[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.

[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.

[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.

[Zonation on non-point source pollution risk in the lower reaches of Zijiang River based on the "source-sink" theory]. / 基于"源-汇"理论的资江下游地区非点源污染风险区划.

Non-point source pollution risk assessment and zonation research are of great significance for the eco-environmental protection and optimization of land use structure. We identified the "source" and "sink" landscape using the "source-sink" landscape pattern theory based on the two phases of land use data in the lower reaches of Zijiang River in 2010 and 2018. We comprehensively considered the non-point source pollution occurrence and migration factors, and used location-weighted landscape contrast index (LCI) and non-point source pollution load index (NPPRI) to analyze non-point source pollution risk spatio-temporal characteristics in the study area. Zonation on non-point source pollution in the lower reaches of Zijiang River was studied by identifying the key factors of non-point source pollution risk. The results showed that the overall risk of non-point source pollution was relatively low. The sub-basin with "sink" landscape was the main type, accounting for 61.2%. Non-point source pollution risk was low in the southwest and was high along the banks of Zhixi River, Taohua River and main stream of Zijiang River, as well as plain in the northeast of the lower Zijiang River. The risk of non-point source pollution from 2010 to 2018 showed an increasing trend. The changes in landscape pattern, especially the expansion of rural settlement, arable land and the shrinkage of forest land had positive and negative responses to the risk of non-point source pollution, respectively. LCI, slope, and distance were the key factors affecting the change of the risk index of non-point source pollution. The lower reaches of the Zijiang River could be divided into four control regions: pollution treatment area near river, low slope pollution control area, ecological restoration-risk prevention and control area, and ecological priority protection area.

[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.