Water Level Fluctuations Modulate the Microbiomes Involved in Biogeochemical Cycling in Floodplains.

Drastic changes in hydrological conditions within floodplain ecosystems create distinct microbial habitats. However, there remains a lack of exploration regarding the variations in microbial function potentials across the flooding and drought seasons. In this study, metagenomics and environmental analyses were employed in floodplains that experience hydrological variations across four seasons. Analysis of functional gene composition, encompassing nitrogen, carbon, and sulfur metabolisms, revealed apparent differences between the flooding and drought seasons. The primary environmental drivers identified were water level, overlying water depth, submergence time, and temperature. Specific modules, e.g., the hydrolysis of ß-1,4-glucosidic bond, denitrification, and dissimilatory/assimilatory nitrate reduction to ammonium, exhibited higher relative abundance in summer compared to winter. It is suggested that cellulose degradation was potentially coupled with nitrate reduction during the flooding season. Phylogenomic analysis of metagenome-assembled genomes (MAGs) unveiled that the Desulfobacterota lineage possessed abundant nitrogen metabolism genes supported by pathway reconstruction. Variation of relative abundance implied its environmental adaptability to both the wet and dry seasons. Furthermore, a novel order was found within Methylomirabilota, containing nitrogen reduction genes in the MAG. Overall, this study highlights the crucial role of hydrological factors in modulating microbial functional diversity and generating genomes with abundant nitrogen metabolism potentials.

Seasonal variation regulate the endogenous phosphorus release in sediments of Shijiuhu Lake via water-level fluctuation.

Freshwater lakes undergo substantial alterations of the phosphorus (P) cycle in the water-sediment ecosystem due to thermal change. The impact process of seasonal fluctuation on P cycling in sediments has been scarcely investigated. P forms in sediments from a freshwater lake in China were analyzed using sequential extraction technique. The vertical distribution of soluble reactive P (SRP), Fe2+, and S2- in the interstitial water was measured using diffusion gradient technique (DGT). Fick's Law and DIFS model were used to obtain the diffusion fluxes of SRP and the kinetic parameters in the water-sediment system. The results showed that total P (TP) concentrations in the solid sediments varied from 207.5, 266.6 and 130.3 mg/kg to 614.7, 1053.1, and 687.6 mg/kg in winter, spring, and summer, respectively. The concentrations of individual P forms in spring were higher than those in other seasons, with Fe-bound P (Fe-P) concentration being the highest across all seasons. Notably, significant variations of SRP concentrations were found in the interstitial water between sedimentary depths of approximately 2 cm and 6 cm, particularly in the summer. Furthermore, higher diffusion fluxes of SRP through the interface were found in summer. A stable anaerobic environment failed to develop in spring with high water level, preventing the desorption of solid Fe-P and diffusion of Fe2+ into the water due to the afflux and deposition of P-containing particulate into deeper sediment layers along with organic material. Under extreme high-temperature in summer, decreased rainfall and rising temperatures boosted the activity of aquatic organisms in the water, thereby reducing P fixation by sediments and leading to P release. This process increased the risk of P excess and potential eutrophication in the water. Generally, clarifying the resupplying processes of endogenous P in sediment systems experiencing seasonal variations is critical for eutrophication management of lakes.

Soil aggregate disintegration effects on soil erodibility in the water level fluctuation zone of the Three Gorges Reservoir, China.

Spatial hydrological alterations can affect soil structural stability. Over time, forces induced by water weaken soil aggregates and this has a negative implication to soil health. The Three Gorges Reservoir (TGR) in particular, experienced a long-term hydrological condition and repetitive seasonal water level fluctuations that could affect soil health. The present study was conducted to investigate the effects of different water levels on soil aggregate disintegration rate over time and its relation to soil erosion susceptibility in water reservoirs. Samples from different elevations (155 m, 160 m, 163 m, 166 m, 172 m, and 180 m) in the water level fluctuation zone (WLFZ) were exposed to continuous wet-shaking for 3, 9, 27, 54, and 81 min resulted to different WLF intensity accordingly. The results showed a comparative difference between aggregates size before and after the experiment where micro-aggregates (<0.25 mm) increased with respect to elevations increase. The exponential prediction proved that aggregate stability decreased with the increase of WLF intensity, insisting the effects of continuous hydrological stress to aggregate break-down. A couple of factors definitely confirmed that soil erodibility (k) is primarily determined by disintegration of soil aggregates for the surface soil of the TGR. Despite the fact that Disintegration rate (Dr) and k showed a positive relationship, R2 = 0.73 (p < 0.05), the results showed that the soil properties decreasing Dr also decreases soil erodibility in the study area. Non-effective role of soil organic matter (SOM) for stabilizing soil aggregates was primarily related to water level fluctuations inhibiting decomposition. Relying on the present findings, environmental problems mostly soil erosion in the TGR could be therefore linked to excessive destabilization of soil aggregates. Therefore, the results of this study should play a major role in determining the factors primarily inducing soil erosion in river reservoirs.

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.

Shifts of soil archaeal nitrification and methanogenesis with elevation in water level fluctuation zone of the three Gorges Reservoir, China.

The water level fluctuation zone is a unique ecological zone exposed to long-term drying and flooding and plays a critical role in the transport and transformation of carbon and nitrogen materials in reservoir-river systems. Archaea are a vital component of soil ecosystems in the water level fluctuation zones, however, the distribution and function of archaeal communities in responde to long-term wet and dry alternations are still unclear. The community structure of archaea in the drawdown areas at various elevations of the Three Gorges Reservoir was investigated by selecting surface soils (0-5 cm) of different inundation durations at three sites from upstream to downstream according to the flooding pattern. The results revealed that prolonged flooding and drying increased the community diversity of soil archaea, with ammonia-oxidizing archaea being the dominant species in non-flooded regions, while methanogenic archaea were abundant in soils that had been flooded for an extended period of time. Long-term alternation of wetting and drying increases methanogenesis but decreases nitrification. It was determined that soil pH, NO3--N, TOC and TN are significant environmental factors affecting the composition of soil archaeal communities (P = 0.02). Long-term flooding and drying changed the community composition of soil archaea by altering environmental factors, which in turn influenced nitrification and methanogenesis in soils at different elevations. These findings contribute to our understanding of soil carbon and nitrogen transport transformation processes in the water level fluctuation zone as well as the effects of long-term wet and dry alternation on soil carbon and nitrogen cycles. The results of this study can provide a basis for ecological management, environmental management, and long-term operation of reservoirs in water level fluctuation zones.

Distribution, sources and risk assessment of PAHs in soil from the water level fluctuation zone of Xiangxi Bay, Three Gorges Reservoir.

Information on PAH distribution in the water level fluctuation zone (WLFZ) of Three Gorges Reservoir is limited. In this study, we investigated PAH distribution and sources and assessed PAH risks, over one annual water level fluctuation cycle (June 2017-June 2018) at four elevations spanning the WLFZ (145 m, 155 m, 165 m and 175 m) at seven locations in the water level fluctuation zone along Xiangxi River. The mean total PAH concentration in June 2018 (953 ng g-1) was significantly higher than in June 2017 (494 ng g-1), and the horizontal and vertical distributions of PAHs changed significantly. The changes in distribution patterns provided evidence for the cause of increased PAH levels, which were attributed to construction of the Xiangxi River Bridge. Thus, this study of PAH dynamics in the WLFZ soils of Xiangxi Bay also provided valuable information on the impact of bridge construction on WLFZ soils. The change in PAH levels among stations implicated sediment disturbance resulting from bridge construction as the major contributor to the increased PAH levels. Source characterization, based on the ratios of certain PAHs, indicates that PAHs are mainly from the combustion of petroleum fuels, biomass and coal. These ratios indicated that the proportion of PAHs from fuel combustion increased from 2017 to 2018, implicating the heavy equipment used during bridge construction as another source of the increased PAH levels. The incremental lifetime cancer risk (ILCR) model was used to assess the health risk of the PAHs and the range among all age groups (10-5-10-4) indicates a potential health risk. The mean effects range-median quotient (M-ERM-Q) was used to assess the ecological risk of PAHs and the range (0.1-0.5) indicates low to medium risk. The increase in PAH levels from 2017 to 2018 increased the risk to public health and the environment. The results of this investigation provide a reference for ecological restoration of the WLFZ and support development of effective policies for environmental and public health. Further, the results provide information on the impact of bridge construction on WLFZ soils and identify research needed to more fully understand PAH dynamics in WLFZ soils.

Effects of Herbaceous Plants on Methylmercury Net Production and Release From Sediment After Flooding in the Water-Level Fluctuation Zone of the Jialing River.

To explore the effects of herbaceous plants on mercury (Hg) behaviors after flooding in the water-level fluctuation zone (WLFZ) of Jialing River, three typical local plants (D. pyramidalis, A. philoxeroides and C. dactylon) and their in-situ sediments were collected for flooding experiments to study the Hg dynamics of water and sediment in different treatments. The results showed that flooding increased sediment MeHg concentrations and flooded plants (especially for the D. pyramidalis) promoted this process. Similarly, the highest dissolved MeHg level and proportion of MeHg to total Hg (%MeHg) were observed in plant-water treatments in the presence of D. pyramidalis, indicating the potential for the methylation of Hg in the water body influenced by the decay process of herbaceous plants. These results suggest that the herbaceous plant D. pyramidalis contributes more to Hg methylation and release in the WLFZ of the Jialing River than other plants studied.

Role of the rhizosphere of a flooding-tolerant herb in promoting mercury methylation in water-level fluctuation zones.

The water-level fluctuation zone (WLFZ) has been considered as a hotspot for mercury (Hg) methylation. Flooding-tolerant herbs are gradually acclimated to this water-land ecotone, tending to form substantial root systems for improving erosion resistance. Accompanying rhizosphere microzone plays crucial but unclear roles in methylmercury (MeHg) formation in the WLFZ. Thus, we conducted this study in the WLFZ of the Three Gorges Reservoir, to explore effects of the rhizosphere of a dominant flooding-tolerant herb (bermudagrass) on MeHg production. The elevated Hg and MeHg in rhizosphere soils suggest that the rhizosphere environment provides favorable conditions for Hg accumulation and methylation. The increased bioavailable Hg and microbial activity in the rhizosphere probably serve as important factors driving MeHg formation in the presence of bermudagrass. Simultaneously, the rhizosphere environments changed the richness, diversity, and distribution of hgcA-containing microorganisms. Here, a typical iron-reducing bacterium (Geobacteraceae) has been screened, however, the majority of hgcA genes detected in rhizosphere, near-, and non-rhizosphere soils of the WLFZ were unclassified. Collectively, these results provide new insights into the elevated MeHg production as related to microbial processes in the rhizosphere of perennial herbs in the WLFZ, with general implications for Hg cycling in other ecosystems with water-level fluctuations.

Evaluation of Hg methylation in the water-level-fluctuation zone of the Three Gorges Reservoir region by using the MeHg/HgT ratio.

In the recent decade, the hydroelectric reservoir is identified as a methylmercury (MeHg) hotspot and gained much attention. The artificial water level management in the Three Gorges Reservoir (TGR) in China formed a water-level-fluctuation zone (WLFZ) undergoing flooding drying rotations annually. However, the mercury (Hg) methylation and major geochemical driving factors at different elevations in the WLFZ remain unclear. Here we use total Hg (HgT) normalized MeHg (MeHg/HgT ratio) to evaluate Hg methylation degree in a one-year field study at 155, 165 m elevations in the WLFZ and with >175 m elevation as the reference. Results demonstrate that MeHg/HgT ratio at the WLFZ could reach 4.1% in soils, and both 155 and 165 m elevations have a higher Hg methylation degree than the >175 m elevation. However, the differences in MeHg/HgT ratios both in soils and waters between 155 and 165 m elevations are not significant. This indicates the influence of different submerging periods on the MeHg/HgT at the WLFZ elevations is not observed. The significant correlation between the MeHg/HgT ratio and soil organic carbon (SOC) content implies a MeHg retention in re-exposed soils after flooding. Decoupling of MeHg/HgT ratios between submerged soil and overlying water are found at both elevations and therefore make MeHg/HgT in waters alone cannot be used to evaluate Hg methylation degree in this study. The calculation of HgT and MeHg partitioning coefficient (Kd) found an immobilization of MeHg by submerged soils at the WLFZ during the flooding period. Major geochemical factors, determined through principal component analysis (PCA), in affecting Hg methylation are the redox cycling of sulfur and the distribution of organic matters in the WLFZ.

A Review of Studies on the Biogeochemical Behaviors of Mercury in the Three Gorges Reservoir, China.

The Three Gorges Reservoir (TGR) is a relatively large reservoir, and its water level management actions produce a widespread water level fluctuation zone (WLFZ), which has characteristics of both terrestrial and aquatic ecosystems. Here, an integrated overview of current knowledge on Hg behaviors in the TGR, especially the WLFZ, as well as exposure risk to local residents was presented. Hg levels in the TGR were comparable with other natural aquatic systems. WLFZ in the TGR was confirmed to be an environment favorable for Hg methylation by enhancing microbial activity, promoting sulfur cycling and increasing the level of low-molecular-weight organic matters. However, elevated fish Hg concentrations did not follow the impoundment of TGR, indicating no obvious reservoir effect, while it is still noteworthy that frequently consuming fish is likely to be a methylmercury (MeHg) exposure pathway for specific populations e.g. fishermen around the TGR.

Water level fluctuations influence microbial communities and mercury methylation in soils in the Three Gorges Reservoir, China.

Reservoirs tend to have enhanced methylmercury (MeHg) concentrations compared to natural lakes and rivers, and water level fluctuations can promote MeHg production. Until now, little research has been conducted on the effects of microorganisms in soils for the formation of MeHg during different drying and flooding alternating conditions in the Three Gorges Reservoir (TGR). This study aimed to understand how water level fluctuations affect soil microbial composition and mercury concentrations, and if such microbial variations are related to Hg methylation. The results showed that MeHg concentrations and the ratios of MeHg to THg (MeHg%) in soils were higher in the seasonally drying and flooding alternating areas (DFAs, 175-155m) than those in the non-inundated (NIAs, >175m) and inundated areas (IAs, <145m). However, MeHg% in all samples was less than 1%, indicating that the Hg methylation activity in the soils of the TGR was under a low level. 454 high-throughput sequencing of 16S rRNA gene amplicons showed that soil bacterial abundance and diversity were relatively higher in DFA compared to those in NIA and IA, and microbial community composition varied in these three areas. At the family level, those groups in Deltaproteobacteria and Methanomicrobia that might have many Hg methylators were also showed a higher relative abundance in DFA, which might be the reason for the higher MeHg production in these areas. Overall, our results suggested that seasonally water level fluctuations can enhance the microbial abundance and diversity, as well as MeHg production in the TGR.

Response of soil physico-chemical properties to restoration approaches and submergence in the water level fluctuation zone of the Danjiangkou Reservoir, China.

With the completion of the Danjiangkou Dam, the impoundment and drainage of dams can significantly alter shorelines, hydrological regime, and sediment and can result in the loss of soil and original riparian vegetation. Revegetation may affect soil properties and have broad important implications both for ecological services and soil recovery. In this work, we investigated the soil properties under different restoration approaches, and before and after submergence in the water level fluctuation zone (WLFZ) of the Danjiangkou Reservoir. Soil physical (bulk density and soil moisture), chemical (pH, soil organic carbon, nitrogen, phosphorus and potassium contents), and heavy metals were determined. This study reported that restoration approaches have impacts on soil moisture, pH, N, soil organic carbon, P, K and heavy metals in the WLFZ of the Danjiangkou Reservoir. Our results indicated that different restoration approaches could increase the soil moisture while decrease soil pH. Higher soil organic carbon in propagule banks transplantation (PBT) and shrubs restoration (SR) indicate that PBT and SR may provide soil organic matter more quickly than trees restoration (TR). SR and TR could significantly improve the soil total P and available P. PBT and SR could improve the soil total K and available K. SR and TR could significantly promote Cu and Zn adsorption, and Pb and Fe release by plant. Submergence could significantly affect the soil pH, NO3－-N, NH4＋-N, total P and available P. Submergence could promote NO3－-N and available P adsorption, and NH4＋-N and total P release by soil. The soil quality index (SQI) values implied that TR and PBT greatly improved soil quality. The present study suggests that PBT and TR could be effective for soil restoration in WLFZ of the Danjiangkou Reservoir.

Characterization of heavy metal contamination in the soil and sediment of the Three Gorges Reservoir, China.

This paper analyzes the concentration, distribution, bioavailability, and potential heavy metal contamination risk of Cu, Pb, Cd, Zn, and Cr in the soil and sediment of the Three Gorges Reservoir (TGR). In this paper, 14 stations that cover the upper reaches to the lower reaches of the TGR were selected. The spatial distribution of heavy metals in the TGR showed that the average concentrations of Cu, Pb, Cd, Zn, and Cr were higher in the upper and lower reaches than those in the middle reaches because of industrial and agricultural activities as well as natural processes (e.g., soil erosion, rock weathering). The results also indicated that multiple pollution sources and complex geomorphological, geochemical and biological processes resulted in remarkably higher heavy metal concentrations in the soils of the water-level-fluctuation zone (WLFZ) than in the soils of the banks. The Cu, Pb, Cd, Zn, and Cr concentrations in the soils of the TGR did not exceed their respective maximum allowable concentration (MAC) values for agricultural soils in China, indicating that the soil in the TGR was not seriously contaminated with Cu, Pb, Cd, Zn, or Cr. However, the mean concentrations of all the studied metals in the sediments were higher than the geochemical background values and much higher than those in the soils, thus indicating the effect of the pollution sources and the altered hydrologic conditions that occurred after the impoundment of the TGR. A geoaccumulation index analysis indicated that the TGR sediments were moderately polluted with Cu and Cd, unpolluted to moderately polluted with Pb and Cr, and unpolluted with Zn. Fractionation studies indicated that Cd was mainly present in the non-residual fractions and exhibited great instability and bioavailability; furthermore, the alternating wetting and drying of the WFLZ soils enhance the mobility and bioavailability of Cd. Thus, greater attention should be paid to Cd pollution in the TGR because of its higher risk assessment values and potentially adverse biological effects.

Nitrite-dependent anaerobic methane oxidizing bacteria along the water level fluctuation zone of the Three Gorges Reservoir.

The nitrite-dependent anaerobic methane oxidation (n-damo) mediated by "Candidatus Methylomirabilis oxyfera" connects the biogeochemical carbon and nitrogen cycles in a novel way. Many environments have been reported to harbor such organism being slow-growing and oxygen-sensitive anaerobes. Here, we focused on the population of n-damo bacteria in a fluctuating habitat being the wetland in the water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) in China. A molecular approach demonstrated positive amplifications when targeting the functional pmoA gene only in the lower sites which endured longer flooding time in an elevation gradient. Only 1 operational taxonomic unit (OTU) in the lower elevation zone targeting the 16S ribosomal RNA (rRNA) gene was clustering into the NC-10 group a, which is presumed to be the true n-damo group. Moreover, a relatively low level of diversity was observed in this study. The abundances were as low as 4.7 × 10(2) to 1.5 × 10(3) copies g(-1) dry soil (ds) in the initial stage, which were almost the lowest reported. However, an increase was observed (3.2 × 10(3) to 5.3 × 10(4) copies g(-1) ds) after nearly 6 months of flooding. Intriguingly, the abundance of n-damo bacteria correlated positively with the accumulated flooding time (AFT). The current study revealed that n-damo bacteria can be detected in a fluctuating environment and the sites with longer flooding time seem to be preferred habitats. The water flooding may be the principal factor in this ecosystem by creating anoxic condition. The wide range of such habitats suggests a high potential of n-damo bacteria to play a key role in natural CH4 consumption.

Visual Sensing for Urban Flood Monitoring.

With the increasing climatic extremes, the frequency and severity of urban flood events have intensified worldwide. In this study, image-based automated monitoring of flood formation and analyses of water level fluctuation were proposed as value-added intelligent sensing applications to turn a passive monitoring camera into a visual sensor. Combined with the proposed visual sensing method, traditional hydrological monitoring cameras have the ability to sense and analyze the local situation of flood events. This can solve the current problem that image-based flood monitoring heavily relies on continuous manned monitoring. Conventional sensing networks can only offer one-dimensional physical parameters measured by gauge sensors, whereas visual sensors can acquire dynamic image information of monitored sites and provide disaster prevention agencies with actual field information for decision-making to relieve flood hazards. The visual sensing method established in this study provides spatiotemporal information that can be used for automated remote analysis for monitoring urban floods. This paper focuses on the determination of flood formation based on image-processing techniques. The experimental results suggest that the visual sensing approach may be a reliable way for determining the water fluctuation and measuring its elevation and flood intrusion with respect to real-world coordinates. The performance of the proposed method has been confirmed; it has the capability to monitor and analyze the flood status, and therefore, it can serve as an active flood warning system.

Influence of anti-seasonal inundation on geochemical processes of arsenic speciation in the water-level-fluctuation zone soil of the Three Gorges Reservoir, China.

Since the completion of Three Gorges Dam, the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) experiences the periodic anti-seasonal inundation. However, knowledge for mechanisms of mobilization and transformation of arsenic (As) in WLFZ soils of the TGR remains scarce. To address this gap, a combination of field observation and simulated flooding experiments attempts to illustrate the As mobilization, the transformation between As(V) and As(III), and the factors driving these processes. The study revealed that anti-seasonal inundation (with a temperature at 13 â„ƒ) mitigated As release from submerged soils. Interestingly, the total As and ratio of As(III) (the more toxic form of As) concentrations in porewater at 13 â„ƒ was lower, and the prevalence of As(III) occurred later than those at 32 °C (imitate the seasonal inundation condition). The results indicated that the As reduction and the corresponding toxic risks in submerged soils were alleviated under anti-seasonal inundation. The study proposes the reduction of As-bearing manganese (Mn) mineral assemblages and competitive adsorption of dissolved organic carbon (DOC) as primary mechanisms for As mobilization. Furthermore, microorganism-mediated detoxification/reduction processes involving DOC, nitrogen, and Mn (oxyhydr)oxides were identified as central pathways for As(III) enrichment under anti-seasonal inundation. This study enhances understandings of the biogeochemical processes and fate of As in WLFZ soils influenced by artificial regulation of the reservoir.

The influence mechanism of water level operation on algal blooms in canyon reservoirs and bloom prevention.

The water level operation of reservoirs affects the spatiotemporal patterns of water quality, light-heat, hydrodynamics and phytoplankton, which have implications for algal bloom prevention. However, the theoretical analysis and practical applications of related research are limited. Based on prototype observations and numerical modeling, data on algae, water level operation and environmental factors in the Zipingpu Reservoir from April and September in 2015 to 2017 and 2020 to 2022 were collected. An in-depth analysis of the causal mechanisms between algal blooms and water level operation was performed, and prevention strategies with practical application assessments were developed. Water level operation control in the reservoir from April to September can be divided into five stages (falling-rising-oscillating-falling-rising), with algal blooms occurring only in the second stage. The rising water level with inflow into the middle layers shapes a closed-loop circulation in the surface waters. This distributes the nutrients that were trapped in the surface layer during the first stage, helping algae avoid to phosphorus limitation and thrive in the closed loop circulation, leading to algal blooms (chlorophyll-a exceeding 10 mg/m3). There is a significant positive correlation (p < 0.05) between algal blooms and the rapid rise in water levels in the second stage, occurring within a span of three days. To contain the algal bloom, a water level operation limit of rising waters on the third day after a two-day consecutive rise in water level was examined. This was found to be effective after its practical application to the case reservoir in 2022, with chlorophyll-a concentrations consistently below 10 mg/m3. This study unveils the mechanisms through which water level operation affects algal blooms and presents a successful case of bloom prevention. Furthermore, it serves as a valuable reference for the management of canyon reservoirs.

The coupling of sulfide and Fe-Mn mineral promotes the migration of lead and zinc in the redox cycle of high pH floodplain soils.

In this study, we investigated the impact of fluctuating water levels on the distribution of lead (Pb) and zinc (Zn) in soil and sediments at a historical Pb-Zn smelting site along the Xiangjiang River. Despite the high pH levels (7 to 11) in the study area, which generally inhibits heavy metal solubility, we found that regular changes in water levels still affect Pb-Zn movement. Soil analysis revealed distinct redox zones within the unconfined aquifer, as shown by the variable Fe/Mn and Ce/Ce* ratios. Advanced techniques such as Mn K-edge XAFS, Mössbauer spectroscopy, and TOF-SIMS indicated persistent Fe-Mn redox cycling and highlighted the presence of Pb and Zn-rich manganese oxides near sulfur-bearing minerals. These findings suggest that acidic microzones produced by the oxidation of sulfur-bearing minerals become "refuges" for microbial and heavy metal activity. Considering that sulfur-containing minerals are widespread waste types in nonferrous metal smelting sites, these findings are instructive for a better understanding of the transformation mechanisms of heavy metal ions in nonferrous metal smelting-polluted environments and for guiding pollution remediation strategies.

UAV-based modelling of vegetation recovery under extreme habitat stresses in the water level fluctuation zone of the Three Gorges Reservoir, China.

Impoundment of the Three Gorges Reservoir on the upper Yangtze River has remarkably altered hydrological regime within the dammed reaches, triggering structural and functional changes of the riparian ecosystem. Up to date, how vegetation recovers in response to compound habitat stresses in the water level fluctuation zone remains inexplicitly understood. In this study, plant above-ground biomass (AGB) in a selected water level fluctuation zone was quantified to depict its spatial and temporal pattern using unmanned aerial vehicle (UAV)-derived multispectral images and screened empirical models. The contributions of multiple habitat stressors in governing vegetation recovery dynamics along the environmental gradient were further explored. Screened random forest models indicated relatively higher accuracy in AGB estimation, with R2 being 0.68, 0.79 and 0.62 during the sprouting, growth, and mature periods, respectively. AGB displayed a significant linear increasing trend along the elevational gradient during the sprouting and early growth period, while it showed an inverted U-shaped pattern during late growth and mature period. Flooding duration, magnitude and timing were found to exert greater negative effects on plant sprouting and biomass accumulation and acted as decisive factors in governing the elevation-dependent pattern of AGB. Localized spatial variations in AGB were modulated by other stressors such as sediment burial, soil erosion, soil moisture and nutrient content. Occurrence of episodic summer floods and vegetation distribution were responsible for an inverted U-shaped pattern of AGB during the late growth and mature period. Generally, AGB reached its peak in August, thereafter an obvious decline by an unprecedent dry-hot climatic event. The water level fluctuations with cumulative flooding effects exerted substantial control on AGB temporal dynamics, while climatic condition played a secondary role. Herein, further restorative efforts need to be directed to screening suitable species, maintaining favorable soil condition, and improving vegetation pattern to balance the many trade-offs.

Elucidation of the dominant factors influencing N2O emission in water-level fluctuation zones in a karst canyon reservoir, southwest China.

The water-level fluctuations zones (WLFZs) are crucial transitional interfaces within river-reservoir systems, serving as hotspots for N2O emission. However, the comprehension of response patterns and mechanisms governing N2O emission under hydrological fluctuation remains limited, especially in karstic canyon reservoirs, which introduces significant uncertainty to N2O flux assessments. Soil samples were collected from the WLFZs of the Hongjiadu (HJD) Reservoir along the water flow direction from transition zone (T1 and T2) to lacustrine zone (T3, T4 and T5) at three elevations for each site. These soil columns were used to conduct simulation experiments under various water-filled pore space gradients (WFPSs) to investigate the potential N2O flux pattern and elucidate the underlying mechanism. Our results showed that nutrient distribution and N2O flux pattern differed significantly between two zones, with the highest N2O fluxes in the transition zone sites and lacustrine zone sites were found at 75 % and 95 % WFPS, respectively. Soil nutrient loss in lower elevation areas is influenced by prolonged impoundment durations. The higher N2O fluxes in the lacustrine zone can be attributed to increased nutrient levels resulting from anthropogenic activities. Furthermore, correlation analysis revealed that soil bulk density significantly impacted N2O fluxes across all sites, while NO3-and SOC facilitated N2O emissions in T1-T2 and T4-T5, respectively. It was evident that N2O production primarily contributed to nitrification in the transition zone and was constrained by the mineralization process, whereas denitrification dominated in the lacustrine zone. Notably, the annual N2O efflux from WLFZs accounted for 27 % of that from the water-air interface in HJD Reservoir, indicating a considerably lower contribution than anticipated. Nevertheless, this study highlights the significance of WLFZs as a vital potential source of N2O emission, particularly under the influence of anthropogenic activities and high WFPS gradient.