Diversity and ecological assembly process of aerobic anoxygenic phototrophic bacteria in a low irradiation area, Three Gorges Reservoir

Aerobic anoxygenic phototrophic bacteria (AAPB) are significant bacterial groups in aquatic ecosystems, known for their rapid growth and photoheterotrophic characteristics. However, the distribution and ecological assembly process of AAPB in low irradiation freshwater basins remain unclear, warranting further investigation. In this study, we present the diversity, abundance, spatial variations, ecological process, and community interaction of AAPB in sediment of Three Gorges Reservoir (TGR) under low irradiation. Our findings demonstrate the dominant genera of AAPB community that exist in the TGR area also are appeared in different waters, with some regional preference. Moreover, the concentration of pufM gene, an indicator for AAPB, maintains a consistently high numerical level ranging from (2.21 ± 0.44) × 104 to (9.98 ± 0.30) × 107 gene copies/g. Although solar irradiation is suggested as the major factor affecting AAPB, it remains unclear whether and how AAPB differ between regions due to varying solar irradiation levels. Our results show spatial differences between total bacteria and AAPB communities, with significant differences observed only in AAPB. Geographical and environmental factor contributed less than 10% to the spatial difference of community, with sediment type and environmental factors being the key factors influencing microbial community structure. The stochastic process plays a dominant role in the aggregation and replacement of AAPB communities, among which the most contribution is dispersal limitation. For AAPB network, Yoonia and Gemmobacter are the hubs for modules. Those results valuable insights into the AAPB communities in TGR with low irradiation.

Influences of water level fluctuation on water exchange and nutrient distribution in a bay: Evidence from the Xiangxi Bay, Three Gorges Reservoir

Following the Three Gorges Reservoir (TGR) impoundment, many tributaries were turned into bays; hydrodynamic conditions of TGR profoundly changed the residence time, temperature, and nutrient distributions of bays, and nutrient enrichment occurred in these bays. However, little research has been done on the effects of water level qqfluctuations (WLFs) of TGR on the bay. In this study, Xiangxi Bay (XXB), one of the tributaries of TGR, was selected as the delegate to construct and calibrate a two-dimensional hydrodynamic-temperature-tracer-water quality model based on the CE-QUAL-W2. The results were the following: 1) In spring, as total nitrogen (TN) in the TGR tended to be higher than that in the XXB, the downward WLF increased water exchange, TGR-XXB nutrient flux and TN in the epilimnion of the XXB, and decreased the water exchange and TN in the hypolimnion of the XXB. The upward WLF did the opposite. The situation would be reversed in autumn. 2) Under a larger magnitude or a shorter period of WLF, its corresponding effects on the water exchange and TN increased. 2) Both the downward and upward modes of WLF helped to decrease the thermal stratification of XXB. 4) The upward/downward WLF could be used to decrease the epilimnetic TN of XXB in spring/autumn, and was suggested to reduce the local algal bloom. The WLFs by the TGR regulation could profoundly change the water exchange and nutrient distribution in the bay, which helped to control nutrient concentrations and prevent algal blooms.

Exposure of the residents around the Three Gorges Reservoir, China to chromium, lead and arsenic and their health risk via food consumption

Hydrological management of the Three Gorges Dam has resulted in the interception of heavy metals in the Three Gorges Reservoir (TGR). However, the exposure to heavy metals and health risks among local residents remained poorly understood. Here we collected 208 biomarker samples (hair) and 20 food species from typical regions in the TGR to assess the exposure levels of three toxic metals (Cr, Pb and As) in residents of the TGR, and subsequently investigated their health risk via dietary intake. Results indicated that hair Cr and As levels were below the reference value for normal people and threshold of skin lesions, respectively, whereas about 22% hair Pb exceeded the reference for clinical medicine, indicating a potential Pb exposure of local residents. Smoking habit and fish consumption were found to be predictors for hair Pb. In addition, the concentrations of heavy metals in all investigated food samples were below the limits of contaminants in food in China, except for Pb in the sweet potato and fish. The estimated daily intake of metals (DIMs) revealed that the intakes of Cr and As from studied food were under the recommended thresholds of Cr and As. However, the intake of Pb via diet exceeded the limit of the prevalence of chronic kidney disease and closed to the threshold for cardiovascular, which was probably associated with the high Pb concentrations of fish and sweet potato. Overall, residents around the TGR were at low exposure to Cr and As, but Pb exposure may need more attention.

[Effects of Farming Practices on Soil Nitrogen and Phosphorus Concentrations and Its Loss in the Drawdown Area of the Tributary Embayment of the Three Gorges Reservoir]

As the Three Gorges Reservoir (TGR) periodically operates at low water levels, its drawdown area has been utilized for cultivation by local farmers due to the overlap of the non-inundated period and the crop-growth period. However, traditional agricultural planting may affect the aquatic environment of the TGR area. To explain the effects of agricultural farming and abandoned farming on the water environment, a study was conducted in the drawdown area in an embayment of the Pengxi River (a tributary of the TGR). Corn, potato, and peanut fields were investigated for nitrogen and phosphorus content in surface soil, during the farming period (March to September 2018) and the conversion period (March to September 2019). Nitrogen and phosphorus balance models were constructed for farmland and abandoned farmland, to compare and analyze the budgets and loss risk of nitrogen and phosphorus from soil in the drawdown area. The results showed that the ammonia nitrogen (NH₄⁺-N), total phosphorus (TP), and inorganic phosphorus (IP) content of soil in the corn field varied significantly across different planting periods. The concentrations of ammonium nitrogen and nitrate nitrogen (NO₃^--N) were significantly higher in farmland soil than in abandoned farmland soil, and the concentrations of total phosphorus (TP), inorganic phosphorus (IP), and calcium-bound phosphorus (Ca-P) were significantly lower in farmland soil than in abandoned farmland soil. The different soils were ranked according to the intensity of nitrogen and phosphorus surplus as follows:corn field>potato field>peanut field. The apparent surplus values in the different farmland soils were 76.89 kg ·hm^-2(corn field), 51.92 kg ·hm^-2(potato field), and 43.74 kg ·hm^-2(peanut field) for nitrogen, and 79.69 kg ·hm^-2(corn field), 75.76 kg ·hm^-2(potato field), and 17.78 kg ·hm^-2(peanut field) for phosphorous. Overall, the surplus intensities of nitrogen and phosphorus in all three croplands were higher than the respective risk thresholds, indicating potential nitrogen and phosphorus pollution in the three farmland types. Agricultural farming in the drawdown area may therefore increase the risk of nitrogen and phosphorus loss and is not conducive to the protection of the aquatic environment.

Total mercury and methylmercury in human hair and food: Implications for the exposure and health risk to residents in the Three Gorges Reservoir Region, China

Three Gorges Dam (TGD) is the largest hydroelectric construction in the world, and its potential impacts on the ecological environment and human health risks have invoked considerable global concern. However, as a mercury (Hg) sensitive system, limited work was conducted on the Hg exposure level of local residents around the Three Gorges Reservoir (TGR). Thus, 540 human hair samples and 22 species of local food samples were collected to assess the Hg exposure and human health risk to the residents located in the Three Gorges Reservoir Region (TGRR) and to investigate their dietary exposure to Hg. The results showed that the geometric mean concentrations of total mercury (THg) and methylmercury (MeHg) in hair were 0.42 ± 0.43 μg g^-1 and 0.23 ± 0.32 μg g^-1, respectively, lower than the reference level (1.0 μg g^-1) recommended by the United States Environmental Protection Agency (US EPA), indicating a low level Hg exposure for residents around the TGR. No significant difference in the accumulation of Hg in hair between the gender subgroups was observed, whereas age difference, smoking and alcohol drinking behavior, and fish consumption frequency were significant predictors of hair Hg level. Besides, THg and MeHg of all the investigated food samples did not exceed the corresponding Chinese national standard. The average probable daily intakes (PDIs) of THg and MeHg were 0.032 μg kg^-1 day^-1 and 0.007 μg kg^-1 day^-1, which were obviously below the recommended values of 0.57 μg kg^-1 day^-1 and 0.1 μg kg^-1 day^-1, respectively. The cereal (mainly rice) contribution of THg (76.0%) and MeHg (74.4%) intakes to the local residents around the TGR was much higher than that of fish (10.7% and 22.9%, respectively) due to the considerable rice consumption. Overall, residents around the TGR were at a low Hg exposure and rice consumption was the major pathway for Hg exposure.

Spatial-temporal distribution of sediment phosphorus with sediment transport in the Three Gorges Reservoir

Suspended sediment is an important phosphorus (P) adsorption medium in river catchments. Early adsorption isotherm models often ignored sediment heterogeneity, resulting in incorrect sediment P estimates in field environments. In the Three Gorges Reservoir (TGR), China, P load assessment is essential to eutrophication risk management, but the sediment P evolution in the TGR is unclear. Herein, the P-adsorption capacity of suspended sediment was estimated with an improved Langmuir model via sediment parameter consideration, and the long-term distribution and variations in simulated sediment P with sediment transport were assessed from 2003- 2016. The results showed that the improved Langmuir model attained a good fit with experimental and field data. The sediment load entering the TGR significantly decreased, especially the median size (D₅₀) fraction smaller than 0.008 mm, resulting in long-term discharged sediment load decline and annual mean D₅₀ increase. Meanwhile, the annual sediment P load in the TGR decreased from 7.46- 22.38 kg/s in 2003 to 1.74- 4.73 kg/s in 2016. The increasing sediment particle size reduced the sediment P load and was sensitive to the low sediment P load in the regular impoundment stage (September 2008- 2016). The flood season (June-September) transported 69.2- 98.6% of the annual sediment P. Around 62.3% of total sediment P load was retained in the TGR from 2003- 2016. The results revealed that the retention role of the Three Gorges Dam (TGD) facilitated the long-term reduction in fine sediment and sediment P in the TGR downstream. This study highlights the importance of the particle size in P-adsorption capacity estimation with suspended sediment transport.

[Tempo-spatial and Retention Analysis of Total Phosphorus in the Three Gorges Reservoir]

Phosphorus, as a limiting and vital macronutrient in water, circulates mainly with sediment through rivers. The construction and operation of large reservoirs can have a profound impact on the characteristics of phosphorus transportation and transformation. Based on measured hydrological and water quality data during 2008-2016 in the Three Gorges Reservoir (TGR), a statistical model of total phosphorus (TP) flux and sediment load in different periods was established. The formula of TP flux was established by analyzing the interpolation of unmeasured TP concentration, tempo-spatial characteristics, flux variation, and the retention effect of TP in the TGR. The results indicated that taking no account of the effects of TP inflows of tributaries, annual TP concentration in the TGR varied between 0.196 mg·L^-1 and 0.290 mg·L^-1 during 2008-2012, and inner-annual changes in TP concentration presented an "M" type with two obvious peaks. TP concentration decreased slowly from upstream to downstream of the TGR, and increased significantly from Cuntan to Qingxichang in some years. The average annual TP flux and retention rate of the TGR were 82300 t and 49.76% during 2008-2012, respectively. During 2013-2016, the average annual TP flux and retention rate decreased significantly, to about 47900 t and 12.03%, respectively.

[Pollution Characteristic of Ni in Sediments in the Three Gorges Reservoir]

The Three Gorges Reservoir (TGR) has achieved the target of water storage of 175 m for eight consecutive years until October 2017. To study the temporal and spatial variation of nickel (Ni) in different water periods under 175 m operating conditions in the TGR area, probe the impact of a large-scale water conservancy project on the Ni enrichment, and establish the pollution evaluation system of Ni in the TGR area, we collected 173 surface sediment samples from the TGR area during four consecutive water periods from December 2015 to June 2017 and measured the Ni concentration by using inductively coupled plasma mass spectrometry (ICP-MS). The mean values of Ni in the four water periods are higher than the background value of Yangtze sediment and soil. At the spatial scale, the Ni content in the mainstream exhibits a clear upward trend from upstream to downstream and is lower than that of the tributary. The tributaries of the lower reaches shows a notably higher Ni content than the tributaries of the upper and middle reaches. At the temporal scale, the water period exerts an insignificant effect on the Ni content. The Ni content is relatively stable and shows a downward tendency at a 175 m water level. The regional geochemical baseline (RGB) value of Ni was obtained through building a geochemical baseline model in the area. The RGB values of Ni in sediments during four consecutive water periods are 47.0, 44.2, 42.9, and 41.9 mg·kg^-1, respectively. The Ni contents in the middle and lower reaches of the mainstream and tributary are significantly affected by human activities. Moreover, the pollution evaluations based on global Ni background values, local background values, and geochemical baseline values as reference values were compared and the geoaccumulation index and potential ecological risk of Ni were determined to comprehensively assess its pollution risk. The assessment data indicate that Ni in the aquatic environment of the TGR area is almost uncontaminated and poses a low ecological risk, except for samples in regions around Fengdu County and Guizhou Town in the Zigui County along the mainstream, which were uncontaminated to moderately contaminated. Relative to global and regional background values, the pollution assessment results obtained using the RGB as a reference value are more scientific and better match the temporal and spatial variation of the study area.

Distribution, sedimentation, and bioavailability of particulate phosphorus in the mainstream of the Three Gorges Reservoir

The transportation and sedimentation of particulate phosphorus (PP) in a huge reservoir such as the Three Gorges Reservoir (TGR) are closely related to the phosphorus distribution characteristics and nutritional status of the water body. In this study, the PP distribution, sedimentation, and bioavailability in the mainstream section of the TGR were investigated through a field survey, indoor simulated settlement experiment, and historical data analysis. The results indicated that PP was the major component of the total phosphorus (TP) and that the Three Gorges Dam (TGD) trapped nearly 76.25% of suspended sediment (SS) and 75.35% of PP in the TGR, even during the flood season. A decline in flow velocity promoted the deposition of PP; additionally, PP concentrations gradually dropped from 0.35 mg/L in Chongqing to 0.02 mg/L in Zigui. The static PP sedimentation process adequately fitted a pseudo-second-order kinetic equation with a maximum correlation coefficient of 0.97. Moreover, more than half of the PP sedimentation process was achieved in less than 60 min for samples collected from the upper river reaches within simulated sedimentation process. The median particle size of SS and absolute value of the water column's zeta potential were negatively and positively related to the t₁₂ values of PP sedimentation, respectively. Compared with the concentration and particle size of SS obtained in the pre-TGR period, the values in the mainstream section of the TGR were lower. However, the TP and Fe/Al-P contents in SS increased several times. Due to the combined effects of flow velocity reduction and SS trapping, the water transparency and bioavailability of water column phosphorus were enhanced. Thus, the risk of water bloom outburst significantly increased when the impounded water level of 175 m in the TGR became the normal state.

Distribution and sources of polycyclic aromatic hydrocarbons and phthalic acid esters in water and surface sediment from the Three Gorges Reservoir

After the impoundment of the Three Gorges Reservoir (TGR), the hydrological situation of the reservoir has changed greatly. The concentration and distribution of typical persistent organic pollutants in water and sediment have also changed accordingly. In this study, the concentration, distribution and potential sources of 16 polycyclic aromatic hydrocarbons (PAHs) and 6 phthalic acid esters (PAEs) during the water drawdown and impoundment periods were investigated in water and sediment from the TGR. According to our results, PAHs and PAEs showed temporal and spatial variations. The mean ΣPAH and ΣPAE concentrations in water and sediment were both higher during the water impoundment period than during the water drawdown period. The water samples from the main stream showed larger ΣPAH concentration fluctuations than those from tributaries. Both the PAH and PAE concentrations meet the Chinese national water environmental quality standard (GB 3838-2002). PAH monomers with 2-3 rings and 4 rings were dominant in water, and 4-ring and 5-6-ring PAHs were dominant in sediment. Di-n-butyl phthalate (DBP) and di-2-ethylhexyl phthalate (DEHP) were the dominant PAE pollutants in the TGR. DBP and DEHP had the highest concentrations in water and sediment, respectively. The main source of PAHs in water from the TGR was petroleum and emissions from coal and biomass combustion, whereas the main sources of PAHs in sediments included coal and biomass combustion, petroleum, and petroleum combustion. The main source of PAEs in water was domestic waste, and the plastics and heavy chemical industries were the main sources of PAEs in sediment.

Occurrence of organotins in the aquatic environment during an operating cycle of the Three Gorges Reservoir, China

Organotins (OTs) pollution in the aquatic environment of the Three Gorges Reservoir (TGR) was assessed during the reservoir's operating cycle. Butyltins (BTs) and phenyltins (PhTs) in the water phase and suspended particulate matter (SPM) at different water levels were analysed. It was found that the distribution of OTs in the surface and bottom water phases were similar, with the dominant OTs being BTs at the low water level and PhTs at the high water level. The detection rates and concentrations of OTs in the water phase at the high water level were both higher than those at the low water level, with most OTs being monobutyltin (MBT) at the low water level and monophenyltin (MPhT) at the high water level. The concentrations of OTs in SPM at the low water level were higher than those at the high water level, and BTs, especially dibutyltin (DBT) and tributyltin (TBT), were the predominant OTs whether surface or bottom layer at each water level. The BTs and SPM concentrations had a significant positive relationship in all samples, indicating that the SPM concentration would determine the distribution of BTs in the aquatic environment of the TGR region (TGRR). The difference in the distribution of OTs at the different water levels indicated that the hydrological and hydraulic behaviour of the TGR influences OTs transport in the aquatic environment of the TGRR.

dominating in the Yangtze Three Gorges Reservoir (TGR), China

Propanil (3',4'-dichloropropionanilide) is a selective post emergence herbicide for controlling broad leaf and grass weeds in rice (Oryza sativa L.). After being taken up by plants, the fate of propanil in decomposing plant material is of particular importance to the phytoremediation of the environment. Therefore, we investigated the biotransformation of propanil in the plant Bidens pilosa under conditions close to those present in the Three Gorges Reservoir (TGR), China. Plants pre-treated with ¹⁴C-ring-labeled propanil were either (treatment a) directly submerged in TGR water for 90 days or (treatment b) pre-extracted with organic solvents, and subsequently only insoluble materials and non-extractable residues (NER) of the pesticide fractions were similarly incubated. After incubation in TGR water (treatment a), 30 % of applied radioactivity was released into water and simultaneously, amounts of NER in the plant debris appeared to increase with time finally amounting to 40 % of applied ¹⁴C. The radioactivity contained in the extractable fractions were identified as propanil, 3,4-dichloroaniline (DCA), and N-β-D-glucopyranosyl-3,4-dichloroaniline (DCA-Glu). In treatment b, significant ¹⁴C amounts were released to the water (6 % of applied ¹⁴C) and the solubilized radioactivity fractions were demonstrated to agree with those found in the extractable fractions. Therefore, if residues of the pesticide propanil are taken up by plants, it may enter again the aquatic environment after plant death and submergence. This phenomenon may have a potential impact on aquatic organisms, which to our knowledge has not been reported before. As plant uptake and degradation of xenobiotics are recognized as detoxification, we consider B. pilosa with its high uptake potential, at least for propanil, as suitable species for phytoremediation.

Characteristics of cadmium remobilization in tributary sediments in Three Gorges Reservoir using chemical sequential extraction and DGT technology

The Three Gorges Reservoir (TGR) is the largest reservoir in China. Cadmium (Cd) is a primary pollutant in the TGR, and its speciation and bioavailability have attracted extensive attention since TGR submergence. In this study, Chelex-100 DGT (diffusive gradient in thin films) and the sequential extraction method were used to investigate the bioavailable Cd in sediments obtained from a typical tributary (Meixi) and mainstream (Yangtze) in the TGR. The total Cd concentrations in sediments of the four stations were also determined. In comparison to the concentrations of labile Cd measured by DGT (C_DGT-Cd) in four profiles, CJ and MX-upstream/downstream were at potential risk for Cd release from surface sediments using the apparent diffusion flux across the interface numerical model. The order of C_DGT-Cd in surface sediments was as follows: CJ > MX-downstream > MX-upstream > MX-midstream. Additionally, a positive correlation was demonstrated between C_DGT-Cd and Cd in the exchangeable fraction (F1) in the surface sediments, indicating that Cd in the exchangeable fraction was readily captured by DGT. A negative correlation was observed between C_DGT-Cd and C_DGT-Fe, C_DGT-Mn in the sediment-water-interface (SWI), suggesting that Fe/Mn oxides did not control the release of labile-Cd from sediments. Furthermore, a positive correlation existed between the C_DGT-Cd in the surface sediments and Cd in the oxidizable fraction (F3), illustrating that Cd sorbed or bound with organic matter or sulfide was labile and released into the water phase from the surface sediments. A dark area was found in the AgI gel, which further demonstrated that Cd simultaneously was released with sulfide in this area.

The fate of the herbicide propanil in plants of the littoral zone of the Three Gorges Reservoir (TGR), China

The anti-seasonal hydrology with 30m water fluctuations in the Three Gorges Reservoir (TGR) of China attracts growing environmental and ecological concerns. We investigated the biotransformation of the herbicide propanil in plants dominating in the littoral zone of the TGR by applying the ¹⁴C-ring-labeled herbicide into non-aseptic hydroponic plant systems (Cynodon dactylon, Nelumbo nucifera and Bidens pilosa), aseptic plants (Lemna minor and Lemna gibba) and cell suspension cultures (C. dactylon and L. minor). (1) Propanil absorbed in plants of the hydroponic systems was (12.46±1.63)% of applied radioactivity (AR) (C. dactylon), (52.36±6.38)% (N. nucifera) and (76.55±6.07)% (B. pilosa), respectively. The ¹⁴C-residues in the plant extractable fractions and the corresponding media were confirmed by radio-Thin Layer Chromatography (TLC), radio-High Performance Liquid Chromatography (HPLC) and Gas Chromatography-Electron Ionization Mass Spectrometry (GC-EIMS) as propanil, 3,4-dichloroaniline (DCA) and N-(3,4-dichlorophenyl)-β-d-glucopyranosylamine (Glu-DCA). (2) About 8% of AR was taken up by both aseptic plants, from which 7.0% of AR was extracted and identified also as propanil, DCA and Glu-DCA. (3) Concerning cell suspension cultures, (39.22±9.39)% of AR was absorbed by C. dactylon after 72hr, whereas the accumulated ¹⁴C-propanil by L. minor cell suspension culture amounted to (65.04±1.72)% after 7days. The identified compounds in cell cultures are consistent with those in the tested plants. Most of the pesticide residues in the intact plants were un-extractable, which are recognized as the end of the detoxification process. We therefore consider these plants as suitable for the phytoremediation of the herbicide propanil in the TGR region.