Effect of a dam on the optical properties of different-sized fractions of dissolved organic matter in a mid-subtropical drinking water source reservoir

Integrating fecal pollution markers and fluorescence analysis for water quality assessment of urban river

Human fecal contamination in urban rivers poses significant health risks, but their potential connections with other substances like dissolved organic matter (DOM) remain underexplored. In this study, five fecal pollution markers related to fecal Bacteroides or human fecal contamination (AllBac, HF183, BacH, Hum2, and Hum163) and DOM along an urban river were analyzed using quantitative polymerase chain reaction (qPCR) and three-dimensional excitation-emission (3D EEM) fluorescence spectrometry. All five markers were detected with average absolute abundance ranging from 2.51 to 6.28 lg gene copies/100 mL, showing a progressive increase along the river (R2 = 0.29-0.92, p < 0.05). Parallel factor analysis identified three dominant DOM components (humic acid-like, fulvic acid-like, and protein-like), with strong positive correlations between protein-like components and all fecal markers (R2 = 0.59-0.66, p < 0.001). Both fecal and DOM distributions consistently showed significant differences between upstream and downstream areas (p < 0.001), suggesting their complementary assessment. While DOM was more sensitive to environmental variables such as rainfall, rubber dam, and tidal dynamic, the combination of fecal pollution markers and 3D EEM analysis allowed a more comprehensive assessment of contamination levels, mitigating potential biases caused by the influence of multiple factors on a single method. Furthermore, due to the strong correlation between protein-like and fecal markers in the DOM, 3D EEM can be used as a pre-detection means for qPCR detection, reducing testing time and costs.

The synergy of environmental and microbial variations caused by hydrologic management affects the carbon emission in the Three Gorges Reservoir

The synergy of environmental and microbiological changes caused by hydrologic management on carbon emissions of river reservoirs remains unknown. Here, we investigated physiochemistry parameters, compositions of dissolved organic matter (DOM), carbon fluxes (CH₄ and CO₂), and microbial communities in the surface waters of the Three Gorges Reservoir (TGR) within one whole hydrological year. The results showed that hydrologic management significantly changed physiochemistry and DOM composition of the TGR water, and further influenced microbial community composition and functions. DOM content during the drainage period was much lower than during the impoundment period. During the impoundment period, humification extent of DOM became decreasing, while biotransformation extent became increasing compared with the drainage period. DOM composition and water pH exhibited significant correlation with the fluxes of CH₄ and CO₂, respectively. Microbial community composition and function significantly differed between the drainage and impoundment periods. Most of the differential microbial taxa were affiliated with functional groups involved in carbon cycle such as methanotrophy and phototrophy, which showed significant correlation with carbon fluxes. CH₄ and CO₂ fluxes can be mostly explained by synergy of microbial function with DOM composition and water pH, respectively. Such synergistic effect may account for the observed temporal variations of CH₄ fluxes and spatial variations of CO₂, and for the relatively low annual carbon emissions in the TGR. In summary, the synergy of environmental and microbial variations caused by hydrologic management affects carbon emissions from river reservoirs.

Impact of the Three Gorges Dam on the Quality of Riverine Dissolved Organic Matter

The Three Gorges Dam (TGD) is the largest hydropower facility in the world, influencing the riverine hydrology and mass flux in the Yangtze River. Little is known about its impact on the riverine dissolved organic matter (DOM) quality. In this work, the water quality and DOM quality for water samples collected from the upstream and downstream sites of TGD were investigated. The presence of TGD significantly affects the quantity and quality of DOM but has no pronounced effect on nutrient concentrations. Upstream DOM had higher concentration but lower average molecular weight and aromaticity than the downstream DOM. The biological processes in the dam reservoir contribute significantly to upstream DOM. In the downstream sites, terrestrial DOM input raises the average molecular weight and aromaticity of the overall DOM pool. These results suggest that TGD will influence not only the mass flux of organic carbon but also its quality and lability, which has both environmental and ecological significance.

Optical and molecular signatures of dissolved organic matter in Xiangxi Bay and mainstream of Three Gorges Reservoir, China: Spatial variations and environmental implications

With the on-going boom in the construction of dam reservoirs all over the world, the sources and composition of dissolved organic matter (DOM) in fluvial networks are expected to be altered. Considering the importance of DOM as a key biogeochemical component in inland waters, this might bring important ecological and environmental influences. However, limited information is available on the molecular composition of DOM in dam reservoirs. In this study, the spatial characteristics of DOM composition were investigated in Xiangxi tributary and mainstream of the Three Gorges Reservoir (TGR), the largest freshwater reservoir in the world. The concentration alteration of conservative cations revealed the water intrusion from mainstream into Xiangxi tributary, which mainly controlled the hydrological gradient. One tyrosine-like (C4), one tryptophan-like (C2), and two humic-like (C1 and C3) fluorescent components were identified in fluorescent DOM (FDOM) by parallel factor analysis (PAFACAC), potentially indicating algal, anthropogenic, and terrestrial inputs, respectively. Decreasing trends of C1, C3 and C4 components and an increasing trend of C2 component were observed from Xiangxi tributary to mainstream, indicating higher terrestrial and algal inputs but lower anthropogenic inputs in Xiangxi tributary compared to mainstream. The Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) characterization further revealed substantial heterogeneity of DOM at the molecular level. Interestingly, S-containing compounds related to synthetic surfactants were consistently detected, and their relative abundances showed an increasing trend from Xiangxi tributary to mainstream, in agreement with the distribution of the anthropogenic derived C2 component. Meanwhile, numerous lignin-like S-containing compounds were identified, likely the result of the incorporation of sulfide ions to lignin-like CHO compounds. This study represents the first molecular level characterization of DOM in the TGR system, which should aid the design and implementation of more detailed future studies.

Remote observation of water clarity patterns in Three Gorges Reservoir and Dongting Lake of China and their probable linkage to the Three Gorges Dam based on Landsat 8 imagery

The Secchi disk depth (Z_SD) plays a critical role in describing water clarity. Several studies have shown linkages between Three Gorges Dam (TGD) and the downstream lacustrine ecosystem in the middle and lower Yangtze River basin. However, the potential influence on the Z_SD fluctuation in the entire anthropogenic reservoirs of Three Gorges (ER) and Dongting Lake (DTL) has not been reported, possibly due to technical obstacles in obtaining statistically significant spatial and temporal results. We addressed this challenge by using remote sensing technology: the Landsat 8 Operational Land Imager (OLI). We proposed a new, robust remote-sensing algorithm to estimate Z_SD from OLI imagery using red and green band-ratio, leading to MAPE of 21.68% and RMSE of 0.076m for Z_SD ranging from 0.1m to 1.05m. After satisfactory image-based validation, the algorithm was implemented on OLI data to derive Z_SD patterns over ER and DTL from 2013 to 2017. Several crucial findings can be drawn: 1) Spatial-temporal patterns of Z_SD exhibited notable fluctuations over both ER and DTL, and they also demonstrated a significant correlation with each other because of the opposite temporal cycle of Z_SD fluctuations between ER and DTL; 2) Temporally, monthly fluctuations of Z_SD between ER and DTL had opposite temporal cycles, which was mainly attributed to the surface runoff and sediment discharge driven by the outbound runoff variations of TGD. Spatially, the heterogeneity of the Z_SD pattern in ER might have resulted from the different geographical regions being divided by large anthropologic hydrological facilities, such as TGD; 3) The relationship between Z_SD and total suspended matter (TSM) showed a significant negative correlation, as did the relationship between Z_SD and K_d(490). These findings demonstrate that TSM often plays a principal role in light attenuation of extremely turbid inland waters; 4) An inversed phenomenon of water clarity was observed at the intersection of DTL and the Yangtze River around Chenglingji site (YRAC), which was due to the opposite temporal cycle of Z_SD fluctuations between DTL and ER after the impoundment of TGD; and 5) Owing to the analysis of noise-equivalent Z_SD, OLI data can be used to derive Z_SD, since the imagery uncertainty is 0.07m by means of our band-ratio algorithm, which demonstrates similar results to MODIS. The proposed Z_SD-derived algorithm in this study could be suitable for other turbid lakes or reservoirs to formulate related strategies of water quality management in the middle and lower Yangtze River basin, and the unveiled findings here improve our understanding of Z_SD spatiotemporal fluctuations in large river-connected lakes, such as Poyang Lake.