{"paper_id":"39790d33-e34f-45e6-864a-183f3b9a2028","body_text":"Sediment Mercury and Methylmercury Dynamics During Reservoir Operation | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Sediment Mercury and Methylmercury Dynamics During Reservoir Operation Shuai Liu, Lei Liao, Jing Liu, Xianglong Wei This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6041669/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 23 May, 2025 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract Mercury (Hg) and methylmercury (MeHg) are highly neurotoxic substances, and reservoirs play a critical role in Hg storage while providing geochemical environments conducive to MeHg production. However, the mechanisms of Hg transport and methylation during reservoir regulation remain poorly understood. This study investigates the spatial distribution of sediment Hg and MeHg in both horizontal and vertical profiles within the Nuozhadu Reservoir, located in the upper Mekong River. Using the diffusive gradients in thin films (DGT) technique, we observed that sediment Hg and MeHg concentrations initially increased and then slightly decreased from the tail to the dam front of the reservoir. This trend is primarily attributed to the transport of fine sediment particles influenced by reservoir interception. MeHg production was observed during sediment transport, but hydraulic conditions driving sediment accumulation emerged as the dominant source of sediment MeHg. Furthermore, vertical fluctuations in DGT-Hg and DGT-MeHg values were noted, likely caused by periodic reservoir operations. Our findings indicate that sediments release Hg into the overlying water, establishing sediments as a significant source of Hg pollution in reservoirs. This study enhances our understanding of Hg transport and methylation processes under the influence of reservoir regulation. Earth and environmental sciences/Environmental sciences Earth and environmental sciences/Biogeochemistry Mercury Methylmercury Sediment Reservoir operation Lancang River Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Mercury (Hg) and methylmercury (MeHg) are highly neurotoxic substances that are widely distributed across the globe 1 – 3 . Since the Industrial Revolution, the background concentration of Hg in the environment has increased 3–5 times, and the MeHg concentration in Atlantic cod surged by as much as 23% between the 1970s and 2000s 4 . Additionally, overfishing driven by dietary changes has exacerbated this issue. Human exposure to Hg and MeHg is linked to long-term neurocognitive deficits, with significant impacts on children that persist into adulthood. The global economic burden of such exposure is estimated to exceed US $ 1,173 billion 5 , 6 . Given the continuous rise in Hg and MeHg levels, this problem is expected to escalate further in the future. Rivers are recognized as hotspots for Hg transportation, methylation, and bioaccumulation and have consequently drawn significant global attention. Dam construction in rivers introduces several environmental changes, including hydrodynamic reduction, substance interception, and the creation of anoxic conditions. These alterations influence the spatial distribution and methylation of Hg 7 , 8 . For instance, studies have reported that MeHg production in reservoirs can lead to a 10-fold increase in riverine MeHg levels and a 2.6-fold increase in concentrations within estuarine surface waters 9 . Sediment is also considered a crucial reservoir for Hg 10 , but the fate of Hg within sediments is highly complex and influenced by a variety of factors 11 , 12 . For example, Azaroff et al. (2019) found that high Hg content is often associated with the accumulation of organic matter and fine sediment particles 13 . Similarly, Kelly et al. (2018) demonstrated that Hg is primarily transported via fine particles, which subsequently settle in low-energy environments, leading to elevated Hg concentrations 14 . Liem-Nguyen et al. (2016) observed that the deposition of autochthonous organic matter in sediment zones enhances MeHg production 15 . Furthermore, Diez et al. (2016) highlighted the role of Hg methylation occurring in suspended particles and organic matter within the water column of Lake Geneva 16 . Hg distribution and methylation in static reservoir systems have been extensively studied 17 . However, the effects of reservoir regulation on Hg distribution and methylation remain poorly understood 18 , 19 . Reservoir regulation, as an environmental factor, may further influence the fate of Hg by altering sediment activity. Specifically, the mobility of Hg in sediments plays a critical role in determining its bioavailability within aquatic ecosystems 20 , 21 . During reservoir regulation, sediment particle transport occurs, and variations in mobile sediment particle Hg and organic carbon (OC) levels may affect Hg methylation processes 22 , 23 . Vertical profiles of Hg species in sediments are easily disturbed by reservoir regulation and can exhibit significant variability within just a few millimeters 24 , 25 . Consequently, high spatial resolution is essential for understanding the Hg biogeochemical cycle during reservoir regulation processes 26 . Sediment transport in reservoirs typically involves the mixing of bottom sediments with the overlying water column, which is often anoxic, within a depth of several centimeters 27 , 28 . This mixing is primarily driven by hydrodynamic disturbances, such as reservoir regulation and wave activity, which can result in several adverse environmental effects. On one hand, Hg redistribution during sediment transport is influenced by the competition between particles of varying grain sizes under hydrodynamic disturbances 14 , 22 . On the other hand, OC is repeatedly incorporated into sediment particles during transport 29 , 30 . Therefore, sediment transport during reservoir regulation can significantly impact Hg redistribution and methylation within sediment particles. To test this hypothesis, we investigated the spatial patterns of sediment Hg and MeHg in both horizontal and vertical profiles using the diffusive gradients in thin films (DGT) technique in the Nuozhadu (NZD) Reservoir. The primary goals of this study were to (1) examine the vertical and spatial distributions of Hg species in sediment profiles and (2) clarify the transport potential of sediment-bound Hg and assess sedimentation processes under the influence of reservoir regulation. Materials and Methods Study Area The Lancang River, located upstream of the Mekong River, flows through Myanmar after traversing Yunnan Province in Southwest China. The river has an annual mean discharge of 2,180 m 3 /s and a catchment area of 167,487 km².As shown in Fig. 1 , the Nuozhadu (NZD) Reservoir, one of the largest dams in the upper Mekong River, spans a 261.5 km stretch of the Lancang River's main channel. It has a storage capacity of 23.7 × 10 9 m³ and a total catchment area of 1.5 × 10 5 km². The NZD Reservoir is part of a cascade system, with the Dachaoshan (DCS) Reservoir located upstream and the Jinghong (JH) Reservoir downstream. The region's climate is classified as temperate or subtropical monsoon, with annual temperatures ranging from 12°C to 27°C. The mean annual precipitation is approximately 1,399.6 mm, of which 80% occurs during the rainy season from May to October. Sample Collection The field survey was conducted in October 2018 across an approximately 230 km valley of the Lancang River. Surface sediment samples were obtained at 10 sites from the reservoir tail to the dam front of the NZD Reservoir at a spacing of approximately 25 km. This arrangement accounts for the spatial heterogeneity in hydrodynamic and sedimentation conditions (Fig. 1 , red circles). The surface sediment samples, denoted as S1 to S10, were collected with an Ekman grab sampler. At each sampling site, samples were collected in triplicate after complete homogenization. Overall, the 200.0 g sediment samples were stored in 50.0 mL tubes and frozen in the dark until Hg, MeHg, OC and grain size analyses were conducted. Separate 5.0 g sediment samples were stored in 5.0 mL Eppendorf (EP) tubes for hgc AB analysis. Notably, sediment cores were extracted with a Uwitec gravity corer near the central river at the reservoir tail, reservoir middle, and dam front sites of the NZD Reservoir in April 2019 (Fig. 1 , yellow circles). The sediment cores were sealed with the intention of preventing interference by air and oxygen. The sediment cores were subsequently maintained upright, delivered to the laboratory and stabilized immediately for at least 24 h. The new flat DGT components included a plastic base, 3-mercaptopropyl-functionalized silica resin, 1.5% agarose gel as the diffusive layer, a plastic cover plate and a 0.13 mm-thick cellulose nitrate filter membrane (Whatman, 0.45 mm pore size). The reactivity of 3-mercaptopropyl-functionalized silica gel is 79%. Before deployment, the DGT probes were deoxygenated with N2 in Milli-Q water for 1 day. Notably, two DGT probes for Hg and MeHg were pushed into each sediment core smoothly and gently until the mark matched the sediment-water interface position. The displaced push-down water was obtained and preserved at a height of approximately 5.0 cm. The DGT probes were laid out for 24 h in the dark at each sample site. During deployment, the pH varied between 6.6 and 7.3, with the temperature remaining stable at 19.6°C ± 0.8°C, which suggests steady environmental conditions. Following retrieval, the probes were washed with Milli-Q water. Each resin gel was then segregated into 2.0 mm (vertical) × 6.0 mm (horizontal) slides in a clean laboratory for Hg and MeHg analysis. Sample Analysis All the sediments were dried at -60°C for 7 days in the dark to remove water with a freeze dryer (Christ ALPHA 2–4 LD plus, Germany). A portion of the dried sediment was passed through a 0.15-mm mesh prior to Hg, MeHg and OC analysis. Hg in the sediment and resin gel was analyzed with a direct mercury analyzer (DMA-80, Milestone, US). The MeHg concentrations in the sediment and resin gel were obtained with a methylmercury analyzer (Tekran 2600, Tekran Inc., Canada). The total OC content in the sediment was analyzed with an elemental profiler (Vario MACRO cube, Elementar Inc., Germany). The size of the sediment particles was determined via a Mastersizer 2000 laser grain size meter (clay: < 4.0 µm m; silt: 4.0–64.0 µm; sand: > 64.0 µm). Following grain size analysis, the sediment samples were distributed via an ultrasonic oscillator (PS-60A, Shenzhun Shenghuatai ultrasonic equipment Co., Ltd., China). The quantitative polymerase chain reaction (qPCR) technique was employed to quantify the microbes involved in Hg methylation in the sediment samples. Several functional genes control the Hg methylation process, of which hgc AB in sulfate-reducing microorganisms is the most common biomarker for Hg methylation and was selected for qPCR analysis in this study. DNA extraction was performed with Powersoil DNA Isolation Kits following the manufacturer’s guidelines. qPCR analysis was performed with primers targeting hgc AB. Gene copies were enlarged and quantified within a Bio-Rad cycler containing the iQ5 real-time fluorescence detection mechanism and software (v 2.0; Bio-Rad). The total reactions were finished within a total volume of 20 µL and included 10 µL of SYBR® Premix Ex TaqTM (TOYOBO, Japan), 0.8 µL of BSA (3 mg mL – 1 , Sigma) (each primer: 0.5 mM), dual-distilled water and template DNA. For hgc A, the following qPCR program was adopted: 95°C for 3 min; 95°C for 15 s; 65°C for 20 s (40 times); and extension for 4 min at 72°C. A standard curve was generated via serial dilution (10–2 ~ 10–8) of the identified density plasmid DNA with target fragments. The total PCRs were performed in triplicate in 96-well plates (Bio-Rad) sealed with optical-quality sealing tape (Bio-Rad). In the absence of a DNA template, negative controls were included for each PCR operation. Data Processing The DGT-Hg and MeHg contents in the sediment column were determined as Eq. 1 : Where M denotes the mass of Hg and MeHg accumulated on the resin gel, △g denotes the diffusive layer thickness, D denotes the diffusive coefficient of Hg of the gel, A denotes the region of the exposed surface, and t denotes the exposure time. The diffusive flux (J) is the sum of the fluxes of sediment (Js) and overlying water (Jw). In accordance with Fick’s law, the diffusive flux of DGT-Hg was determined as Eq. 2: Where J w is the DGT-Hg flux between water and the sediment water interface, J s is the DGT-Hg flux from the sediment to the sediment water interface, and D s and D w denote the diffusive coefficients of Hg of the porewater and natural water, respectively. In accordance with the research of Gao et al. (2018), a sediment porosity (φ) of 0.9 was selected 31 . Moreover, (δCDGT/δxs)(x = 0) and (δCDGT/δxw)(x = 0) denote the DGT-Hg content gradients within the sediment and overlying water columns, respectively. Notably, gradients within 1 cm between the sediment and water interface were determined. All analyses were conducted in SPSS 22.0, and any differences were considered statistically significant at p < 0.05. Results Sediment Hg in the NZD Reservoir In the NZD Reservoir, sediment Hg content exhibited a spatial pattern, first increasing and then slightly decreasing from the tail to the dam front, while sediment grain size (d 50 ) showed a gradual decrease (Fig. 2 A). In the reservoir tail area, sediment Hg content remained relatively stable. However, it increased from 77.12 ng g⁻¹ at site S4 to 114.14 ng g⁻¹ at site S8 in the reservoir middle area before decreasing to 85.68 ng g⁻¹ at site S10 in the dam-front area. Similarly, sediment grain size (d 50 ) remained unchanged in the reservoir tail area but decreased from 7.61 µm at site S4 to a minimum of 5.01 µm at site S8 in the reservoir middle area, followed by an increase to 6.33 µm at site S10 in the dam-front area. Grain size analysis revealed that silt dominated the sediments in the NZD Reservoir, with clay, silt, and sand accounting for 32.09%, 64.18%, and 3.73%, respectively. The proportion of clay in the sediments showed minimal variation in the reservoir tail area but increased slightly from 27.82% at site S4 to a maximum of 38.21% at site S8 in the reservoir middle area, before decreasing to 33.15% at site S10 in the dam-front area. Correlation analysis indicated that sediment Hg content was positively correlated with clay content (r² = 0.66, p < 0.05) and negatively correlated with silt and sand contents (Fig. 2 B). In contrast to the reservoir tail and dam-front sediment columns, the reservoir middle sediment column exhibited greater spatial heterogeneity in DGT-Hg concentrations. The mean (± SD) DGT-Hg concentrations in porewater at the reservoir tail, reservoir middle, and dam-front sampling sites were 29.14 ± 3.93 ng L⁻¹, 33.87 ± 8.68 ng L⁻¹, and 29.66 ± 2.27 ng L⁻¹, respectively. Similarly, the DGT-Hg concentrations in sediment at the reservoir tail, reservoir middle, and dam front sites were 33.99 ± 1.51 ng L⁻¹, 34.87 ± 5.10 ng L⁻¹, and 15.55 ± 2.72 ng L⁻¹, respectively. The DGT-Hg concentrations at the reservoir tail and reservoir middle sites were more than double those at the dam-front site. The net Hg flux from sediment to the overlying water varied across sites, reaching 7.36, 3.25, and 3.69 ng d⁻¹ cm⁻² at the dam front, reservoir middle, and reservoir tail sites, respectively (Fig. 5 ). Notably, the Hg flux at the reservoir tail site was approximately twice as high as that at the reservoir middle and dam front sites. Sediment MeHg and MeHg/Hg Ratio in the NZD Reservoir In the NZD Reservoir, sediment MeHg and %MeHg values were lower at the reservoir tail but increased from 0.41 ng g⁻¹ and 0.53% at site S4 to 0.68 ng g⁻¹ and 0.74% at site S7, respectively. These values then slightly decreased to 0.44 ng g⁻¹ and 0.60%, respectively, at the dam front (Fig. 3 A). In contrast, sediment organic carbon (OC) content increased from 11.26 mg g⁻¹ at site S1 in the reservoir tail to a maximum of 25.17 mg g⁻¹ at site S8 in the reservoir middle, before slightly decreasing to 24.30 mg g⁻¹ at the dam front. Additionally, the abundance of hgc AB genes, associated with Deltaproteobacteria, generally increased along the reservoir, from 19.14 × 10⁵ gene copies per gram of sediment at the reservoir tail to 48.24 × 10⁵ gene copies per gram at the dam front (Fig. 3 B). The spatial variations in DGT-MeHg concentrations during the first sediment layer (0–60 mm depth) were significantly greater than those in the deeper layers and decreased rapidly below a depth of 60 mm (Fig. 3 ). Mean (± SD) DGT-MeHg concentrations in porewater during the first 60 mm were 0.25 ± 0.17 ng L − 1 , 0.59 ± 0.22 ng L − 1 , and 0.17 ± 0.14 ng L − 1 at the reservoir tail, reservoir middle, and dam front sites, respectively. In the reservoir tail sediment column, DGT-MeHg concentrations slightly increased near the sediment water interface at an 8 mm depth, followed by a sharp decrease at the 10 mm depth. This trend repeated at depths of 10–16 mm, 18–24 mm, 42–52 mm, 54–88 mm, and 90–100 mm. In contrast, the dam front sediment column exhibited an opposite trend, with intermittent sharp increases followed by decreases near the bottom. Specifically, DGT-MeHg concentrations sharply increased at the 10-mm depth, slightly decreased until the 30-mm depth, and repeated this pattern at depths of 32–54 mm, 56–94 mm, and 96–100 mm. Meanwhile, the reservoir middle sediment column showed continuous fluctuations in DGT-MeHg concentrations throughout the profile. The DGT-MeHg concentrations in the overlying water column were 0.05 ± 0.03 ng L − 1 , 0.55 ± 0.18 ng L − 1 , and 0.04 ± 0.02 ng L − 1 at the reservoir tail, reservoir middle, and dam front sampling sites, respectively. Notably, the DGT-MeHg concentration at the reservoir middle site was more than 11 times higher than those at the dam front and reservoir tail sites. However, due to the insufficient linearity of the DGT-MeHg concentrations in the overlying water, the MeHg diffusion flux could not be calculated. Discussion Both Hg and MeHg in Reservoir Sediments Are Determined by Hydrodynamics The Hg content in the sediment of the NZD Reservoir was high at the middle of the reservoir, where fine particles settled because of the weak hydrodynamics. The sediment Hg content decreased in the reservoir tail and dam-front areas, where coarse-grained sediment is enriched. Hence, at the basin scale, the spatial variation in Hg levels was affected by hydrodynamics and grain size variables (Fig. 2 A). The NZD Reservoir is located in an environment with limited anthropogenic disturbance, in which sediment Hg is derived mainly from nonpoint sources. With dam construction in rivers, the flow velocity decreases, and Hg adsorbed onto particulates eventually settles onto the sediment 32 . However, hydrodynamic factors influence the geographic pattern of the Hg content 13 , 14 . During hydrodynamic reinforcement, sediment Hg can be released into the overlying water column and redistributed via competition between grain particles of different sizes 20 , 22 . In the resettlement process, coarse particles resorb limited Hg and reach a steady state relatively quickly, whereas fine particles may resorb more Hg but be transported across greater distances or become deposited in nearby areas after a prolonged period to reach a steady state in the unsteady flow process of the reservoir 13 , 33 . This process can function as a “sieve” for size grouped particles, thus affecting the spatial pattern of the Hg content in sediment. Hence, a remarkable positive relationship was observed between the sediment Hg and clay contents but not with the silt and sand contents (Fig. 2 B). The decrease in the sediment Hg content between the reservoir tail and dam front can be attributed to water released from upstream via discharge and the dam outlet, which makes it comparatively difficult for fine sediment particles to settle in these areas. Inside the NZD Reservoir, the sediment MeHg content increased from the reservoir tail to the reservoir middle and subsequently decreased to the dam front, exhibiting spatial patterns similar to those of the %MeHg and Hg contents in the sediment. However, the abundance of hgc AB genes and the OC content in the sediment increased between the reservoir tail and the dam front, exhibiting different spatial patterns from those of the MeHg and %MeHg contents in the sediment (Fig. 4 ). Hg methylation might have occurred during the transport of mobile sediment particles and their redeposition in the area with weak hydrodynamics because fresh carbon was continuously reloaded in the water column, thereby providing a microanoxic environment for mobile sediment particles, which is ideal for Hg methylation 16 , 22 , 34 . Therefore, MeHg in the sediment comprises two parts, namely, MeHg from in situ production and MeHg from offsite accumulation. In the NZD Reservoir, the abundance of hgc AB and the OC content in the sediment increased between the reservoir tail and the dam front, indicating that MeHg production increased (Fig. 4 ). However, MeHg and %MeHg in the sediment were dominated by accumulated Hg. This finding indicates that mobile sediment particles are an important source of MeHg in sediments. Thus, the spatial distributions of both Hg and MeHg in sediments are determined by hydrodynamics. Deposition and Risk of Exposure to Hg and MeHg during Reservoir Operation The sediment column samples from different depths provide historical reservoir operation information 35 , 36 . Both DGT-Hg and DGT-MeHg exhibited periodic fluctuations along the vertical direction, but the fluctuations in DGT-MeHg were greater, indicating that the Hg methylation intensity varied during reservoir operation. Moreover, DGT-MeHg occurred at the middle and in the tail of the reservoir, with the first 60 mm exhibiting the largest spatial differences. In this study, the DGT-Hg distribution in the sediment column of the NZD Reservoir can be clarified on the basis of the behavior of mobile sediment particles sorted by hydrodynamics 37 . At the reservoir tail and dam front sites of the NZD Reservoir, particles were carried out because of the changing hydrodynamic conditions due to the discharge of the upstream DCS Reservoir and the dam outlet of the NZD Reservoir, respectively. Hence, the DGT-Hg concentration in the sediment column exhibited relatively high spatial heterogeneity. At the reservoir middle site of the NZD Reservoir, considerable amounts of fine particles were deposited due to the weak hydrodynamics. Thus, a low spatial heterogeneity in DGT-Hg was observed in the reservoir middle sediment column. The data revealed that Hg methylation is closely related to the characteristics of particles because the DGT-MeHg concentration was positively correlated with the %MeHg concentration in the bulk sediment but not with the DGT-Hg concentration. The irregular distribution of DGT-MeHg in the overlying water column supports the possibility that Hg methylation may occur in oxygen deficient zones of stratified water columns, whereas the grain size “sieve” phenomenon during reservoir regulation further affects the changes in DGT-MeHg in sediments. Moreover, the discontinuous change in the DGT-MeHg distribution within the sediment vertical profiles indicates that the DGT-MeHg exposure risk is influenced by human activities. The dam front zone is generally the deepest area of the reservoir, and it is particularly prone to particle resuspension and resettlement because of the operation height of turbines, which may prevent fine particle transport downstream. Thus, the DGT-MeHg concentration at the dam front sharply increased intermittently and then decreased at the bottom during reservoir regulation. However, in the reservoir tail sediment column, ripple transport caused the preferential burial of fine particles. Thus, in the reservoir tail sediment column, the DGT-MeHg concentration slightly increased from the sediment water interface to the bottom and then sharply decreased. Completely opposite characteristics of the vertical distribution were recorded compared with those at the dam front site. Compared with nonmobile sediment particles, mobile sediment particles exposed to water usually better reflect the MeHg exposure risk. Thus, the potential ecological implication for this finding is that the risk of exposure to MeHg increases during reservoir regulation. Moreover, the DGT-MeHg concentrations during the first period (0–60 mm) were significantly greater than those during the second period but decreased rapidly below 60 mm. Two main mechanisms may explain this phenomenon. One hypothesis states that seasonal MeHg variations govern the process. For example, high seasonal variations in the reservoir sediment MeHg content have been obtained at most sites in the Northern Hemisphere, with minima in winter and peaks in summer 19 , 38 . Another hypothesis is that DGT-MeHg desorption is related to organic matter mineralization advancing more slowly since organic matter in deeper sediment is more recalcitrant 39 , 40 . We can accept the second assumption, since seasonal MeHg changes commonly occur synchronously with Hg changes. In addition, a seasonal change was observed in the region, in which the change in the NZD Reservoir was very small. However, our data and information do not support the first hypothesis. Risk of Exposure to MeHg during Reservoir Regulation Hg resuspension, desorption, resorption, and deposition in mobile sediment particles are ongoing processes that occur in balance 41 . Under the change in hydrodynamics caused by reservoir regulation, fine particles can carry more Hg substances than coarse particles can, are resuspended first, and experience a prolonged period of exposure to fresh OC in the water column during transport. However, coarse particles exhibit the opposite trend. We attributed the differences in %MeHg in the DGT probes to mobile sediment particles being driven during transport. Our data revealed that Hg methylation is closely related to the characteristics of particles because the DGT-MeHg concentration was positively correlated with the %MeHg concentration in the bulk sediment but not with the DGT-Hg concentration. The irregular distribution of DGT-MeHg in the overlying water column supports the possibility that Hg methylation may have occurred in oxygen-deficient zones of stratified water columns, whereas the grain size “sieve” phenomenon during reservoir regulation may have further affected the changes in DGT-MeHg in the sediment. Moreover, the discontinuous change in the DGT-MeHg distribution within the sediment vertical profiles indicates that the DGT-MeHg exposure risk is influenced by human activities. The dam-front zone is generally the deepest area of the reservoir, and it is particularly prone to particle resuspension and resettlement because of the operation height of turbines, which may prevent fine particle transport downstream. Thus, the DGT-MeHg concentration at the dam front sharply increased intermittently and then decreased at the bottom during reservoir regulation. However, in the reservoir tail sediment column, ripple transport caused the preferential burial of fine particles. Thus, in the reservoir tail sediment column, the DGT-MeHg concentration slightly increased from the sediment water interface to the bottom and then sharply decreased. Completely opposite characteristics of the vertical distribution were recorded compared with those at the dam front site. Compared with nonmobile sediment particles, mobile sediment particles exposed to water usually better reflect the MeHg exposure risk. Thus, the potential ecological implication for this finding is that the risk of exposure to MeHg increases during reservoir regulation. Risk of Exposure to Hg and MeHg during Reservoir Regulation In the NZD Reservoir, DGT-Hg was released from the sediments into the overlying water, which indicates that sediments increasingly became the source of DGT-Hg in the overlying water. Within the overlying water column, the density gradient of DGT-derived MeHg is not suitable for linear testing, so the flux cannot be calculated. However, the density of DGT-generated MeHg within the overlying water is greater than that in the sediment at the corresponding point. These findings indicate the risk of MeHg release from sediments. Furthermore, the sediment Hg content in front of the dam and at the tail of the reservoir decreased, but there was greater release of DGT-Hg, indicating that hydrodynamic enhancement promoted Hg release from the sediment. The strong reduction conditions at the tail of the reservoir further facilitate the release of DGT-Hg, resulting in significantly greater release of DGT-Hg at the reservoir tail than in the other regions. The risk of Hg and MeHg among reservoirs is usually determined by their concentration in the water body 42 , 43 . Therefore, when studying the hazards of Hg and MeHg in reservoirs, the Hg and MeHg contents in sediments can serve as a reference, and more importantly, attention should be given to the tail area of reservoirs with strong hydrodynamics and reducing conditions. Conclusion Under the control of reservoir regulation, active transfer of sediment particles occurs along a reservoir within a canyon river, which may influence the spatial patterns of Hg in sediment. This study revealed that reservoir transport can facilitate local storage and methylation of Hg above its source point. The main conclusions are as follows: Hydrodynamics control Hg and MeHg distribution: The spatial distributions of both Hg and MeHg in the sediments of the NZD Reservoir are driven primarily by hydrodynamic conditions and the particle size. In fine particles, which settle in weak-hydrodynamics zones, relatively high Hg and MeHg levels can accumulate, whereas coarse particles, which are enriched in the reservoir tail and dam front, exhibit reduced Hg and MeHg contents due to their limited adsorption capacity. Risk of exposure to Hg and MeHg during reservoir operation: The spatial distribution of MeHg reflects both in situ production and offsite accumulation, whereas mobile sediment particles contribute significantly to MeHg accumulation. The hydrodynamic changes induced by reservoir operation affect the flux of Hg in sediments, particularly in reservoir tail areas. These hydrodynamic fluctuations increase the Hg exposure risk. Declarations Competing interests The authors declare no competing interests. Author Contribution S.L. contributed to the study’s conceptualization, investigation, review and editing, and initial drafting of the manuscript. L.L. was responsible for data curation and formal analysis, interpretation of results, and writing the initial draft of the manuscript. J.L. analyzed the results and contributed to the interpretation of findings. X.W. provided supervision and guidance throughout the study, offering critical insights and feedback on research design and analysis. All authors reviewed the manuscript. Acknowledgement This work was supported by National Natural Science Foundation of China (No. 52309098, 52309085), Natural Science Foundation of Jiangsu Province (BK20230960), Internal Talent Project of Xihua University(Z222059). Data Availability Xianglong Wei (corresponding author) has to be contacted in case of any queries or requirement of data. The datasets used and/or analysed during the current study available from the corresponding author on reasonable request. References Ke, T., Junior, F. B., Santamaria, A., Bowman, A. B. & Aschner, M. Genetic Factors in Methylmercury-Induced Neurotoxicity: What Have We Learned from Caenorhabditis Elegans Models? Adv. neurotoxicology . 9 , 271–290 (2023). Ke, T. et al. Epigenetics and Methylmercury-Induced Neurotoxicity, Evidence from Experimental Studies. Toxics 11 , 72 (2023). Chen, L. et al. Trans-Provincial Health Impacts of Atmospheric Mercury Emissions in China. Nat. Commun. 10 , 1484 (2019). 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Environ. Sci. 74 , 79–87 (2018). Gibson, B. D., Ptacek, C. J., Blowes, D. W. & Daugherty, S. D. Sediment Resuspension under Variable Geochemical Conditions and Implications for Contaminant Release. J. Soils Sediments . 15 , 1644–1656 (2015). Wang, P. et al. Impact of Macrozoobenthic Bioturbation and Wind Fluctuation Interactions on Net Methylmercury in Freshwater Lakes. Water Res. 124 , 320–330 (2017). Cui, Z. et al. Vertical Distribution Characteristics and Ecological Risk Assessment of Mercury and Arsenic in Ice, Water, and Sediment at a Cold-Arid Lake. Toxics 12 , 540 (2024). He, T., Lu, J., Yang, F. & Feng, X. Horizontal and Vertical Variability of Mercury Species in Pore Water and Sediments in Small Lakes in Ontario. Sci. Total Environ. 386 , 53–64 (2007). Poulin, B. A. et al. Biogeochemical and Hydrologic Synergy Control Mercury Fate in an Arid Land River-Reservoir System. Environ. Sci. Processes impacts . 25 , 912–928 (2023). Cieśla, M. & Gruca-Rokosz, R. Implications of Suspended Sediment in the Migration of Nutrients at the Water-Sediment Interface in Retention Reservoirs. Sci. Rep. 14 , 24924 (2024). Mermillod-Blondin, F. et al. Interactions between Sediment Characteristics and Oxygen Conditions at the Sediment–Water Interface of Reservoirs: Influences on Nutrient Dynamics and Eutrophication. Hydrobiologia 851 , 3433–3452 (2024). Dong, J. et al. Effect of Water–Sediment Regulation of the Xiaolangdi Reservoir on the Concentrations, Bioavailability, and Fluxes of PAHs in the Middle and Lower Reaches of the Yellow River. J. Hydrol. 527 , 101–112 (2015). Xia, X. et al. Effect of Water-Sediment Regulation of the Xiaolangdi Reservoir on the Concentrations, Characteristics, and Fluxes of Suspended Sediment and Organic Carbon in the Yellow River. Sci. Total Environ. 571 , 487–497 (2016). Gao, B. et al. Assessment of Cr Pollution in Tributary Sediment Cores in the Three Gorges Reservoir Combining Geochemical Baseline and in Situ DGT. Sci. Total Environ. 628–629 , 241–248 (2018). Feng, X. et al. Biogeochemical Cycle of Mercury in Reservoir Systems in Wujiang River Basin, Southwest China. Springer Singapore (2018). Willacker, J. J. et al. Reservoir Stratification Modulates the Influence of Impoundments on Fish Mercury Concentrations along an Arid Land River System. Environ. Sci. Technol. 57 , 21313–21326 (2023). Seelen, E. A., Massey, G. M. & Mason, R. P. Role of Sediment Resuspension on Estuarine Suspended Particulate Mercury Dynamics. Environ. Sci. Technol. 52 , 7736–7744 (2018). Lintern, A. et al. Sediment Cores as Archives of Historical Changes in Floodplain Lake Hydrology. Sci. Total Environ. 544 , 1008–1019 (2016). Snyder, N. P. et al. Reconstructing Depositional Processes and History from Reservoir Stratigraphy: Englebright Lake, Yuba River, Northern California. J. Geophys. Research: Earth Surf. 111 , (2006). F04003-1-F04003-16. Neal-Walthall, N., Ndu, U. & Jr, N. A. R. Utility of Diffusive Gradient in Thin-Film Passive Samplers for Predicting Mercury Methylation Potential and Bioaccumulation in Freshwater Wetlands. Environ. Sci. Technol. 56 , 1743–1752 (2022). Yao, H. et al. Mercury and methylmercury concentrations in two newly constructed reservoirs in the Wujiang River, Guizhou, China. Environ. Toxicol. Chem. 30 , 530–537 (2011). Bigham, G. N., Murray, K. J., Masue-Slowey, Y. & Henry, E. A. Biogeochemical Controls on Methylmercury in Soils and Sediments: Implications for Site Management. Integr. Environ. Assess. Manag . 13 , 249–263 (2016). Zhang, L. et al. Mercury Sorption and Desorption on Organo-Mineral Particulates as a Source for Microbial Methylation. Environ. Sci. Technol. 53 , 2426–2433 (2019). Kim, E. H., Mason, R. P., Porter, E. T. & Soulen, H. L. The Impact of Resuspension on Sediment Mercury Dynamics, and Methylmercury Production and Fate: A Mesocosm Study. Mar. Chem. 102 , 300–315 (2006). Basta, P. et al. Risk Assessment of Mercury-Contaminated Fish Consumption in the Brazilian Amazon: An Ecological Study. Toxics 11 , 800 (2023). Mao, L. et al. Occurrence and Risk Assessment of Total Mercury and Methylmercury in Surface Seawater and Sediments from the Jiaozhou Bay, Yellow Sea. Sci. Total Environ. 714 , 136539 (2020). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 23 May, 2025 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 22 Apr, 2025 Reviews received at journal 17 Apr, 2025 Reviewers agreed at journal 08 Apr, 2025 Reviews received at journal 28 Mar, 2025 Reviewers agreed at journal 10 Mar, 2025 Reviewers invited by journal 02 Mar, 2025 Editor assigned by journal 20 Feb, 2025 Editor invited by journal 20 Feb, 2025 Submission checks completed at journal 19 Feb, 2025 First submitted to journal 16 Feb, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-6041669\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":false,\"archivedVersions\":[],\"articleType\":\"Article\",\"associatedPublications\":[],\"authors\":[{\"id\":418772353,\"identity\":\"13e47ce1-ea70-4fce-83d5-c407764246ed\",\"order_by\":0,\"name\":\"Shuai Liu\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Xihua University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Shuai\",\"middleName\":\"\",\"lastName\":\"Liu\",\"suffix\":\"\"},{\"id\":418772354,\"identity\":\"4ec3c78f-c2ce-4db2-b546-0175464f0d86\",\"order_by\":1,\"name\":\"Lei Liao\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Anhui and Huaihe River Institute of Hydraulic Research\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Lei\",\"middleName\":\"\",\"lastName\":\"Liao\",\"suffix\":\"\"},{\"id\":418772355,\"identity\":\"9526d220-dfdd-4f22-94cc-ce51defefe8d\",\"order_by\":2,\"name\":\"Jing Liu\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Nanjing Hydraulic Research Institute\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Jing\",\"middleName\":\"\",\"lastName\":\"Liu\",\"suffix\":\"\"},{\"id\":418772356,\"identity\":\"93b127ae-d0cf-4f52-adf1-2193cd2734f3\",\"order_by\":3,\"name\":\"Xianglong Wei\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAApklEQVRIiWNgGAWjYDACdgbGx4wNIFYCsVqYGZiNSdbCJk2aFoPDPGbVhTsOM/Cz5xgw/NxBhBbJZh6z2zPPHGaQ7HljwNh7hggt/My8227zth1mMLiRY8DM2EaEFjaglmKQFnuitYBsYQbbIkGsFslm/s/SvG3pPBJnnhUc7CVGi8HxtsTPvG3WcvztyRsf/CRGCwzwgIgDJGgYBaNgFIyCUYAPAACIdS3o6RntPQAAAABJRU5ErkJggg==\",\"orcid\":\"\",\"institution\":\"Hohai University\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Xianglong\",\"middleName\":\"\",\"lastName\":\"Wei\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2025-02-16 14:23:20\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-6041669/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-6041669/v1\",\"draftVersion\":[],\"editorialEvents\":[{\"content\":\"https://doi.org/10.1038/s41598-025-03255-6\",\"type\":\"published\",\"date\":\"2025-05-23T15:58:34+00:00\"}],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":76834217,\"identity\":\"938bff0c-85b2-4938-b108-811b213349e2\",\"added_by\":\"auto\",\"created_at\":\"2025-02-21 08:57:07\",\"extension\":\"png\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":98257,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eLocations of the sampling sites and dams within the Nuozhadu (NZD) Reservoir.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"1.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6041669/v1/f41d44811dc319ec6ec72391.png\"},{\"id\":76834219,\"identity\":\"a877d668-971b-4e40-a063-aa5fd730782d\",\"added_by\":\"auto\",\"created_at\":\"2025-02-21 08:57:08\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":125211,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eSpatial pattern of the sediment Hg content and grain sizes in the NZD Reservoir. (A) Spatial modes of the sediment Hg content and different grain sizes in the NZD Reservoir. (B). Correlations between the sediment Hg content and the sand, silt, and clay contents in sediments. A grain size \\u0026lt; 4 μm is classified as clay, a size of 4-64 μm is classified as silt, and a size \\u0026gt; 64 μm is classified as sand. The error bars denote the standard deviations (DSs).\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"2.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6041669/v1/890d6ded8b2eff149782c887.png\"},{\"id\":76834224,\"identity\":\"bef02227-f279-4fa8-9f5f-f62acf30cc5c\",\"added_by\":\"auto\",\"created_at\":\"2025-02-21 08:57:08\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":20625,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eSediment DGT-Hg concentration and Hg diffusion flux in the NZD Reservoir. (A) Sediment DGT-Hg concentration. (B) Diffusive flux of DGT-Hg throughout the sediment-water interface.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6041669/v1/73ea38c86257f5242c234c6f.png\"},{\"id\":76834220,\"identity\":\"6650b404-2fec-4381-ade5-dc7e09e1dd46\",\"added_by\":\"auto\",\"created_at\":\"2025-02-21 08:57:08\",\"extension\":\"png\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":105150,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eSpatial patterns of MeHg and %MeHg within the NZD Reservoir. (A) Sediment MeHg and %MeHg contents in the sediment of the NZD Reservoir. (B) Spatial patterns of \\u003cem\\u003ehgc\\u003c/em\\u003eAB genes associated with Deltaproteobacteria abundance and organic carbon in the sediment of the NZD Reservoir. The error bars denote the DSs.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"4.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6041669/v1/66989e0ae8d820ab722831e7.png\"},{\"id\":76834218,\"identity\":\"48666b50-1bc1-4a8e-acd2-088cccf31a32\",\"added_by\":\"auto\",\"created_at\":\"2025-02-21 08:57:07\",\"extension\":\"png\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":42267,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eDGT-MeHg concentration and %MeHg level in the sediment with depth calculated from the DGT measurements. (A) DGT-MeHg. (B) %MeHg. The sediment–water interface is shown by the dotted line at the zero depth.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"5.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6041669/v1/59140528a5660c0ac3f0bc90.png\"},{\"id\":83460332,\"identity\":\"5994df39-4086-445e-a92a-3ba5f5aba480\",\"added_by\":\"auto\",\"created_at\":\"2025-05-26 16:12:29\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":1013931,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6041669/v1/92897f2a-2da5-4a2e-960b-129f4d3492b5.pdf\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"Sediment Mercury and Methylmercury Dynamics During Reservoir Operation\",\"fulltext\":[{\"header\":\"Introduction\",\"content\":\"\\u003cp\\u003eMercury (Hg) and methylmercury (MeHg) are highly neurotoxic substances that are widely distributed across the globe\\u003csup\\u003e \\u003cspan additionalcitationids=\\\"CR2\\\" citationid=\\\"CR1\\\" class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR3\\\" class=\\\"CitationRef\\\"\\u003e3\\u003c/span\\u003e \\u003c/sup\\u003e. Since the Industrial Revolution, the background concentration of Hg in the environment has increased 3\\u0026ndash;5 times, and the MeHg concentration in Atlantic cod surged by as much as 23% between the 1970s and 2000s\\u003csup\\u003e \\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e4\\u003c/span\\u003e \\u003c/sup\\u003e. Additionally, overfishing driven by dietary changes has exacerbated this issue. Human exposure to Hg and MeHg is linked to long-term neurocognitive deficits, with significant impacts on children that persist into adulthood. The global economic burden of such exposure is estimated to exceed US\\u003cspan\\u003e$\\u003c/span\\u003e1,173 billion\\u003csup\\u003e \\u003cspan citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR6\\\" class=\\\"CitationRef\\\"\\u003e6\\u003c/span\\u003e \\u003c/sup\\u003e. Given the continuous rise in Hg and MeHg levels, this problem is expected to escalate further in the future.\\u003c/p\\u003e \\u003cp\\u003eRivers are recognized as hotspots for Hg transportation, methylation, and bioaccumulation and have consequently drawn significant global attention. Dam construction in rivers introduces several environmental changes, including hydrodynamic reduction, substance interception, and the creation of anoxic conditions. These alterations influence the spatial distribution and methylation of Hg\\u003csup\\u003e\\u003cspan citationid=\\\"CR7\\\" class=\\\"CitationRef\\\"\\u003e7\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR8\\\" class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e\\u003c/sup\\u003e. For instance, studies have reported that MeHg production in reservoirs can lead to a 10-fold increase in riverine MeHg levels and a 2.6-fold increase in concentrations within estuarine surface waters\\u003csup\\u003e\\u003cspan citationid=\\\"CR9\\\" class=\\\"CitationRef\\\"\\u003e9\\u003c/span\\u003e\\u003c/sup\\u003e. Sediment is also considered a crucial reservoir for Hg\\u003csup\\u003e\\u003cspan citationid=\\\"CR10\\\" class=\\\"CitationRef\\\"\\u003e10\\u003c/span\\u003e\\u003c/sup\\u003e, but the fate of Hg within sediments is highly complex and influenced by a variety of factors\\u003csup\\u003e\\u003cspan citationid=\\\"CR11\\\" class=\\\"CitationRef\\\"\\u003e11\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e\\u003c/sup\\u003e. For example, Azaroff et al. (2019) found that high Hg content is often associated with the accumulation of organic matter and fine sediment particles\\u003csup\\u003e\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e\\u003c/sup\\u003e. Similarly, Kelly et al. (2018) demonstrated that Hg is primarily transported via fine particles, which subsequently settle in low-energy environments, leading to elevated Hg concentrations\\u003csup\\u003e\\u003cspan citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e\\u003c/sup\\u003e. Liem-Nguyen et al. (2016) observed that the deposition of autochthonous organic matter in sediment zones enhances MeHg production\\u003csup\\u003e\\u003cspan citationid=\\\"CR15\\\" class=\\\"CitationRef\\\"\\u003e15\\u003c/span\\u003e\\u003c/sup\\u003e. Furthermore, Diez et al. (2016) highlighted the role of Hg methylation occurring in suspended particles and organic matter within the water column of Lake Geneva\\u003csup\\u003e\\u003cspan citationid=\\\"CR16\\\" class=\\\"CitationRef\\\"\\u003e16\\u003c/span\\u003e\\u003c/sup\\u003e.\\u003c/p\\u003e \\u003cp\\u003eHg distribution and methylation in static reservoir systems have been extensively studied\\u003csup\\u003e\\u003cspan citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e\\u003c/sup\\u003e. However, the effects of reservoir regulation on Hg distribution and methylation remain poorly understood\\u003csup\\u003e\\u003cspan citationid=\\\"CR18\\\" class=\\\"CitationRef\\\"\\u003e18\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e\\u003c/sup\\u003e. Reservoir regulation, as an environmental factor, may further influence the fate of Hg by altering sediment activity. Specifically, the mobility of Hg in sediments plays a critical role in determining its bioavailability within aquatic ecosystems\\u003csup\\u003e\\u003cspan citationid=\\\"CR20\\\" class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR21\\\" class=\\\"CitationRef\\\"\\u003e21\\u003c/span\\u003e\\u003c/sup\\u003e. During reservoir regulation, sediment particle transport occurs, and variations in mobile sediment particle Hg and organic carbon (OC) levels may affect Hg methylation processes\\u003csup\\u003e\\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR23\\\" class=\\\"CitationRef\\\"\\u003e23\\u003c/span\\u003e\\u003c/sup\\u003e. Vertical profiles of Hg species in sediments are easily disturbed by reservoir regulation and can exhibit significant variability within just a few millimeters\\u003csup\\u003e\\u003cspan citationid=\\\"CR24\\\" class=\\\"CitationRef\\\"\\u003e24\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR25\\\" class=\\\"CitationRef\\\"\\u003e25\\u003c/span\\u003e\\u003c/sup\\u003e. Consequently, high spatial resolution is essential for understanding the Hg biogeochemical cycle during reservoir regulation processes \\u003csup\\u003e\\u003cspan citationid=\\\"CR26\\\" class=\\\"CitationRef\\\"\\u003e26\\u003c/span\\u003e\\u003c/sup\\u003e. Sediment transport in reservoirs typically involves the mixing of bottom sediments with the overlying water column, which is often anoxic, within a depth of several centimeters\\u003csup\\u003e\\u003cspan citationid=\\\"CR27\\\" class=\\\"CitationRef\\\"\\u003e27\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR28\\\" class=\\\"CitationRef\\\"\\u003e28\\u003c/span\\u003e\\u003c/sup\\u003e. This mixing is primarily driven by hydrodynamic disturbances, such as reservoir regulation and wave activity, which can result in several adverse environmental effects. On one hand, Hg redistribution during sediment transport is influenced by the competition between particles of varying grain sizes under hydrodynamic disturbances\\u003csup\\u003e\\u003cspan citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e\\u003c/sup\\u003e. On the other hand, OC is repeatedly incorporated into sediment particles during transport\\u003csup\\u003e\\u003cspan citationid=\\\"CR29\\\" class=\\\"CitationRef\\\"\\u003e29\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e\\u003c/sup\\u003e. Therefore, sediment transport during reservoir regulation can significantly impact Hg redistribution and methylation within sediment particles.\\u003c/p\\u003e \\u003cp\\u003eTo test this hypothesis, we investigated the spatial patterns of sediment Hg and MeHg in both horizontal and vertical profiles using the diffusive gradients in thin films (DGT) technique in the Nuozhadu (NZD) Reservoir. The primary goals of this study were to (1) examine the vertical and spatial distributions of Hg species in sediment profiles and (2) clarify the transport potential of sediment-bound Hg and assess sedimentation processes under the influence of reservoir regulation.\\u003c/p\\u003e\"},{\"header\":\"Materials and Methods\",\"content\":\"\\u003cdiv id=\\\"Sec3\\\" class=\\\"Section2\\\"\\u003e\\n \\u003ch2\\u003eStudy Area\\u003c/h2\\u003e\\n \\u003cp\\u003eThe Lancang River, located upstream of the Mekong River, flows through Myanmar after traversing Yunnan Province in Southwest China. The river has an annual mean discharge of 2,180 m\\u003csup\\u003e3\\u003c/sup\\u003e/s and a catchment area of 167,487 km\\u0026sup2;.As shown in Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e, the Nuozhadu (NZD) Reservoir, one of the largest dams in the upper Mekong River, spans a 261.5 km stretch of the Lancang River\\u0026apos;s main channel. It has a storage capacity of 23.7 \\u0026times; 10\\u003csup\\u003e9\\u003c/sup\\u003e m\\u0026sup3; and a total catchment area of 1.5 \\u0026times; 10\\u003csup\\u003e5\\u003c/sup\\u003e km\\u0026sup2;.\\u003c/p\\u003e\\n \\u003cp\\u003eThe NZD Reservoir is part of a cascade system, with the Dachaoshan (DCS) Reservoir located upstream and the Jinghong (JH) Reservoir downstream. The region\\u0026apos;s climate is classified as temperate or subtropical monsoon, with annual temperatures ranging from 12\\u0026deg;C to 27\\u0026deg;C. The mean annual precipitation is approximately 1,399.6 mm, of which 80% occurs during the rainy season from May to October.\\u003c/p\\u003e\\n\\u003c/div\\u003e\\n\\u003ch3\\u003eSample Collection\\u003c/h3\\u003e\\n\\u003cp\\u003eThe field survey was conducted in October 2018 across an approximately 230 km valley of the Lancang River. Surface sediment samples were obtained at 10 sites from the reservoir tail to the dam front of the NZD Reservoir at a spacing of approximately 25 km. This arrangement accounts for the spatial heterogeneity in hydrodynamic and sedimentation conditions (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e, red circles). The surface sediment samples, denoted as S1 to S10, were collected with an Ekman grab sampler. At each sampling site, samples were collected in triplicate after complete homogenization. Overall, the 200.0 g sediment samples were stored in 50.0 mL tubes and frozen in the dark until Hg, MeHg, OC and grain size analyses were conducted. Separate 5.0 g sediment samples were stored in 5.0 mL Eppendorf (EP) tubes for \\u003cem\\u003ehgc\\u003c/em\\u003eAB analysis.\\u003c/p\\u003e\\n\\u003cp\\u003eNotably, sediment cores were extracted with a Uwitec gravity corer near the central river at the reservoir tail, reservoir middle, and dam front sites of the NZD Reservoir in April 2019 (Fig. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e, yellow circles). The sediment cores were sealed with the intention of preventing interference by air and oxygen. The sediment cores were subsequently maintained upright, delivered to the laboratory and stabilized immediately for at least 24 h.\\u003c/p\\u003e\\n\\u003cp\\u003eThe new flat DGT components included a plastic base, 3-mercaptopropyl-functionalized silica resin, 1.5% agarose gel as the diffusive layer, a plastic cover plate and a 0.13 mm-thick cellulose nitrate filter membrane (Whatman, 0.45 mm pore size). The reactivity of 3-mercaptopropyl-functionalized silica gel is 79%. Before deployment, the DGT probes were deoxygenated with N2 in Milli-Q water for 1 day. Notably, two DGT probes for Hg and MeHg were pushed into each sediment core smoothly and gently until the mark matched the sediment-water interface position. The displaced push-down water was obtained and preserved at a height of approximately 5.0 cm. The DGT probes were laid out for 24 h in the dark at each sample site. During deployment, the pH varied between 6.6 and 7.3, with the temperature remaining stable at 19.6\\u0026deg;C\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;0.8\\u0026deg;C, which suggests steady environmental conditions. Following retrieval, the probes were washed with Milli-Q water. Each resin gel was then segregated into 2.0 mm (vertical) \\u0026times; 6.0 mm (horizontal) slides in a clean laboratory for Hg and MeHg analysis.\\u003c/p\\u003e\\n\\u003ch3\\u003eSample Analysis\\u003c/h3\\u003e\\n\\u003cp\\u003eAll the sediments were dried at -60\\u0026deg;C for 7 days in the dark to remove water with a freeze dryer (Christ ALPHA 2\\u0026ndash;4 LD plus, Germany). A portion of the dried sediment was passed through a 0.15-mm mesh prior to Hg, MeHg and OC analysis. Hg in the sediment and resin gel was analyzed with a direct mercury analyzer (DMA-80, Milestone, US). The MeHg concentrations in the sediment and resin gel were obtained with a methylmercury analyzer (Tekran 2600, Tekran Inc., Canada).\\u003c/p\\u003e\\n\\u003cp\\u003eThe total OC content in the sediment was analyzed with an elemental profiler (Vario MACRO cube, Elementar Inc., Germany). The size of the sediment particles was determined via a Mastersizer 2000 laser grain size meter (clay: \\u0026lt; 4.0 \\u0026micro;m m; silt: 4.0\\u0026ndash;64.0 \\u0026micro;m; sand: \\u0026gt; 64.0 \\u0026micro;m). Following grain size analysis, the sediment samples were distributed via an ultrasonic oscillator (PS-60A, Shenzhun Shenghuatai ultrasonic equipment Co., Ltd., China).\\u003c/p\\u003e\\n\\u003cp\\u003eThe quantitative polymerase chain reaction (qPCR) technique was employed to quantify the microbes involved in Hg methylation in the sediment samples. Several functional genes control the Hg methylation process, of which \\u003cem\\u003ehgc\\u003c/em\\u003eAB in sulfate-reducing microorganisms is the most common biomarker for Hg methylation and was selected for qPCR analysis in this study. DNA extraction was performed with Powersoil DNA Isolation Kits following the manufacturer\\u0026rsquo;s guidelines. qPCR analysis was performed with primers targeting \\u003cem\\u003ehgc\\u003c/em\\u003eAB. Gene copies were enlarged and quantified within a Bio-Rad cycler containing the iQ5 real-time fluorescence detection mechanism and software (v 2.0; Bio-Rad). The total reactions were finished within a total volume of 20 \\u0026micro;L and included 10 \\u0026micro;L of SYBR\\u0026reg; Premix Ex TaqTM (TOYOBO, Japan), 0.8 \\u0026micro;L of BSA (3 mg mL\\u003csup\\u003e\\u0026ndash;\\u003cspan class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e\\u003c/sup\\u003e, Sigma) (each primer: 0.5 mM), dual-distilled water and template DNA. For \\u003cem\\u003ehgc\\u003c/em\\u003eA, the following qPCR program was adopted: 95\\u0026deg;C for 3 min; 95\\u0026deg;C for 15 s; 65\\u0026deg;C for 20 s (40 times); and extension for 4 min at 72\\u0026deg;C. A standard curve was generated via serial dilution (10\\u0026ndash;2\\u0026thinsp;~\\u0026thinsp;10\\u0026ndash;8) of the identified density plasmid DNA with target fragments. The total PCRs were performed in triplicate in 96-well plates (Bio-Rad) sealed with optical-quality sealing tape (Bio-Rad). In the absence of a DNA template, negative controls were included for each PCR operation.\\u003c/p\\u003e\\n\\u003ch3\\u003eData Processing\\u003c/h3\\u003e\\n\\u003cp\\u003eThe DGT-Hg and MeHg contents in the sediment column were determined as Eq. \\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e:\\u003c/p\\u003e\\n\\u003cdiv id=\\\"Equ1\\\" class=\\\"Equation\\\"\\u003e\\n \\u003cdiv class=\\\"EquationNumber\\\"\\u003e\\u003cimg src=\\\"data:image/png;base64,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\\\" width=\\\"581\\\" height=\\\"77\\\"\\u003e\\u003c/div\\u003e\\n\\u003c/div\\u003e\\n\\u003cp\\u003eWhere M denotes the mass of Hg and MeHg accumulated on the resin gel, △g denotes the diffusive layer thickness, D denotes the diffusive coefficient of Hg of the gel, A denotes the region of the exposed surface, and t denotes the exposure time.\\u003c/p\\u003e\\n\\u003cp\\u003eThe diffusive flux (J) is the sum of the fluxes of sediment (Js) and overlying water (Jw). In accordance with Fick\\u0026rsquo;s law, the diffusive flux of DGT-Hg was determined as Eq.\\u0026nbsp;2:\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cimg src=\\\"data:image/png;base64,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\\\" width=\\\"581\\\" height=\\\"75\\\"\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eWhere J\\u003csub\\u003ew\\u003c/sub\\u003e is the DGT-Hg flux between water and the sediment water interface, J\\u003csub\\u003es\\u003c/sub\\u003e is the DGT-Hg flux from the sediment to the sediment water interface, and D\\u003csub\\u003es\\u003c/sub\\u003e and D\\u003csub\\u003ew\\u003c/sub\\u003e denote the diffusive coefficients of Hg of the porewater and natural water, respectively. In accordance with the research of Gao et al. (2018), a sediment porosity (\\u0026phi;) of 0.9 was selected\\u003csup\\u003e\\u003cspan class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e\\u003c/sup\\u003e. Moreover, (\\u0026delta;CDGT/\\u0026delta;xs)(x\\u0026thinsp;=\\u0026thinsp;0) and (\\u0026delta;CDGT/\\u0026delta;xw)(x\\u0026thinsp;=\\u0026thinsp;0) denote the DGT-Hg content gradients within the sediment and overlying water columns, respectively. Notably, gradients within 1 cm between the sediment and water interface were determined.\\u003c/p\\u003e\\n\\u003cp\\u003eAll analyses were conducted in SPSS 22.0, and any differences were considered statistically significant at p\\u0026thinsp;\\u0026lt;\\u0026thinsp;0.05.\\u003c/p\\u003e\"},{\"header\":\"Results\",\"content\":\"\\u003cdiv id=\\\"Sec8\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eSediment Hg in the NZD Reservoir\\u003c/h2\\u003e \\u003cp\\u003eIn the NZD Reservoir, sediment Hg content exhibited a spatial pattern, first increasing and then slightly decreasing from the tail to the dam front, while sediment grain size (d\\u003csub\\u003e50\\u003c/sub\\u003e) showed a gradual decrease (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eA). In the reservoir tail area, sediment Hg content remained relatively stable. However, it increased from 77.12 ng g⁻\\u0026sup1; at site S4 to 114.14 ng g⁻\\u0026sup1; at site S8 in the reservoir middle area before decreasing to 85.68 ng g⁻\\u0026sup1; at site S10 in the dam-front area. Similarly, sediment grain size (d\\u003csub\\u003e50\\u003c/sub\\u003e) remained unchanged in the reservoir tail area but decreased from 7.61 \\u0026micro;m at site S4 to a minimum of 5.01 \\u0026micro;m at site S8 in the reservoir middle area, followed by an increase to 6.33 \\u0026micro;m at site S10 in the dam-front area. Grain size analysis revealed that silt dominated the sediments in the NZD Reservoir, with clay, silt, and sand accounting for 32.09%, 64.18%, and 3.73%, respectively. The proportion of clay in the sediments showed minimal variation in the reservoir tail area but increased slightly from 27.82% at site S4 to a maximum of 38.21% at site S8 in the reservoir middle area, before decreasing to 33.15% at site S10 in the dam-front area. Correlation analysis indicated that sediment Hg content was positively correlated with clay content (r\\u0026sup2; = 0.66, p\\u0026thinsp;\\u0026lt;\\u0026thinsp;0.05) and negatively correlated with silt and sand contents (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eB).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eIn contrast to the reservoir tail and dam-front sediment columns, the reservoir middle sediment column exhibited greater spatial heterogeneity in DGT-Hg concentrations. The mean (\\u0026plusmn;\\u0026thinsp;SD) DGT-Hg concentrations in porewater at the reservoir tail, reservoir middle, and dam-front sampling sites were 29.14\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;3.93 ng L⁻\\u0026sup1;, 33.87\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;8.68 ng L⁻\\u0026sup1;, and 29.66\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;2.27 ng L⁻\\u0026sup1;, respectively. Similarly, the DGT-Hg concentrations in sediment at the reservoir tail, reservoir middle, and dam front sites were 33.99\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;1.51 ng L⁻\\u0026sup1;, 34.87\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;5.10 ng L⁻\\u0026sup1;, and 15.55\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;2.72 ng L⁻\\u0026sup1;, respectively. The DGT-Hg concentrations at the reservoir tail and reservoir middle sites were more than double those at the dam-front site. The net Hg flux from sediment to the overlying water varied across sites, reaching 7.36, 3.25, and 3.69 ng d⁻\\u0026sup1; cm⁻\\u0026sup2; at the dam front, reservoir middle, and reservoir tail sites, respectively (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e). Notably, the Hg flux at the reservoir tail site was approximately twice as high as that at the reservoir middle and dam front sites.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003c/div\\u003e\\n\\u003ch3\\u003eSediment MeHg and MeHg/Hg Ratio in the NZD Reservoir\\u003c/h3\\u003e\\n\\u003cp\\u003eIn the NZD Reservoir, sediment MeHg and %MeHg values were lower at the reservoir tail but increased from 0.41 ng g⁻\\u0026sup1; and 0.53% at site S4 to 0.68 ng g⁻\\u0026sup1; and 0.74% at site S7, respectively. These values then slightly decreased to 0.44 ng g⁻\\u0026sup1; and 0.60%, respectively, at the dam front (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eA). In contrast, sediment organic carbon (OC) content increased from 11.26 mg g⁻\\u0026sup1; at site S1 in the reservoir tail to a maximum of 25.17 mg g⁻\\u0026sup1; at site S8 in the reservoir middle, before slightly decreasing to 24.30 mg g⁻\\u0026sup1; at the dam front. Additionally, the abundance of \\u003cem\\u003ehgc\\u003c/em\\u003eAB genes, associated with Deltaproteobacteria, generally increased along the reservoir, from 19.14 \\u0026times; 10⁵ gene copies per gram of sediment at the reservoir tail to 48.24 \\u0026times; 10⁵ gene copies per gram at the dam front (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eB).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eThe spatial variations in DGT-MeHg concentrations during the first sediment layer (0\\u0026ndash;60 mm depth) were significantly greater than those in the deeper layers and decreased rapidly below a depth of 60 mm (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e). Mean (\\u0026plusmn;\\u0026thinsp;SD) DGT-MeHg concentrations in porewater during the first 60 mm were 0.25\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;0.17 ng L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, 0.59\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;0.22 ng L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, and 0.17\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;0.14 ng L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e at the reservoir tail, reservoir middle, and dam front sites, respectively.\\u003c/p\\u003e \\u003cp\\u003eIn the reservoir tail sediment column, DGT-MeHg concentrations slightly increased near the sediment water interface at an 8 mm depth, followed by a sharp decrease at the 10 mm depth. This trend repeated at depths of 10\\u0026ndash;16 mm, 18\\u0026ndash;24 mm, 42\\u0026ndash;52 mm, 54\\u0026ndash;88 mm, and 90\\u0026ndash;100 mm. In contrast, the dam front sediment column exhibited an opposite trend, with intermittent sharp increases followed by decreases near the bottom. Specifically, DGT-MeHg concentrations sharply increased at the 10-mm depth, slightly decreased until the 30-mm depth, and repeated this pattern at depths of 32\\u0026ndash;54 mm, 56\\u0026ndash;94 mm, and 96\\u0026ndash;100 mm. Meanwhile, the reservoir middle sediment column showed continuous fluctuations in DGT-MeHg concentrations throughout the profile.\\u003c/p\\u003e \\u003cp\\u003eThe DGT-MeHg concentrations in the overlying water column were 0.05\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;0.03 ng L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, 0.55\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;0.18 ng L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, and 0.04\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;0.02 ng L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e at the reservoir tail, reservoir middle, and dam front sampling sites, respectively. Notably, the DGT-MeHg concentration at the reservoir middle site was more than 11 times higher than those at the dam front and reservoir tail sites. However, due to the insufficient linearity of the DGT-MeHg concentrations in the overlying water, the MeHg diffusion flux could not be calculated.\\u003c/p\\u003e\"},{\"header\":\"Discussion\",\"content\":\"\\u003cdiv id=\\\"Sec11\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eBoth Hg and MeHg in Reservoir Sediments Are Determined by Hydrodynamics\\u003c/h2\\u003e \\u003cp\\u003eThe Hg content in the sediment of the NZD Reservoir was high at the middle of the reservoir, where fine particles settled because of the weak hydrodynamics. The sediment Hg content decreased in the reservoir tail and dam-front areas, where coarse-grained sediment is enriched. Hence, at the basin scale, the spatial variation in Hg levels was affected by hydrodynamics and grain size variables (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eA).\\u003c/p\\u003e \\u003cp\\u003eThe NZD Reservoir is located in an environment with limited anthropogenic disturbance, in which sediment Hg is derived mainly from nonpoint sources. With dam construction in rivers, the flow velocity decreases, and Hg adsorbed onto particulates eventually settles onto the sediment\\u003csup\\u003e\\u003cspan citationid=\\\"CR32\\\" class=\\\"CitationRef\\\"\\u003e32\\u003c/span\\u003e\\u003c/sup\\u003e. However, hydrodynamic factors influence the geographic pattern of the Hg content\\u003csup\\u003e\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e\\u003c/sup\\u003e. During hydrodynamic reinforcement, sediment Hg can be released into the overlying water column and redistributed via competition between grain particles of different sizes\\u003csup\\u003e\\u003cspan citationid=\\\"CR20\\\" class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e\\u003c/sup\\u003e. In the resettlement process, coarse particles resorb limited Hg and reach a steady state relatively quickly, whereas fine particles may resorb more Hg but be transported across greater distances or become deposited in nearby areas after a prolonged period to reach a steady state in the unsteady flow process of the reservoir\\u003csup\\u003e\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR33\\\" class=\\\"CitationRef\\\"\\u003e33\\u003c/span\\u003e\\u003c/sup\\u003e. This process can function as a \\u0026ldquo;sieve\\u0026rdquo; for size grouped particles, thus affecting the spatial pattern of the Hg content in sediment. Hence, a remarkable positive relationship was observed between the sediment Hg and clay contents but not with the silt and sand contents (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eB). The decrease in the sediment Hg content between the reservoir tail and dam front can be attributed to water released from upstream via discharge and the dam outlet, which makes it comparatively difficult for fine sediment particles to settle in these areas.\\u003c/p\\u003e \\u003cp\\u003eInside the NZD Reservoir, the sediment MeHg content increased from the reservoir tail to the reservoir middle and subsequently decreased to the dam front, exhibiting spatial patterns similar to those of the %MeHg and Hg contents in the sediment. However, the abundance of \\u003cem\\u003ehgc\\u003c/em\\u003eAB genes and the OC content in the sediment increased between the reservoir tail and the dam front, exhibiting different spatial patterns from those of the MeHg and %MeHg contents in the sediment (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eHg methylation might have occurred during the transport of mobile sediment particles and their redeposition in the area with weak hydrodynamics because fresh carbon was continuously reloaded in the water column, thereby providing a microanoxic environment for mobile sediment particles, which is ideal for Hg methylation\\u003csup\\u003e\\u003cspan citationid=\\\"CR16\\\" class=\\\"CitationRef\\\"\\u003e16\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR34\\\" class=\\\"CitationRef\\\"\\u003e34\\u003c/span\\u003e\\u003c/sup\\u003e. Therefore, MeHg in the sediment comprises two parts, namely, MeHg from in situ production and MeHg from offsite accumulation. In the NZD Reservoir, the abundance of \\u003cem\\u003ehgc\\u003c/em\\u003eAB and the OC content in the sediment increased between the reservoir tail and the dam front, indicating that MeHg production increased (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e). However, MeHg and %MeHg in the sediment were dominated by accumulated Hg. This finding indicates that mobile sediment particles are an important source of MeHg in sediments. Thus, the spatial distributions of both Hg and MeHg in sediments are determined by hydrodynamics.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec12\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eDeposition and Risk of Exposure to Hg and MeHg during Reservoir Operation\\u003c/h2\\u003e \\u003cp\\u003eThe sediment column samples from different depths provide historical reservoir operation information\\u003csup\\u003e\\u003cspan citationid=\\\"CR35\\\" class=\\\"CitationRef\\\"\\u003e35\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR36\\\" class=\\\"CitationRef\\\"\\u003e36\\u003c/span\\u003e\\u003c/sup\\u003e. Both DGT-Hg and DGT-MeHg exhibited periodic fluctuations along the vertical direction, but the fluctuations in DGT-MeHg were greater, indicating that the Hg methylation intensity varied during reservoir operation. Moreover, DGT-MeHg occurred at the middle and in the tail of the reservoir, with the first 60 mm exhibiting the largest spatial differences.\\u003c/p\\u003e \\u003cp\\u003eIn this study, the DGT-Hg distribution in the sediment column of the NZD Reservoir can be clarified on the basis of the behavior of mobile sediment particles sorted by hydrodynamics\\u003csup\\u003e\\u003cspan citationid=\\\"CR37\\\" class=\\\"CitationRef\\\"\\u003e37\\u003c/span\\u003e\\u003c/sup\\u003e. At the reservoir tail and dam front sites of the NZD Reservoir, particles were carried out because of the changing hydrodynamic conditions due to the discharge of the upstream DCS Reservoir and the dam outlet of the NZD Reservoir, respectively. Hence, the DGT-Hg concentration in the sediment column exhibited relatively high spatial heterogeneity. At the reservoir middle site of the NZD Reservoir, considerable amounts of fine particles were deposited due to the weak hydrodynamics. Thus, a low spatial heterogeneity in DGT-Hg was observed in the reservoir middle sediment column.\\u003c/p\\u003e \\u003cp\\u003eThe data revealed that Hg methylation is closely related to the characteristics of particles because the DGT-MeHg concentration was positively correlated with the %MeHg concentration in the bulk sediment but not with the DGT-Hg concentration. The irregular distribution of DGT-MeHg in the overlying water column supports the possibility that Hg methylation may occur in oxygen deficient zones of stratified water columns, whereas the grain size \\u0026ldquo;sieve\\u0026rdquo; phenomenon during reservoir regulation further affects the changes in DGT-MeHg in sediments. Moreover, the discontinuous change in the DGT-MeHg distribution within the sediment vertical profiles indicates that the DGT-MeHg exposure risk is influenced by human activities. The dam front zone is generally the deepest area of the reservoir, and it is particularly prone to particle resuspension and resettlement because of the operation height of turbines, which may prevent fine particle transport downstream. Thus, the DGT-MeHg concentration at the dam front sharply increased intermittently and then decreased at the bottom during reservoir regulation. However, in the reservoir tail sediment column, ripple transport caused the preferential burial of fine particles. Thus, in the reservoir tail sediment column, the DGT-MeHg concentration slightly increased from the sediment water interface to the bottom and then sharply decreased. Completely opposite characteristics of the vertical distribution were recorded compared with those at the dam front site. Compared with nonmobile sediment particles, mobile sediment particles exposed to water usually better reflect the MeHg exposure risk. Thus, the potential ecological implication for this finding is that the risk of exposure to MeHg increases during reservoir regulation.\\u003c/p\\u003e \\u003cp\\u003eMoreover, the DGT-MeHg concentrations during the first period (0\\u0026ndash;60 mm) were significantly greater than those during the second period but decreased rapidly below 60 mm. Two main mechanisms may explain this phenomenon. One hypothesis states that seasonal MeHg variations govern the process. For example, high seasonal variations in the reservoir sediment MeHg content have been obtained at most sites in the Northern Hemisphere, with minima in winter and peaks in summer\\u003csup\\u003e\\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR38\\\" class=\\\"CitationRef\\\"\\u003e38\\u003c/span\\u003e\\u003c/sup\\u003e. Another hypothesis is that DGT-MeHg desorption is related to organic matter mineralization advancing more slowly since organic matter in deeper sediment is more recalcitrant\\u003csup\\u003e\\u003cspan citationid=\\\"CR39\\\" class=\\\"CitationRef\\\"\\u003e39\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR40\\\" class=\\\"CitationRef\\\"\\u003e40\\u003c/span\\u003e\\u003c/sup\\u003e. We can accept the second assumption, since seasonal MeHg changes commonly occur synchronously with Hg changes. In addition, a seasonal change was observed in the region, in which the change in the NZD Reservoir was very small. However, our data and information do not support the first hypothesis.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec13\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eRisk of Exposure to MeHg during Reservoir Regulation\\u003c/h2\\u003e \\u003cp\\u003eHg resuspension, desorption, resorption, and deposition in mobile sediment particles are ongoing processes that occur in balance\\u003csup\\u003e\\u003cspan citationid=\\\"CR41\\\" class=\\\"CitationRef\\\"\\u003e41\\u003c/span\\u003e\\u003c/sup\\u003e. Under the change in hydrodynamics caused by reservoir regulation, fine particles can carry more Hg substances than coarse particles can, are resuspended first, and experience a prolonged period of exposure to fresh OC in the water column during transport. However, coarse particles exhibit the opposite trend. We attributed the differences in %MeHg in the DGT probes to mobile sediment particles being driven during transport. Our data revealed that Hg methylation is closely related to the characteristics of particles because the DGT-MeHg concentration was positively correlated with the %MeHg concentration in the bulk sediment but not with the DGT-Hg concentration. The irregular distribution of DGT-MeHg in the overlying water column supports the possibility that Hg methylation may have occurred in oxygen-deficient zones of stratified water columns, whereas the grain size \\u0026ldquo;sieve\\u0026rdquo; phenomenon during reservoir regulation may have further affected the changes in DGT-MeHg in the sediment.\\u003c/p\\u003e \\u003cp\\u003eMoreover, the discontinuous change in the DGT-MeHg distribution within the sediment vertical profiles indicates that the DGT-MeHg exposure risk is influenced by human activities. The dam-front zone is generally the deepest area of the reservoir, and it is particularly prone to particle resuspension and resettlement because of the operation height of turbines, which may prevent fine particle transport downstream. Thus, the DGT-MeHg concentration at the dam front sharply increased intermittently and then decreased at the bottom during reservoir regulation. However, in the reservoir tail sediment column, ripple transport caused the preferential burial of fine particles. Thus, in the reservoir tail sediment column, the DGT-MeHg concentration slightly increased from the sediment water interface to the bottom and then sharply decreased. Completely opposite characteristics of the vertical distribution were recorded compared with those at the dam front site. Compared with nonmobile sediment particles, mobile sediment particles exposed to water usually better reflect the MeHg exposure risk. Thus, the potential ecological implication for this finding is that the risk of exposure to MeHg increases during reservoir regulation.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec14\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eRisk of Exposure to Hg and MeHg during Reservoir Regulation\\u003c/h2\\u003e \\u003cp\\u003eIn the NZD Reservoir, DGT-Hg was released from the sediments into the overlying water, which indicates that sediments increasingly became the source of DGT-Hg in the overlying water. Within the overlying water column, the density gradient of DGT-derived MeHg is not suitable for linear testing, so the flux cannot be calculated. However, the density of DGT-generated MeHg within the overlying water is greater than that in the sediment at the corresponding point. These findings indicate the risk of MeHg release from sediments. Furthermore, the sediment Hg content in front of the dam and at the tail of the reservoir decreased, but there was greater release of DGT-Hg, indicating that hydrodynamic enhancement promoted Hg release from the sediment. The strong reduction conditions at the tail of the reservoir further facilitate the release of DGT-Hg, resulting in significantly greater release of DGT-Hg at the reservoir tail than in the other regions. The risk of Hg and MeHg among reservoirs is usually determined by their concentration in the water body\\u003csup\\u003e\\u003cspan citationid=\\\"CR42\\\" class=\\\"CitationRef\\\"\\u003e42\\u003c/span\\u003e,\\u003cspan citationid=\\\"CR43\\\" class=\\\"CitationRef\\\"\\u003e43\\u003c/span\\u003e\\u003c/sup\\u003e. Therefore, when studying the hazards of Hg and MeHg in reservoirs, the Hg and MeHg contents in sediments can serve as a reference, and more importantly, attention should be given to the tail area of reservoirs with strong hydrodynamics and reducing conditions.\\u003c/p\\u003e \\u003c/div\\u003e\"},{\"header\":\"Conclusion\",\"content\":\"\\u003cp\\u003eUnder the control of reservoir regulation, active transfer of sediment particles occurs along a reservoir within a canyon river, which may influence the spatial patterns of Hg in sediment. This study revealed that reservoir transport can facilitate local storage and methylation of Hg above its source point. The main conclusions are as follows:\\u003c/p\\u003e \\u003cp\\u003e \\u003col\\u003e \\u003cspan\\u003e \\u003cli\\u003e \\u003cp\\u003eHydrodynamics control Hg and MeHg distribution: The spatial distributions of both Hg and MeHg in the sediments of the NZD Reservoir are driven primarily by hydrodynamic conditions and the particle size. In fine particles, which settle in weak-hydrodynamics zones, relatively high Hg and MeHg levels can accumulate, whereas coarse particles, which are enriched in the reservoir tail and dam front, exhibit reduced Hg and MeHg contents due to their limited adsorption capacity.\\u003c/p\\u003e \\u003c/li\\u003e \\u003c/span\\u003e \\u003cspan\\u003e \\u003cli\\u003e \\u003cp\\u003eRisk of exposure to Hg and MeHg during reservoir operation: The spatial distribution of MeHg reflects both in situ production and offsite accumulation, whereas mobile sediment particles contribute significantly to MeHg accumulation. The hydrodynamic changes induced by reservoir operation affect the flux of Hg in sediments, particularly in reservoir tail areas. These hydrodynamic fluctuations increase the Hg exposure risk.\\u003c/p\\u003e \\u003c/li\\u003e \\u003c/span\\u003e \\u003c/ol\\u003e \\u003c/p\\u003e \"},{\"header\":\"Declarations\",\"content\":\"\\u003ch2\\u003eCompeting interests\\u003c/h2\\u003e \\u003cp\\u003eThe authors declare no competing interests.\\u003c/p\\u003e \\u003ch2\\u003eAuthor Contribution\\u003c/h2\\u003e\\u003cp\\u003eS.L. contributed to the study\\u0026rsquo;s conceptualization, investigation, review and editing, and initial drafting of the manuscript. L.L. was responsible for data curation and formal analysis, interpretation of results, and writing the initial draft of the manuscript. J.L. analyzed the results and contributed to the interpretation of findings. X.W. provided supervision and guidance throughout the study, offering critical insights and feedback on research design and analysis. All authors reviewed the manuscript.\\u003c/p\\u003e\\u003ch2\\u003eAcknowledgement\\u003c/h2\\u003e\\u003cp\\u003eThis work was supported by National Natural Science Foundation of China (No. 52309098, 52309085), Natural Science Foundation of Jiangsu Province (BK20230960), Internal Talent Project of Xihua University(Z222059).\\u003c/p\\u003e\\u003ch2\\u003eData Availability\\u003c/h2\\u003e\\u003cp\\u003eXianglong Wei (corresponding author) has to be contacted in case of any queries or requirement of data. The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\u003cli\\u003e\\u003cspan\\u003eKe, T., Junior, F. B., Santamaria, A., Bowman, A. B. \\u0026amp; Aschner, M. Genetic Factors in Methylmercury-Induced Neurotoxicity: What Have We Learned from Caenorhabditis Elegans Models? \\u003cem\\u003eAdv. neurotoxicology\\u003c/em\\u003e. \\u003cb\\u003e9\\u003c/b\\u003e, 271\\u0026ndash;290 (2023).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eKe, T. et al. Epigenetics and Methylmercury-Induced Neurotoxicity, Evidence from Experimental Studies. \\u003cem\\u003eToxics\\u003c/em\\u003e \\u003cb\\u003e11\\u003c/b\\u003e, 72 (2023).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eChen, L. et al. Trans-Provincial Health Impacts of Atmospheric Mercury Emissions in China. \\u003cem\\u003eNat. Commun.\\u003c/em\\u003e \\u003cb\\u003e10\\u003c/b\\u003e, 1484 (2019).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eSchartup, A. T. et al. Climate Change and Overfishing Increase Neurotoxicant in Marine Predators. \\u003cem\\u003eNature\\u003c/em\\u003e \\u003cb\\u003e572\\u003c/b\\u003e, 648 (2019).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eLi, M. L. et al. Global Fishing Patterns Amplify Human Exposures to Methylmercury. \\u003cem\\u003eProc. Natl. Acad. Sci. U.S.A.\\u003c/em\\u003e \\u003cb\\u003e121\\u003c/b\\u003e, e2405898121 (2024).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eDack, K. et al. Prenatal Mercury Exposure and Neurodevelopment up to the Age of 5 Years: A Systematic Review. \\u003cem\\u003eInt. J. Environ. Res. Public Health\\u003c/em\\u003e. \\u003cb\\u003e19\\u003c/b\\u003e, 1976 (2022).\\u003c/span\\u003e\\u003c/li\\u003e \\u003cli\\u003e\\u003cspan\\u003eBest, J. Anthropogenic Stresses on the World\\u0026rsquo;s Big Rivers. \\u003cem\\u003eNat. 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Total Environ.\\u003c/em\\u003e \\u003cb\\u003e714\\u003c/b\\u003e, 136539 (2020).\\u003c/span\\u003e\\u003c/li\\u003e\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":false,\"hideJournal\":false,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":true,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"scientific-reports\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"scirep\",\"sideBox\":\"Learn more about [Scientific Reports](http://www.nature.com/srep/)\",\"snPcode\":\"\",\"submissionUrl\":\"\",\"title\":\"Scientific Reports\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"Scientific Reports\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":true},\"keywords\":\"Mercury, Methylmercury, Sediment, Reservoir operation, Lancang River\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-6041669/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-6041669/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eMercury (Hg) and methylmercury (MeHg) are highly neurotoxic substances, and reservoirs play a critical role in Hg storage while providing geochemical environments conducive to MeHg production. However, the mechanisms of Hg transport and methylation during reservoir regulation remain poorly understood. This study investigates the spatial distribution of sediment Hg and MeHg in both horizontal and vertical profiles within the Nuozhadu Reservoir, located in the upper Mekong River. Using the diffusive gradients in thin films (DGT) technique, we observed that sediment Hg and MeHg concentrations initially increased and then slightly decreased from the tail to the dam front of the reservoir. This trend is primarily attributed to the transport of fine sediment particles influenced by reservoir interception. MeHg production was observed during sediment transport, but hydraulic conditions driving sediment accumulation emerged as the dominant source of sediment MeHg. Furthermore, vertical fluctuations in DGT-Hg and DGT-MeHg values were noted, likely caused by periodic reservoir operations. Our findings indicate that sediments release Hg into the overlying water, establishing sediments as a significant source of Hg pollution in reservoirs. This study enhances our understanding of Hg transport and methylation processes under the influence of reservoir regulation.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Sediment Mercury and Methylmercury Dynamics During Reservoir Operation\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2025-02-21 08:57:03\",\"doi\":\"10.21203/rs.3.rs-6041669/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0},{\"type\":\"decision\",\"content\":\"Revision requested\",\"date\":\"2025-04-22T11:09:44+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"editorInvitedReview\",\"content\":\"\",\"date\":\"2025-04-17T12:43:17+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"reviewerAgreed\",\"content\":\"148711218512558979059587398526187688153\",\"date\":\"2025-04-08T04:09:39+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"editorInvitedReview\",\"content\":\"\",\"date\":\"2025-03-28T17:36:18+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"reviewerAgreed\",\"content\":\"35357855654632876763301338818314235999\",\"date\":\"2025-03-10T10:55:40+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"reviewersInvited\",\"content\":\"\",\"date\":\"2025-03-03T01:08:30+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"editorAssigned\",\"content\":\"\",\"date\":\"2025-02-20T22:09:36+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"editorInvited\",\"content\":\"\",\"date\":\"2025-02-20T14:06:23+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"checksComplete\",\"content\":\"\",\"date\":\"2025-02-19T09:15:04+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"submitted\",\"content\":\"Scientific Reports\",\"date\":\"2025-02-16T14:14:32+00:00\",\"index\":\"\",\"fulltext\":\"\"}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"scientific-reports\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"scirep\",\"sideBox\":\"Learn more about [Scientific Reports](http://www.nature.com/srep/)\",\"snPcode\":\"\",\"submissionUrl\":\"\",\"title\":\"Scientific Reports\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"Scientific Reports\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":true}}],\"origin\":\"\",\"ownerIdentity\":\"efb3a7cb-963e-40f4-8020-ce95d965350c\",\"owner\":[],\"postedDate\":\"February 21st, 2025\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"published-in-journal\",\"subjectAreas\":[{\"id\":44651870,\"name\":\"Earth and environmental sciences/Environmental sciences\"},{\"id\":44651871,\"name\":\"Earth and environmental sciences/Biogeochemistry\"}],\"tags\":[],\"updatedAt\":\"2025-05-26T16:08:07+00:00\",\"versionOfRecord\":{\"articleIdentity\":\"rs-6041669\",\"link\":\"https://doi.org/10.1038/s41598-025-03255-6\",\"journal\":{\"identity\":\"scientific-reports\",\"isVorOnly\":false,\"title\":\"Scientific Reports\"},\"publishedOn\":\"2025-05-23 15:58:34\",\"publishedOnDateReadable\":\"May 23rd, 2025\"},\"versionCreatedAt\":\"2025-02-21 08:57:03\",\"video\":\"\",\"vorDoi\":\"10.1038/s41598-025-03255-6\",\"vorDoiUrl\":\"https://doi.org/10.1038/s41598-025-03255-6\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-6041669\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-6041669\",\"identity\":\"rs-6041669\",\"version\":[\"v1\"]},\"buildId\":\"8U1c8b4HqxoKbykW_rLl7\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}