[Spectral Characteristics and Source Analysis of Chromophoric Dissolved Organic Matter in Surface Water of Taihu Lake Before Cyanobacterial Blooming].

Chromophoric dissolved organic matter (CDOM) is an important part of the nutrient biogeochemical cycle in aquatic ecosystems. To explore the characteristics and sources of CDOM components in the surface water of Taihu Lake, UV-visible spectroscopy and excitation emission matrix fluorescence spectroscopy-parallel factor analysis were used to analyze CDOM components in surface water. Combined with CDOM optical parameters (a355, SUVA254, a250/a365, FI, BIX, and HIX), the spatial differences and pollution sources were identified, and a preliminary comparison was made between this study and the historical data of CDOM components in Taihu Lake. According to the results, a355, SUVA254, and a250/a365 showed the characteristics of high concentration, high aromatic ability, and low relative molecular weight of CDOM in the surface water of the eastern part of Taihu Lake; however, the northern part showed the opposite characteristics. Four components were isolated from CDOM using parallel factor analysis:one tyrosine-like (C1), two types of tryptophan (C2 and C4), and one fulionic acid (C3). The main component C1 had a strong linear relationship with the C2 and C3 components, suggesting that different components originated from similar pollution sources. The fluorescence index showed that CDOM in different areas of Taihu Lake were differently affected by endogenous and terrestrial inputs; however, the overall humification degree was low. This indicated that the CDOM components in Taihu Lake were primarily protein-like (C1, C2, and C4) (>85%) and autogenous, with good biochemical availability.

Riverine dissolved organic matter (DOM) as affected by urbanization gradient.

Rapid urbanization has considerably altered carbon biogeochemical cycle and river hydrology. However, the influences of urban land use and urban-induced nutrient increase on dissolved organic matter (DOM) characteristics are poorly understood. Here we hypothesize that the alterations significantly change sources and levels of DOM in river systems that drain the urban areas. To test the hypothesis, we investigated DOM in headwater rivers with varied urban intensities in the Three Gorges Reservoir Area (TGRA), China, through field sampling conducted in the dry and wet seasons. We found positive relationships of urban land (%Urban) with DOC concentration and chromophoric DOM (CDOM) absorption coefficients a254, a280 and a350, as well as fluorescence index (FI370), indicating the significantly increased levels of DOM and autochthonous sources along an urbanization gradient. A stepwise regression analysis demonstrated that occurrences of DOC and CDOM can be predicted by %Urban, while increasing autochthonous source is predictable by the increase in riverine nitrogen. Moreover, a254, a280 and FI370 values showed distinct seasonal variations, with significantly higher CDOM concentration in the wet season and with much higher autochthonous signal in the dry season with high nitrogen loading. Based on the findings, we conclude that urbanization influences occurrences and sources of DOM, with increasing urbanization making an important and direct contribution to DOM, and an indirect effect of urban induced nutrient enrichment, i.e., enhanced nutrient loadings increase autochthonous DOM production in rivers.

[Optical Composition and Potential Driving Factors of Chromophoric Dissolved Organic Matter in Large Lakes and Reservoirs in the Eastern Region of China].

Intensified urbanization has been occurring in the eastern region of China in recent decades, and excessive industrial and household sewage has been discharged into lakes and reservoirs, which has directly lowered water quality and destructed the functions of aquatic ecosystems. Lakes and reservoirs are typically drinking water sources supplying water for metropolitan areas as well as large- and medium-sized cities. Chromophoric dissolved organic matter (CDOM) is the colored fraction of DOM, and its source and optical composition strongly affect water supply safety and the health of surrounding citizens. In April 2021, we collected 68 samples from Reservoir Changtan (n=11), Lake Taihu (n=25), Lake Hongze (n=18), and Lake Gaoyou (n=14), and we further carried out 28 days of laboratory bio-incubation, together with optical measurements and parallel factor analysis (EEMs-PARAFAC) to analyze the bio-degradability ω(BDOC), sources, and optical composition of CDOM in these waters. The results showed that after 28 days of laboratory bio-incubation, the bioavailability of dissolved organic carbon (BDOC) of the four lakes and reservoirs were all higher than 50%. PARAFAC results showed that CDOM collected from the four lakes was composed of four fluorescent components, including a terrestrial humic-like C1, a tryptophan-like C2, and tyrosine-like C3 and C4. Protein-like components (C2, C3, and C4) contributed importantly to the CDOM pool in the four waterbodies, and in Lake Hongze the contribution of C2-C4 was as high as (90.0±2.2)%. In Lake Hongze, total phosphorus (TP) correlated closely with C1-C3, indicating that those components can be used to trace the variability of TP. Among the four waterbodies, a254 was positively correlated with DOC (R2=0.96, P<0.01), indicating that a254 can be used to estimate the dynamics of DOC in these waters. In Lake Taihu, we found a low level of humification index (HIX) and a high level of biological index (BIX), indicating that autochthonous substances contributed importantly to the CDOM pool in this lake. In comparison, allochthonous sources contributed importantly to the CDOM pool in the remaining three waters. The enhanced monitoring of the discharge of industrial and agricultural effluents in the upstream watersheds of the four waters can help to protect the water quality and maintain healthy aquatic ecosystems.

[Source and Optical Dynamics of Chromophoric Dissolved Organic Matter in the Watershed of Lake Qinghai].

Lake Qinghai is the largest lake in China and is of great significance to maintain the ecological security of the Qinghai-Tibet Plateau. Few studies have been carried out to investigate the optical composition and source of chromophoric dissolved organic matter (CDOM) in large lakes on the Qinghai-Tibet Plateau. It is of great significance to study the source and optical dynamics of CDOM in Lake Qinghai watershed for water quality protection and filling in the gaps in the knowledge of CDOM variability in a remote area. Two sampling campaigns in the Lake Qinghai watershed were carried out, and excitation-emission matrices coupled with parallel factor analysis (EEMs-PARAFAC) were used to unravel the optical composition and the sources of CDOM. Our results indicated that the mean dissolved organic carbon (DOC) concentration, a250:a365, and the spectral slope of CDOM absorption S275-295 in the lake were significantly higher than that in the inflow river (P<0.0001, t-test), whereas the mean absorption coefficient of CDOM a350, humification index (HIX), fluorescence peak integration ratio IC:IT, and specific ultraviolet absorbance at 254 nm SUVA254 of CDOM were shown to be lower in the lake than in the inflow river (P<0.0001), indicating that compared with the lake itself, CDOM in the inflow was humic-rich and highly aromatic. Four fluorescent components were obtained using PARAFAC, including a terrestrial human-like component C1, a microbial human-like component C2, a tyrosine-like C3, and a tryptophan-like C4. The mean DOC concentration, S275-295, and a250:a365in the headwater streams of the Lake Qinghai watershed were lower than those in the downstream estuary, indicating that the CDOM abundance increased, and the molecular weight decreased, from the headwaters to the downstream river mouths. The mean of SUVA254, C1, and the first axis of principal component analysis were positively related to terrestrial input (i.e., the PC1 values were significantly higher in rivers than in lakes (P<0.001)), indicating that the aromaticity of CDOM in rivers was higher than that in lakes. Particularly, the contribution of terrestrial humic-like C1 was higher in the Quanji River, Shaliu River, and Khargai River compared with that in other tributaries due to an intensified cultivated land use at the downstream estuary of these rivers.

[Influences of Hydrological Scenarios on the Bioavailability, Fate, and Balance of Chromophoric Dissolved Organic Matter in Lake Poyang].

Lake Poyang has significant differences in hydrological characteristics between the flood and dry seasons. Unraveling the optical composition, bioavailability, fate, and balance of chromophoric dissolved organic matter (CDOM) and organic carbon fluxes in Lake Poyang under different hydrological conditions can help provide advanced schemes on carbon cycling, the transfer and transformation of organic matter, and water resource management of the lake. Three fluorescent components, including a humic-like (C1), a tryptophan-like (C2), and a tyrosine-like (C3) component, were obtained using three-dimensional fluorescence spectroscopy coupled with parallel factor analysis. Prior to and after 28 days of laboratory biodegradation, the means of a254 and the terrestrial humic-like (C1) component in the flood season were both significantly higher than that in the dry season (t-test, P<0.01), indicating that the terrestrial humic-like (C1) component contributed importantly to the CDOM pool. The contribution percentages of protein-like components in the dry season were 81.7% of the summed fluorescent components of CDOM, indicating that there might be discharge of domestic wastewater from areas surrounding the lake in the dry season. The bioavailabilities of the humic-like (C1) component and DOC were 14.0% and 43.2%, respectively, in the dry season. This can be explained by a declined-dilution effect in the lake during the dry rather than in the flood season. We observed no significant difference in the bioavailability of protein-like components under different hydrological conditions. The bioavailability of C1 (i.e.,%ΔC1) showed a decreasing trend from the southern inflowing river mouths to the downstream northern outlet at Hukou in both the flood and dry seasons, indicating that the bioavailability of the C1 decreased following the migration of CDOM in the lake. In the dry season and flood season, Lake Poyang was the source of DOC with fluxes of 14.0×103 t·mon-1 and 1.4×103 t·mon-1, respectively, whereas CDOM fluxes in corresponding periods were the source and weak sink with corresponding fluxes of 9.3×1010 m3·(m·mon)-1 and 1.1×1010 m3·(m·mon)-1, respectively. Therefore, the lake released substantial organic matter to the downstream receiving waters during the dry season, whereas in the flood season, the higher water level in the Yangtze River resulted in a prolonged water residence time of the lake, and a fraction of CDOM was bio-degraded into inorganic nutrients, favoring the metabolisms and the eutrophication process of the lake ecosystem.

Inventory of riverine dissolved organic carbon in the Bohai Rim.

Riverine carbon (C) composition and export are closely related to changes in the coastal environment and climate. Excessive C inputs from rivers to seas and their subsequent decomposition could result in harmful algal blooms and ecosystem degradation in the coastal sea. In this study, we explored the C transportation and composition in the 24 major rivers of the Bohai Sea (BS) Rim based on the investigation of dissolved organic carbon (DOC), carbon stable isotopes (δ13CDOC) and chromophoric dissolved organic matter (CDOM). The results showed that the riverine DOC concentrations were high (10.6 ± 6.04 mg/L) in the BS Rim compared with the DOC levels in the main rivers in Eastern China (4.98 ± 2.45 mg/L). The δ13CDOC ranged from -28.29‰ to -25.32‰ in the rivers of the BS Rim, suggesting that the DOC mainly originated from riverine plankton, soil organic matter mainly induced by C3 plants, and sewage. The excitation-emission matrix fluorescence spectroscopy of the CDOM indicated that a soluble, microbial by product-like material accounted for the largest proportion (approximately 40%) of CDOM in these rivers and that CDOM mainly originated from autochthonous riverine sources with high protein-like components. The rivers in the BS Rim transported approximately 0.55 Tg C of DOC to the BS each year, with more than 70% of reactive C based on the CDOM composition. The DOC yields in terms of unit drainage area transported from the small rivers to the BS were higher compared to those of the larger rivers in the world, which indicated that the small rivers in the Bohai Rim could be an important source of the C in the BS. This study would enrich our understanding of environmental evolution in coastal areas with numerous small rivers.

Variability of dissolved organic matter in two coastal wetlands along the Changjiang River Estuary: Responses to tidal cycles, seasons, and degradation processes.

Dissolved organic matter (DOM) in the coastal tidal marsh-estuary systems are complex mixtures with different source materials that vary with hydrological regimes, seasons, and environmental conditions and are modified by removal processes including photochemical and microbial degradations. Here, monthly surveys of DOM and its optical properties (i.e., absorbance and fluorescence of DOM) covering a complete semi-diurnal tidal cycle were conducted in two coastal marshes with distinct hydrological regimes (i.e., one freshwater and one brackish marsh) in the Changjiang River Estuary (CRE). Four fluorescent components were identified by excitation-emission matrix fluoresces combined with parallel factor analysis (EEMs-PARAFAC) as two terrestrial humic-like components and two autochthonous protein-like components. Results indicated that ebbing waters draining the marshes were consistently enriched with highly absorbing, more humic and highly aromatic DOM compared to the flood tidewaters. On a seasonal basis, DOM dynamics were largely modulated by the marsh productivity and the seasonal Changjiang runoff. Protein-like fluorophores, however, demonstrated a constant, less variable pattern on both the tidal and seasonal timescales. Onsite water incubation experiments with photochemical and microbial alterations revealed that photochemistry was primarily responsible for the removal of optically active components in the marsh DOM pools whereas the impact of microbes was minor. Principal component analysis (PCA) illustrated the processes regulating the DOM dynamics at the marsh-estuarine interface and allowed a clear distinction of samples between the two marshes, in addition to the samples under the influence of episodic weather events (i.e., Super Typhoon Lekima in summer 2009 and the basin-wide severe flood event occurred in summer 2020). This study underscores the importance of world large-rivers such as Changjiang on estuarine marsh DOM dynamics and also highlights the substantial heterogeneity of the marsh DOM across a river-dominated estuary.

Assessment of Eutrophication and DOC Sources Tracing in the Sea Area around Dajin Island Using CASI and MODIS Images Coupled with CDOM Optical Properties.

The sea area around Dajin Island in the Pearl River Estuary is the second-largest habitat in China for the Indo-Pacific humpback dolphin (Sousa Chinensis). However, the rapid economic development of this area brings potential threats to the aquatic ecology around Dajin Island. Real-time monitoring and evaluation of the ecological health of the sea area are urgent. In this study, band ratio and single-band inversion algorithms were performed to obtain Chlorophyll-a (Chl-a) and Suspended Sediment Concentration (SSC), relying on both Compact Airborne Spectrographic Imager (CASI) and Moderate resolution Imaging Spectrometer (MODIS) images. The CASI/Chl-a with high spatial resolution was adopted to assess the eutrophication status, while the dissolved organic carbon (DOC) concentration and chromophoric dissolved organic matter (CDOM) optical properties were used to derive the material composition and sources. The results suggest that the study area is under a low to medium eutrophication state with evenly distributed low Chl-a concentration. However, higher Chl-a is observed in the outer estuary with MODIS/Chl-a. The relatively high DOC concentration, especially in the north, where aquaculture is practiced, and near the estuary's main axis, i.e., east Dajin Island, indicates that the eutrophication state might be underestimated using satellite chlorophyll alone. CDOM optical properties indicated that terrestrial materials are the DOC's primary material sources, but the DOC derived from fishery aquaculture cannot be ignored. The low Chl-a concentration is likely due to the turbulent hydrodynamic regime caused by jet flow and reciprocating flow in this marine area. Comprehensive observation, including the assessment of different technological platforms, is suggested for the aquatic environment.

[Characterizing Sources and Composition of Chromophoric Dissolved Organic Matter in a Key Drinking Water Reservoir Lake Tianmu].

Lake Tianmu is an important source of drinking water, and its water quality can influence ecosystem service functions. Unraveling the sources and composition of chromophoric dissolved organic matter (CDOM) that can affect water treatment processes is necessary to maintain water supply safety and ecosystem service functioning of Lake Tianmu. Samples were collected monthly in 2017 and analyzed for CDOM absorbance and fluorescent spectra using parallel factor analysis (PARAFAC) to investigate the spatial and temporal variations of CDOM sources and composition in Lake Tianmu. PARAFAC results showed that CDOM in Lake Tianmu was mainly composed of a microbial humic-like component C1 (44.2%±9.8%), followed by a tryptophan-like component C2 (29.2%±4.3%), tyrosine-like component C3 (17.2%±13.1%), and terrestrial humic-like component, C4 was the lowest (9.4%±2.4%). The CDOM abundance a(254) and fluorescence intensities of C1 and C2 were significantly higher in the river mouths than in the downstream lake regions, whereas the spectral slope S275-295 was significantly lower in the river mouths (t-test, P<0.05), indicating that allochthonous inputs cause an elevated degree of humification and relative increase in the molecular weight of CDOM in the inflowing river mouths. Seasonal differences in CDOM composition were mainly ascribed to the a(254) and fluorescence intensities of C1, C2, and C4 being significantly higher in the summer and autumn than in the winter and spring (t-test, P<0.05). Our results showed that the influences of different seasons on CDOM composition comprise differences in rainfall and runoff input, as well as water temperature, thermal stratification, phytoplankton biomass, and mineralization of CDOM by light and microbes.

[Sources and Optical Dynamics of Chromophoric Dissolved Organic Matter in Different Types of Urban Water Bodies].

In the past few decades, China's rapid industrial activities and urbanization processes have greatly impacted the urban surface water ecosystem. The changes in the quality of urban surface water directly affect the supply and carbon cycling of urban waters. We collected 50 water samples from urban rivers, lakes, and reservoirs in the city of Changchun in June 2020. Three-dimensional fluorescence spectroscopy coupled with parallel factor analysis (EEMs-PARAFAC) was used to unravel the optical characteristics, composition, and sources of chromophoric dissolved organic matter (CDOM). Our results indicated that the mean concentration of DOC is significantly higher in urban rivers than in reservoirs (t-test, P<0.05), and the mean UV absorption coefficient of CDOM a254 of urban rivers is significantly larger than that of park lakes and reservoirs (t-test, P<0.05), indicating that urban rivers have the highest concentration of CDOM. The spectral slope of CDOM absorption S275-295 and the spectral slope ratio SR were shown to be higher in park lakes than in reservoirs, and even higher than in urban rivers (t-test, P<0.001). Three fluorescent components were obtained using PARAFAC, namely terrestrial human-like (C1), microbial human-like (C2) and tryptophan-like (C3) components. The mean fluorescence intensity of C1-C3 was significantly higher in urban rivers than in both the park lakes and reservoirs (t-test, P<0.005), and the mean fluorescence intensity of C1 in the reservoir water body was significantly higher than that of C2 and C3 (t-test, P<0.005), indicating that the discharge of municipal wastewater likely contributes significantly to the CDOM pool of urban rivers in Changchun, and the contribution percentages of highly bio-labile protein-like components to the CDOM pool in these waters are high. Urban wastewater treatment should be strengthened to effectively protect water quality, as well as the economic, environmental, and ecological functions of urban waters in Changchun City.

Chromophoric dissolved organic matter composition and load from a coastal river system under variable flow regimes.

Chromophoric dissolved organic matter (CDOM) exported from riverine catchments can influence biogeochemical processes in coastal environments with implications for water quality and carbon budget. Despite recent efforts to quantify C fluxes during high flow events, knowledge gaps exist regarding the fluxes and yield of terrestrial, reactive vs. recalcitrant CDOM under episodic to base-flow conditions from uplands to downstream estuaries. We used stream dissolved organic carbon (DOC) concentrations and CDOM optical properties using parallel factor analysis to characterize composition and fluxes under variable flow conditions for a coastal river basin in the SE USA. Our findings showed that episodic flows (>75th percentile) were marked by the elevated flux of humic acid-like CDOM and lower in-stream autochthonous production, or microbial degradation. Further, 70% of the terrestrial CDOM was exported during high flows, with a 3-fold increase in CDOM flux during episodic events, including Hurricane Irene in 2011. While, low flows (<25th percentile) were marked by an increased abundance of microbial, humic CDOM that can be easily processed within the estuary. Due to greater wetland coverage in the Neuse, the annual CDOM yield was 5-6 times higher than the larger rivers, such as the Mississippi, USA, and Changjiang, China. We suggest that similar coastal watersheds in SE USA or elsewhere may contribute substantial amounts of reactive CDOM to the estuaries during high flow conditions and can have negative water quality implications for the coastal C dynamics. These findings can help predict the evolution of coastal C cycling under projected climate change and inform the development of appropriate management strategies.

[CDOM Optical Characteristics and Related Environmental Factors of High-turbidity Waters on the Loess Plateau].

Chromophoric dissolved organic matter (CDOM) optical absorption characteristics, CDOM spectral slopes (S275-295), contribution of each component to water absorption, and the effects of environmental factors on them were analyzed to determine the sources and composition of CDOM in high-turbidity waters on the Loess Plateau. Samples in rivers and lakes (saline and freshwater) in the Loess Plateau area of Shaanxi and Inner Mongolia were collected in May 2018. The results demonstrated significant differences in CDOM absorption optical characteristics between rivers and lakes. The average aCDOM(440) (CDOM concentration, 8.45 m-1) in lakes was higher than that of rivers (2.70 m-1), and the saline lakes showed a higher CDOM concentration (13.52 m-1) than the freshwater lakes (3.38 m-1). Moreover, the light utilization efficiency of freshwater lakes is higher than that of saline lakes and turbid rivers. Great differences in pH and dissolved organic carbon (DOC) were observed between different types of water (P<0.01). The differences in electrical conductivity (EC), turbidity (Tur), and total suspended matter concentration (TSM) were significant with no statistical significances (P>0.1). The chlorophyll a concentration (Chla) in saline lakes was close to the Chla concentration in freshwater lakes if extreme values were excluded. In addition, the CDOM molecular weights of lake water were lower than those in the rivers, while CDOM molecular weights in saline lakes were lower than those in freshwater lakes based on the S275-295. Analyses of specific ultraviolet absorbance (SUVA254) were also conducted to determine the sources of CDOM in different water types, and the results showed that the more terrigenous humus were brought into the rivers and saline lakes compared with the freshwater lakes. Redundancy analysis (RDA) indicated that river and lake water quality parameters of the cumulative variance explained rates were 35.2% for river samples and 61.4% for lake samples, and 100% for samples in the saline and freshwater lakes individually. The results of RDA showed that dissolved oxygen (DO), water temperature, and EC exerted significant effects on CDOM optical properties of the river (P<0.01), while DOC, TSM, and Tur had a great influence on the CDOM optical properties of lakes (P<0.01). There was a strong correlation between pH and CDOM in the saline lakes, while DOC was significantly correlated with CDOM in the freshwater lakes (P<0.05).

[Temporal and Spatial Distribution Characteristics and Difference Analysis of Chromophoric Dissolved Organic Matter in Sediment Interstitial Water from Gangnan Reservoir].

The chromophoric dissolved organic matter (CDOM), the main component of dissolved organic matter, affects the morphological characteristics, migration, and conversion of pollutants in water. Based on UV-vis spectra and excitation emission matrix spectroscopy (EEMs) combined with the parallel factor analysis (PARAFAC), the spatial distribution and spectral characteristics were investigated and source analysis of CDOM was performed. Thus, the spatiotemporal differences in the CDOM in Gangnan Reservoir were analyzed. Results showed that a254, a260, a280, and a355 exhibited significant seasonal differences in Gangnan Reservoir, and the order of CDOM concentrations was summer > spring > autumn > winter. There are significant seasonal differences in the E2/E3, E3/E4, E4/E6, and SR of interstitial water CDOM. The concentrations of E2/E3, E3/E4, E4/E6, and SR were high in winter and low in summer. E2/E3 and E3/E4 in autumn and winter were significantly higher than those in spring and summer, and the E3/E4 in autumn and winter was greater than 3.5, which indicates that the CDOM of the autumn and winter sediments has a smaller molecular weight and a lower degree of humification. Protein-like substances (C1), short-wave fulvic acid (C2), and degraded humic substances (C3) were identified by the PARAFAC model, and there was a significant positive correlation among the three fluorescent components (P<0.001). The total fluorescence intensity of CDOM and the fluorescence intensity of each fluorescent component show significant seasonal differences. The total fluorescence intensity and the fluorescence intensity of each component show the highest levels in spring, followed by autumn and winter, and the lowest levels in summer. The proportion of each fluorescent component in autumn and winter and that of each fluorescent component in spring and summer showed no significant difference. There was a significant difference in the proportion of each fluorescent component between autumn/winter and spring/summer. The BIX and FI of CDOM for autumn and winter were higher than those for spring and summer, indicating that the autogenous source of CDOM in autumn and winter is stronger than that in spring and summer, which was consistent with the result of HIX. PCA and Adonis analysis showed that the spectral characteristics of CDOM exhibited obvious seasonal differences (P<0.001). Moreover, the C1, C2, and C3 and water quality parameters (NH4+, NO3-, NO2-, TDN, and TDP) exhibited significant correlation based on linear regression. The results could provide technical support for the control of organic carbon pollution sources and water quality management in Gangnan Reservoir.

[Bioavailability Characteristics of Chromophoric Dissolved Organic Matter in Lake Gaoyou, Lake Nansi, and Lake Dongping Under Different Hydrological Scenarios].

The bio-lability of chromophoric dissolved organic matter (CDOM) directly reflects its biodegradability potential, and also affects the migration and conversion of pollutants and impacts water quality. This study combines excitation-emission matrices and parallel factor analysis (EEMs-PARAFAC) with laboratory 28 days of bio-incubation experiments, and analyzed the bioavailability characteristics of CDOM samples collected from Lake Gaoyou, Lake Nansi and Lake Dongping in flood season and dry season. Our results showed that:① four fluorescent components were obtained using EEMs-PARAFAC, including a microbial humic-like C1, a terrestrial humic-like C4, a tryptophan-like C2, and a tyrosine-like C3. ② The differences of CDOM absorption pre-and post-incubation, i.e. Δa(254) of the three lakes were positive in the three lakes in the flood season, while partially negative in the dry season, indicating a quite different response of CDOM bioavailability to hydrological seasons. ③ Under different hydrological scenarios, the two humic-like components C1 and C4 increased post-bio-incubation compared with that pre-incubation for the samples collected from Lake Nansi and Lake Dongping, and the two protein-like components in Lake Nansi in both the flood and dry seasons and in Lake Dongping in the flood season (t-test, P<0.001, P=0.005) were lower in the post-than those pre-incubation. In Lake Gaoyou, C1-C3 post-incubation were significantly lower than pre-incubation (t-test, P=0.008, P=0.005). In the dry season, in comparison, C1-C4 except for C2 increased post-incubation than pre-incubation for Lake Gaoyou. This indicated that the protein-like components are unstable and more easily uptaken by microorganisms and may be potentially converted into more stable humic-like components. HIX and IC:IT of the three lakes increased post-incubation while the spectral slope S275-295 decreased, which further confirmed the aforementioned conclusion. ④ During both the flood and dry seasons, the bioavailability of the protein-like components C2-C3 and the fluorescence intensity of C1 and C4 in the inflowing river mouths of the three lakes were higher than in the remaining lake regions. It is therefore necessary to strengthen the water quality management in the inflowing river mouths of the three lakes to maintain the water quality of the lakes.

[Response of Chromophoric Dissolved Organic Matter Dynamics to Different Hydrological Scenarios in the Two Largest Freshwater Lakes Connected to the Yangtze River].

Poyang Lake and Dongting Lake are the two largest freshwater lakes in China connected to the Yangtze River. Changes in the water quality of the two lakes are critical to the water security of the residents surrounding the lakes. Analyses of the optical properties, including chromophoric dissolved organic matter (CDOM) absorption and fluorescence spectroscopy coupled by parallel factor analysis (PARAFAC), were carried out to investigate the dynamics of CDOM in the two lakes in different hydrological scenarios. Our results indicated that different hydrological scenarios have more notable effects on the CDOM dynamics in Poyang Lake compared to those in Dongting Lake. In Poyang Lake, the mean CDOM absorption a(254) and dissolved organic carbon (DOC) were higher in the wet season than in the dry-to-wet transition season, and higher still than in the dry season (t-test, P<0.01), and the mean of the CDOM absorption spectral slope S275-295 was higher in the dry season than in the dry-to-wet transition season and higher still than in the wet season (t-test, P<0.01). In Dongting Lake, the mean of a(254) was not significantly different between different hydrological periods, and SUVA254 reached its maximum in the dry-to-wet transition season. Four fluorescent components were identified using parallel factor analysis. The contribution percentage of CDOM protein-like components in the two lakes was higher during the dry season, and the protein-like components and humic-like components contributed roughly the same amount in the dry-to-wet season, whereas the humic-like components accounted for the main proportion in both lakes during the wet season. From the perspective of spatial distribution, the fluorescence intensity of the four components of Poyang Lake was lower in the southern upstream than in the northern downstream lake regions during the dry season, whereas in the wet season a contrast pattern was found, i.e., with high values found in the upstream lake regions. The spatial difference of fluorescence intensity of the four components in the east of Dongting Lake during the dry season was greater than that in the wet season. We found that DOC increased with increasing water level (r2=0.99, P<0.01) in Poyang Lake and tryptophan-like C2 decreased with increasing water level (r2=0.99, P<0.05) in Dongting Lake. Therefore, the water quality of the two lakes should be managed in a targeted manner according to the response characteristics of CDOM in the two lakes under different hydrological scenarios.

[Distribution Characteristics and Influencing Factors of Chromophoric Dissolved Organic Matter in a Northern-Side River of the Qinling Mountains in Summer].

UV-visible absorption spectroscopy, fluorescence spectroscopy, and parallel factor analysis were used to analyze the composition of chromophoric dissolved organic matter (CDOM) in the waters of the Wangchuan River in summer, and the source of this CDOM was explored. The redundant analysis method and Pearson correlation were used to analyze the correlation between optical parameters and water quality parameters. The results showed that the CDOM of the Wangchuan River is composed of the tryptophan-like component C1 (245, 300/335 nm), the short-wave humus component C2 (240, 320-340/405 nm), and the long-wave humus component C3 (270, 350-370/470 nm), in which components C1 and C2 have some homology (r=0.859, P<0.001). CDOM absorption coefficient α(355) indicates that the CDOM concentration in the water body of the Wangchuan River is at a low level, and the correlation between α(355) and DOC concentration is significant (r=0.850, P<0.001), which is conducive to the establishment of a DOC inversion model. Water fluorescence index FI (2.36±0.20), HIX (3.66±2.47), BIX (1.56±0.82), and freshness index (ß:α) (1.33±0.62), and the spectral slope ratio SR (0.76±0.25) indicate that the CDOM of the Wangchuan River has strong self-generated characteristics, weak humification characteristics, and more new CDOM. Redundancy analysis showed that the humic components (C2, C3) are affected by algae metabolism and microbial action, while tryptophan-like components (C1) are related to land-based input, and negatively correlated with dissolved total nitrogen. The humic components C2 and C3 are positively correlated with total phosphorus, dissolved total phosphorus, and dissolved organic carbon. This paper clarifies the characteristics and influencing factors of CDOM in the Qinling valley, and provides a theoretical basis for water body management in the Qinling valley.

Linking Shifts in Bacterial Community Composition and Function with Changes in the Dissolved Organic Matter Pool in Ice-Covered Baiyangdian Lake, Northern China.

The relationship between CDOM (Chromophoric dissolved organic matter) and the bacterial community was investigated in ice-covered Baiyangdian Lake. The results showed that environmental parameters significantly differed in Baiyangdian Lake, whereas a-diversity was not significantly different. Moreover, the microbial and functional communities exhibited significant differences, and T (Temperature), pH, ORP (Oxidation-reduction potential), DO (Dissolved oxygen), NO3--N, NH4+-N, and Mn (Manganese) were the main environmental factors of these differences, based on redundancy analysis and the Mantel test. Biomarkers of the microbial and functional communities were investigated through linear discriminant analysis effect size and STAMP analysis. The number of biomarkers in the natural area was highest among the typical zones, and most top functions were related to carbohydrate metabolism. Two protein-like components (C1 and C2) and one humic-like component (C3) were identified by parallel factor analysis, and C1 was positively related to C2 (R = 0.99, p < 0.001), indicating the same sources. Moreover, CDOM significantly differed among the typical zones (p < 0.001). The high biological index, fluorescence index, ß:α, and low humification index indicated a strong autochthonous component and aquatic bacterial origin, which was consistent with the results of UV-vis absorption spectroscopy. Network analysis revealed non-random co-occurrence patterns. The bacterial and functional communities interacted closely with CDOM. The dominant genera were CL500-29_marine_group, Flavobacterium, Limnohabitans, and Candidatus_Aquirestis. Random forest analysis showed that C1, C2, and C3 are important predictors of α- and ß-diversity in the water bacterial community and its functional composition. This study provides insight into the interaction between bacterial communities and DOM (Dissolved organic matter) in ice-covered Baiyangdian Lake.

[Effects of Photosynthesis of Submerged Aquatic Plants on CDOM in a Karst Water System: A Case Study from Xueyu Cave, Chongqing, China].

Dissolved organic matter (DOM) in karst water is one of the most important carbon sink components, whose origins, distributions, and transport processes are of significance to carbon sink studies. Chromophoric dissolved organic matter (CDOM) can be utilized to express the composition and structural properties of DOM. In this paper, water samples were collected monthly from Xueyu Cave in a karst underground river from both inside and outside the cave. The changes in hydrochemistry and CDOM spectral characteristics as well as the influencing factors for CDOM changes were studied, with the aim of providing experimental support for carbon sink fluxes in karst processes. The main results were as follows. ① The dominant type of CDOM in Xueyu Cave karst water consisted of small molecular weight organic matter that was autochthonously derived and easily bio-degraded, and the tryptophan-like and tyrosine-like components accounted for more than 60% of the total CDOM. ② The most effective influencing factor inside the cave was microbes, which degraded the small molecular DOM in groundwater and induced slight decreases in the TOC and DOC concentrations; they also increased the humic index (HIX). ③ However, the dominant factor outside the cave was submerged aquatic plant photosynthesis, which induced significant increases in the TOC and DOC concentrations and autochthonous small molecular weight DOM contents.

[Characterizing Chromophoric Dissolved Organic Matter in Key Lakes in the Middle Reaches of the East Route of the South-North Water Diversion Project].

Lake Hongze and Lake Luoma are two key lakes located in the middle reaches of the east line of the South-to-North Water Diversion Project. We attempted to unravel the sources and optical composition of CDOM for samples collected from these lakes using excitation-emission matrices (EEMs) and parallel factor analysis (PARAFAC). ① Three fluorescent components were obtained using PARAFAC, including a terrestrial humic-like C1, a tryptophan-like C2, and a tyrosine-like C3. The sources and optical composition of CDOM in the two lakes were, to a large extent, affected by upstream inflow. ② Specifically, fluorescence intensity (Fmax) of the three components C1-C3 in the inflowing river mouths of the two lakes was notably higher than in the other lake regions, and Fmax of the three components during the flood season was significantly higher than during the dry season (t-test, P<0.01). During the flood season, the fluorescence intensity of the terrestrial humic-like component was the highest. This indicates that the source and composition of CDOM in the two lakes are greatly affected by the inflow from the upstream water system, and that the hydrological processes control the abundance and sources of CDOM, especially the terrestrial humic-like C1.③ Significant positive relationships were found between the terrestrial humic-like C1 and the DOC concentrations and CDOM absorption a(254) (r2=0.60, P<0.01; r2=0.88, P<0.01), and the correlation was higher than the other two components. This indicated that the terrestrial humic-like component was the main source of CDOM. In addition, the terrestrial humic-like C1 had a significant positive correlation with SUVA, S275-295, and the integration ratio of the fluorescence peak C to peak T (IC:IT) (r2=0.49, P<0.01; r2=0.61, P<0.01; r2=0.93, P<0.01). It is further revealed that the source and composition of CDOM in the two lakes are greatly affected by land sources. This study reveals the response of CDOM source and composition in Lake Hongze and Lake Luoma to different hydrological scenarios and water transfer processes. Based on these results, the water quality management of the rivers entering the lake should be strengthened during the flood season.

Autochthonous dissolved organic matter potentially fuels methane ebullition from experimental lakes.

Shallow lakes are hotspots for carbon processing and important natural sources of methane (CH4) emission. Ebullitive CH4 flux may constitute the overwhelming majority of total CH4 flux, but the episodic nature of ebullition events makes determining both quantity and the controlling factors challenging. Here we used the world's longest running shallow-lake mesocosm facility, where the experimental treatments are low and high nutrients crossed with three temperatures, to investigate the quantity and drivers of CH4 ebullition. The mean CH4 ebullition flux in the high nutrient treatment (41.5 ±â€¯52.3 mg CH4-C m-2 d-1) mesocosms was significantly larger than in the low nutrient treatment (3.6 ±â€¯5.4 mg CH4-C m-2 d-1) mesocosms, varying with temperature scenarios. Over eight weeks from June to August covered here warming resulted in a weak, but insignificant enhancement of CH4 ebullition. We found significant positive relationships between ebullition and chlorophyll-a, dissolved organic carbon (DOC), biodegradable DOC, δ2H, δ18O and δ13C-DOC, autochthonous dissolved organic matter (DOM) fluorescent components, and a fraction of lipids, proteins, and lignins revealed using ultrahigh-resolution mass spectrometry, and a negative relationship between ebullitive CH4 flux and the percentage volume inhabited of macrophytes. A 24 h laboratory bio-incubation experiment performed at room temperature (20 ±â€¯2 °C) in the dark further revealed a rapid depletion of algal-DOM concurrent with a massive increased CH4 production, whereas soil-derived DOM had a limited effect on CH4 production. We conclude that eutrophication likely induced the loss of macrophytes and increase in algal biomass, and the resultant accumulation algal derived bio-labile DOM potentially drives enhanced outgassing of ebullitive CH4 from the shallow-lake mesocosms.