a mean of 4.22 ± 0.80 L mg −1 m −1 . Other studies showed that the SUVA 254 values of inland water dominated by terrestrial inputs were 3.64 ± 0.77 L mg −1 m −1 in a humus lake in Sweden [47], 4.91 ± 0.40 L mg −1 m −1 in Poyang Lake [16], and 3.4-4.5 L mg −1 m −1 in the Congo River [48]. Furthermore, the SUVA 254 values of water dominated by autochthonous contribution were found to be 1.7 L mg −1 m −1 in Pony Lake [35] and 1.8 L mg −1 m −1 in Fryxell Lake [49]. In the present study, it was found that the SUVA 254 values of the Poyang Lake wetlands were significantly higher than those of water environments dominated by autochthonous contribution; however, these SUVA 254 values were similar to the values of water environments dominated by terrestrial inputs.
Helms et al. [36] found that S 275-295 values were inversely proportional to the molecular weight of CDOM. Additionally, the S 275-295 values in inland water systems were found to be 0.022-0.028 nm −1 in the northern Gulf of Mexico [50], 0.0302 nm −1 in the prairie lakes of the US Great Plains [51], and 0.02 ± 0.004 nm −1 in the terminal lakes of the Hulun Buir Plateau [32]. In this study, it was found that the value of S 275-295 in the Poyang Lake wetlands ranged from 0.0152 to 0.0194 nm −1 , with a mean value of 0.0170 ± 0.0009 nm −1 . The results showed that the S 275-295 values of the Poyang Lake wetlands were relatively low.
Previous studies showed that S R is an important indicator of allochthonous and autochthonous source inputs. S R < 1 indicates that CDOM is derived from allochthonous inputs, while S R > 1 indicates that CDOM is derived from autochthonous contributions [52]. The S R values in the Poyang Lake wetlands ranged from 0.940 to 1.108, with a mean value of 0.994 ± 0.039, which indicates that the CDOM of the Poyang Lake wetlands is mainly derived from allochthonous inputs.
The analysis presented in this paper indicates that the CDOM in the Poyang Lake wetlands has a relatively high aromaticity and molecular weight, which were mainly affected by allochthonous inputs. The majority of sample sites were located in the Nanji Wetland Nature Reserve of Poyang Lake, the Poyang Lake migratory birds National Nature Reserve, the mouth of rivers, and wetland waters with frequent human activities. These results suggest that due to the influence of rainfall runoff, river input, and human activities, the sources of CDOM in Poyang Lake wetlands are mainly from allochthonous inputs. The spatial distribution trend of E 2 /E 3 in different parts of the Poyang Lake wetlands was as follows: east > north > west > south (Figure 3). The E 2 /E 3 in the south of the lake was significantly lower than that in the east (p = 0.039). The increase of E 2 /E 3 values indicates the decrease of the molecular weight and aromaticity of CDOM [39]. The variation of E 2 /E 3 values in the Poyang Lake wetlands indicate that the molecular weight and aromatization degree of CDOM are low in the east and north, while the molecular weight and aromatization degree of CDOM is higher in the south. The distribution trend of the SUVA 254 of CDOM in the Poyang Lake wetlands was as follows: south > west > east > north (Figure 3). The SUVA 254 was significantly higher in the south than in the east (p = 0.01) and north (p = 0.039). Hur et al. [46] found that the increase of SUVA 254 values indicated the increase in the molecular weight and aromaticity of CDOM. In the present study, it was directly proven that the degree of aromaticity and humification in the south of Poyang Lake wetland were higher than in other parts of the lake. Meanwhile, the CDOM in the east and north of the lake had relatively low aromaticity. The distribution trend of S 275-295 in the Poyang Lake wetlands was as follows: east > west > north > south (Figure 3). The variation of S 275-295 indicated that the allochthonous inputs were the strongest in the south of the lake. The distribution trend of S R in the Poyang Lake wetlands was as follows: north > east > west > south ( Figure 3). The S R was significantly lower in the south than in the east (p = 0.016) and north (p = 0.013). The values of S R in the south were mostly less than 1, which means that the southern CDOM had stronger allochthonous inputs and higher molecular weight and aromatization.
The results showed that the molecular weight and aromaticity of CDOM were higher in the south of the Poyang Lake wetlands than in other parts. The CDOM in the south of the wetlands was mainly from terrestrial inputs, while the CDOM in the east and north had relatively low aromaticity. The sampling sites in the south were located in the Nanji Wetland National Nature Reserve and the estuary of Ganjiang and Fuhe rivers. There are abundant vascular plants around these areas [26]. The CDOM in the south of the Poyang Lake wetlands could be mainly affected by terrestrial vascular vegetation and river contribution. Most of the sampling sites in the east and north were located in sub-lakes. The phytoplankton content in the sub-lakes of Poyang Lake has previously been shown to be higher than that in rivers [53]. In the present study, the maximum Chl-a concentration was found to be higher in the east of Poyang Lake than in the south, and the average value of Chl-a concentration was higher in the north than in the south (Table 1). Therefore, the CDOM in the east and north of the Poyang Lake wetlands could be produced by autochthonous such as microscopic heterotrophs and phytoplankton, which cause the CDOM in these areas to have relatively low aromaticity.
In this study, the source of CDOM in Poyang Lake wetland was discussed by quantitative and qualitative analysis of aromaticity and molecular weight of CDOM using UV-vis absorption characteristics. In addition, the fluorescence excitation-emission matrix spectroscopy (Fluorescence EEMs) and pyrolysis gas chromatography/mass spectrometry (Pyrolysis GC/MS) can provide abundant and precise information of CDOM characteristics [54,55]. In the future work, we will focus on CDOM source tracing through Fluorescence EEMs and Pyrolysis GC/MS as they have promising potential for characterizing the high-resolution composition of CDOM in the wetland water of Poyang Lake.

Conclusions
Wetlands are the key node in the connection between rivers and lakes. Studying the CDOM optical properties of the wetlands around Poyang Lake is necessary to understand the water quality changes in the lake and identify the health of its ecosystem. In this study, CDOM absorption spectrum curves and water quality parameters were collected from 30 sampling sites in the Poyang Lake wetlands in October 2016. The following conclusions can be drawn regarding the CDOM optical absorption characteristics of the wetland waters surrounding Poyang Lake: (1) The CDOM absorption spectrum presented significant spatial variation, with the absorption coefficient showing a trend of south > west > north > east.
(2) There was a strong linear correlation between the CDOM absorption coefficient and the DOC concentration (a g (355) = 1.075DOC-0.659 (r 2 = 0.723, p < 0.001, n = 30)), and the average concentration of DOC and the average value of a g (355) followed the spatial pattern of south > west > north > east.
(3) The analysis of the spectral characteristic parameters of E 2 /E 3 , SUVA 254 , and spectral slopes showed that the CDOM in the Poyang Lake wetlands has relatively high aromaticity and molecular weight, which were mainly affected by terrestrial inputs. The results show that the molecular weight and aromaticity of CDOM were higher in the south of the Poyang Lake wetlands than in other parts.