Intense methane diffusive emissions in eutrophic urban lakes, Central China.

Urban lakes are hotspots of methane (CH4) emissions. Yet, actual field measurements of CH4 in these lakes are rather limited and our understanding of CH4 response to urban lake eutrophication is still incomplete. In this study, we measured dissolved CH4 concentrations and quantified CH4 diffusion from four urban lakes in subtropical China during wet and dry seasons. We found that these lakes were constantly CH4-saturated, contributing the greenhouse gas (GHG) to the atmosphere. Nutrient enrichment significantly increased CH4 concentrations and diffusive fluxes. Average CH4 flux rate in the highly-eutrophic lake zones (4.18 ± 7.68 mmol m-2 d-1) was significantly higher than those in the mesotrophic (0.19 ± 0.18 mmol m-2 d-1) and lightly/moderately-eutrophic zones (0.72 ± 2.22 mmol m-2 d-1). Seasonally, CH4 concentrations and fluxes were significantly higher in the wet season than in the dry season in the mesotrophic and the lightly/moderately-eutrophic lake zones, but an inverse pattern existed in the highly-eutrophic lake zones. CH4 concentrations and fluxes increased with elevated levels of nitrogen, phosphorus and dissolved organic carbon (DOC). The accumulation of nutrients provided autochthonous substrate for methanogenesis, indicated by a negative correlation between CH4 and the C:N ratio. Ammonium-nitrogen (NH4+-N) was the best predictor for spatial fluctuation of CH4 concentrations and diffusive fluxes in the mesotrophic and the lightly/moderately-eutrophic lake zones, while total nitrogen (TN) and total phosphorus (TP) levels showed the highest predictability in the highly-eutrophic lake zones. Based on the findings, we conclude that nutrient enrichment in urban lakes can largely increase CH4 diffusion, and that urban sewage inflow is a key concern for eutrophication boosting CH4 production and diffusive emission. Furthermore, our study reveals that small urban lakes may be an important missing source of GHG emissions in the global C accounting, and that the ratio of littoral-to-pelagic zones can be important for predicting lake-scale estimation of CH4 emission.

Patterns and drivers of CH4 concentration and diffusive flux from a temperate river-reservoir system in North China.

Freshwater reservoirs are regarded as an important anthropogenic source of methane (CH4) emissions. The temporal and spatial variability of CH4 emissions from different reservoirs results in uncertainty in the estimation of the global CH4 budget. In this study, surface water CH4 concentrations were measured and diffusive CH4 fluxes were estimated via a thin boundary layer model in a temperate river-reservoir system in North China, using spatial (33 sites) and temporal (four seasons) monitoring; the system has experienced intensive aquaculture disturbance. Our results indicated that the dissolved CH4 concentration in the reservoir ranged from 0.07 to 0.58 µmol/L, with an annual average of 0.13 ± 0.10 µmol/L, and the diffusive CH4 flux across the water-air interface ranged from 0.66 to 3.61 µmol/(m2•hr), with an annual average of 1.67 ± 0.75 µmol/(m2•hr). During the study period, the dissolved CH4 concentration was supersaturated and was a net source of atmospheric CH4. Notably, CH4 concentration and diffusive flux portrayed large temporal and spatial heterogeneity. The river inflow zone was determined to be a hotspot for CH4 emissions, and its flux was significantly higher than that of the tributary and main basin; the CH4 flux in autumn was greater than that in other seasons. We also deduced that the CH4 concentration/diffusive flux was co-regulated mainly by water temperature, water depth, and water productivity (Chla, trophic status). Our results highlight the importance of considering the spatiotemporal variability of diffusive CH4 flux from temperate reservoirs to estimate the CH4 budget at regional and global scales.

Distribution of Nitrogen and Phosphorus in Pore Water Profiles and Estimation of Their Diffusive Fluxes and Annual Loads in Guanting Reservoir (GTR), Northern China.

Improvement of water quality has frequently been delayed due to high recycling rates of nutrients across the sediment-water interface in a reservoir. Diffusive fluxes and annual loads of ammonia nitrogen (NH4+-N) and phosphate (PO43--P) in sediments from Guanting Reservoir (GTR) were estimated according to their vertical distribution. The average contents of NH4+-N and PO43--P in surface pore water were higher by factors of 6.9 - 11.7 and 1.3 - 6.4 than those in overlying water, respectively. The ranges of fluxes were 1.59 - 13.0 (mg m2 d-1) for NH4+-N, and 0.002 - 0.196 (mg m2 d-1) for PO43--P. The annual load contributions from sediments of GTR were 659 t a-1 for NH4+-N and 4.83 t a-1 for PO43--P. Notably, the upstream of GTR accounted for 50.7% annual loads for NH4+-N, while the downstream contributed 71.2% loads to PO43--P. This study will better inform future environmental management for the reservoir.

Concurrent assessment of diffusive and advective PAH movement strongly affected by temporal and spatial changes.

Chemical movement influences exposure, remediation and interventions. Understanding chemical movement in addition to chemical concentrations at contaminated sites is critical to informed decision making. Using seepage meters and passive sampling devices we assessed both diffusive and advective flux of bioavailable polycyclic aromatic hydrocarbons (PAHs) at three time points, across two seasons, at a former creosote site in St. Helens, Oregon, United States. To our knowledge, this is the first time both diffusive and advective fluxes have been measured simultaneously at a contaminated site. Concentrations of 39 parent PAHs were determined by gas chromatography triple quadrupole mass spectrometry. Across both seasons and all sites, diffusive flux of PAHs was up to three orders of magnitude larger than advective flux. Release of PAHs from sediments and water were identified, likely from legacy contamination, as well as deposition from the air into the site from contemporary and other sources. The majority of PAH movement was comprised of three and four ring PAHs. Chemical movement on the site was found to be spatially and temporally variable. Volatilization decreased and atmospheric deposition increased from summer to fall. At the locations with higher levels of contamination, sum PAH release from sediments decreased by more than two orders of magnitude from summer to late fall. These data reflect the spatial heterogeneity and temporal variability of this site and demonstrate the importance of seasonality in assessing chemical movement at contaminated sites. Results from this study can inform future legacy site assessments to optimize remediation strategies and assess remediation effectiveness.

Ebullition dominates methane emissions in stratified coastal waters.

Coastal areas are an important source of methane (CH4). However, the exact origins of CH4 in the surface waters of coastal regions, which in turn drive sea-air emissions, remain uncertain. To gain a comprehensive understanding of the current and future climate change feedbacks, it is crucial to identify these CH4 sources and processes that regulate its formation and oxidation. This study investigated coastal CH4 dynamics by comparing water column data from six stations located in the brackish Tvärminne Archipelago, Baltic Sea. The sediment biogeochemistry and microbiology were further investigated at two stations (i.e., nearshore and offshore). These stations differed in terms of stratification, bottom water redox conditions, and organic matter loading. At the nearshore station, CH4 diffusion from the sediment into the water column was negligible, because nearly all CH4 was oxidized within the upper sediment column before reaching the sediment surface. On the other hand, at the offshore station, there was significant benthic diffusion of CH4, albeit the majority underwent oxidation before reaching the sediment-water interface, due to shoaling of the sulfate methane transition zone (SMTZ). The potential contribution of CH4 production in the water column was evaluated and was found to be negligible. After examining the isotopic signatures of δ13C-CH4 across the sediment and water column, it became apparent that the surface water δ13C-CH4 values observed in areas with thermal stratification could not be explained by diffusion, advective fluxes, nor production in the water column. In fact, these values bore a remarkable resemblance to those detected below the SMTZ. This supports the hypothesis that the source of CH4 in surface waters is more likely to originate from ebullition than diffusion in stratified brackish coastal systems.

Significant inter-annual fluctuation in CO2 and CH4 diffusive fluxes from subtropical aquaculture ponds: Implications for climate change and carbon emission evaluations.

Aquaculture ponds are potential hotspots for carbon cycling and emission of greenhouse gases (GHGs) like CO2 and CH4, but they are often poorly assessed in the global GHG budget. This study determined the temporal variations of CO2 and CH4 concentrations and diffusive fluxes and their environmental drivers in coastal aquaculture ponds in southeastern China over a five-year period (2017-2021). The findings indicated that CH4 flux from aquaculture ponds fluctuated markedly year-to-year, and CO2 flux varied between positive and negative between years. The coefficient of inter-annual variation of CO2 and CH4 diffusive fluxes was 168% and 127%, respectively, highlighting the importance of long-term observations to improve GHG assessment from aquaculture ponds. In addition to chlorophyll-a and dissolved oxygen as the common environmental drivers, CO2 was further regulated by total dissolved phosphorus and CH4 by dissolved organic carbon. Feed conversion ratio correlated positively with both CO2 and CH4 concentrations and fluxes, showing that unconsumed feeds fueled microbial GHG production. A linear regression based on binned (averaged) monthly CO2 diffusive flux data, calculated from CO2 concentrations, can be used to estimate CH4 diffusive flux with a fair degree of confidence (r2 = 0.66; p < 0.001). This algorithm provides a simple and practical way to assess the total carbon diffusive flux from aquaculture ponds. Overall, this study provides new insights into mitigating the carbon footprint of aquaculture production and assessing the impact of aquaculture ponds on the regional and global scales.

Arsenic distribution characteristics and release mechanisms in aquaculture lake sediments.

It is well known that aquaculture can alter the microenvironments of lakes at sediment-water interface (SWI). However, the main mechanisms underlying the effects of aquaculture activities on arsenic (As) transformations are still unclear. In this context, the present study aims to investigate the variations in the sediment As contents in Yangcheng Lake, as well as to assess its chemical transformations, release fluxes, and release mechanisms. The results showed substantial spatial differences in the dissolved As concentrations in the sediment pore water. The As release fluxes at the SWI ranged from 1.32 to 112.09 µg/L, with an average value of 33.68 µg/L. In addition, the highest As fluxes were observed in the aquaculture areas. The transformation of crystalline hydrous Fe oxide-bound As to adsorbed-As in the aquaculture lake sediments increased the ability of As release. The Partial least squares path modeling results demonstrated the great contributions of organic matter (OM) to the As transformations by influencing the sediment microbial communities and Fe/Mn minerals. The changes in the As fractionation and competing adsorption increased the dissolved As concentrations in the 0-10 mm surface sediment. Non-specifically and specifically adsorbed As were the major sources of dissolved As in the sediments. Specifically, microbial reduction of As[V] and dissolution of Fe oxides increased the dissolved As concentrations at the SWI (20 to -20 mm). The results of the current study highlight the positive enhancement effects of aquaculture on As release from sediments.

Hydrogenotrophic pathway dominates methanogenesis along the river-estuary continuum of the Yangtze River.

Rivers are considered as an important source of methane (CH4) to the atmosphere, but our understanding for the methanogenic pathway in rivers and its linkage with CH4 emission is very limited. Here, we investigated the diffusive flux of CH4 and its stable carbon isotope signature (δ13C-CH4) along the river-estuary continuum of the Yangtze River. The diffusive CH4 flux was estimated to 27.9 ± 11.4 µmol/m2/d and 36.5 ± 24.4 µmol/m2/d in wet season and dry season, respectively. The δ13C-CH4 values were generally lower than -60‰, with the fractionation factor (αc) higher than 1.055 and the isotope separation factor (εc) ranged from 55 to 100. In situ microbial composition showed that hydrogenotrophic methanogens accounts for over 70% of the total reads. Moreover, the incubation test showed that the headspace CH4 concentration by adding CO2/H2 to the sediment was orders of magnitude higher than that by adding trimethylamine and sodium acetate. These results jointly verified the river-estuary continuum is a minor CH4 source and dominated by hydrogenotrophic pathway. Based on the methanogenic pathway here and previous reported data in the same region, the historical variation of diffusive CH4 flux was calculated and results showed that CH4 emission has reduced 82.5% since the construction of Three Gorges Dam (TGD). Our study verified the dominant methanogenic pathway in river ecosystems and clarified the effect and mechanism of dam construction on riverine CH4 emission.

Remobilization characteristics and diffusion kinetic processes of sediment zinc (Zn) in a tidal reach of the Pearl River Estuary, South China.

Exploration of the remobilization mechanism of trace metals in estuarine sediments remain challenging because of dynamic hydrochemical conditions. This study integrated a chemical sequential extraction procedure (BCR), the diffusive gradient in thin films (DGT) and high-resolution dialysis techniques, and Visual MINTEQ ver.3.1 to identify the seasonal mobilization characteristics of sediment Zn within a tidal reach, South China. The mobility of sediment Zn based on the BCR procedure contradicted the results of DGT analysis. In summer, reductive dissolution of Fe/Mn oxides was the key driver of sediment Zn remobilization; during winter, cation exchange reactions facilitated the mobilization of Zn in the brackish water zone. The time-dependence ratios of DGT-labile Zn and dissolved Zn concentrations (mean: 0.34-0.81) indicated the sediment solid phase had partially sustained capacity to resupply Zn to the porewater in both seasons. Sediments generally functioned as a source of Zn in the freshwater zone with organically complexed Zn being diffusively released into the water column at rates of 0.3-15.5 µg·m-2·d-1. In the brackish water zone, the dominant Zn species were transformed into free Zn ions and Zn-inorganic complexes and migrated into sediment, with respective influxes of 18.9-70.7 µg·m-2·d-1 and 18.9-68.3 µg·m-2·d-1, which shifted to a sink of Zn.

Phosphorus internal loading and sediment diagenesis in a large eutrophic lake (Lake Chaohu, China).

Sediment phosphorus (P) release and retention are important in controlling whole-system P dynamics and budget in eutrophic lakes. Here we combine short- (seasonal) and long-term (years to decades) studies to quantify the internal P loading and P release potential in the sediments of Lake Chaohu and explore their controlling mechanisms. In the west region of the lake, short-term P diffusive fluxes ranged from 0.2 mg/m2·d-1 to 6.69 mg/m2·d-1 (averaged 2.76 mg/m2·d-1) and long-term net P release ranged from 2.25 mg/m2·d-1 to 8.94 mg/m2·d-1 (averaged 5.34 mg/m2·d-1); in the east region, short-term P diffusive fluxes varied from 0.73 mg/m2·d-1 to 1.76 mg/m2·d-1 (averaged 1.05 mg/m2·d-1) and long-term P release ranged from 0.13 mg/m2·d-1 to 4.15 mg/m2·d-1 (averaged 1.3 mg/m2·d-1). Both short- and long-term P releases were in the same order of magnitudes as the external P inputs (3.56 mg/m2·d-1). Comparison of the long-term and short-term sediment P release indicates that while the high summer P release in the east might only represent a snapshot value, the sediments in the west contribute to large P release for years or even decades, impeding water quality recovery under lake management. Mobilization of surface sediment legacy P accounted for 81% of short-term P release. The long-term release was dominated by remobilization of iron bond P (BD-P) (average 52.1%) at all sites, while Aluminium-bound P (NaOH-rP) exhibited partly reactive and potentially mobile, releasing P to the water column in most sites in the west. Our study demonstrates the importance of sediments as P sources in lake Chaohu. The combination of short- and long-term P release studies can help understand the roles of sediments in regulating the water quality and eutrophication.

Dual diffusive gradients in the thin films (DGT) probes provide insights into speciation and mobility of sediment chromium (Cr) from the Xizhi River basin, South China.

Investigation of the speciation and remobilization mechanisms of chromium (Cr) in sediment is essential for accurate estimation of its ecological risks in aquatic systems. In this work, a three-step chemical extraction procedure and diffusive gradient in thin films (DGT) technique were combined to investigate the geochemical speciation, mobility potentials, and release characteristics of sediment Cr. The geochemical speciation of sediment Cr decreased in the following order: oxidizable > reducible > residual > acid-soluble fraction. Dissociation of OM-bound Cr(III) and oxidation by Mn oxides contributed to higher labile Cr(III) and Cr(VI) levels in winter, with the labile Cr(III) being the dominant species and accounting for 48.1%65.5% of the total concentration of labile Cr; whereas, reductive dissolution of Mn oxides was responsible for the remobilization of labile Cr(VI) in summer, leading to a shift in dominant Cr species to Cr(VI) (48.9%65.7%) due to rapid precipitation of Cr(III). Sediment acted as a major sink for labile Cr(VI) in two sampling campaigns. For labile Cr(III), sediment converted from source in winter to sink in summer. The diffusive release of labile Cr(III) deserves preferential concern due to its potential to be re-oxidized to more toxic Cr(VI) under the oxic conditions of river water in winter.

Greenhouse gas emissions from Baltic coastal lakes.

Coastal lakes (CL) act as limnetic-ß-oligohaline systems located on non-tidal coastlines in fresh and salt water mixing zone. Owing to considerable terrestrial nutrient input and a high autochthonous productivity CLs release greenhouse gases (GHG) to the ambient atmosphere, however, neither emission from the system was assessed nor controls on the emission were recognized so far. In this study we attempted to quantify diffusive emissions of CH4, CO2 and N2O from CLs based on data collected from seven lakes located on a south coast of the Baltic Sea in Poland. Lake water samples were collected with quarterly resolution along salinity, water depth and wind fetch gradients. From our data it emerged that the concentrations of GHGs were determined by temperature. CH4 showed dependence on salinity, lake water depth and wind fetch. N2O was controlled by dissolved O2 and NO3- and CO2 was largely related to wind fetch. It also appeared that concentrations of N2O and CO2 were influenced by terrestrial nutrient input. The mean fluxes of CH4, CO2 and N2O for the whole system were 21.7 mg·m-2·d-1, 12.7 g·m-2·d-1 and 0.74 mg·m-2·d-1, respectively which was equivalent to 7.9 g CH4·m-2·y-1, 4.6 kg CO2·m-2·y-1 and 269 mg N2O·m-2·y-1. CH4 and N2O were released throughout the year and CO2 was predominantly emitted during winter. We showed that diffusive emissions of the GHGs showed relationships to the surface area of the lakes as well as the ratio of catchment area to lake area (CA/LA). The study would benefit from further extension with higher resolution analyses of the lakes over longer timescales and quantification of ebullitive GHG emission (CH4 in particular).

Seasonal variations of cadmium (Cd) speciation and mobility in sediments from the Xizhi River basin, South China, based on passive sampling techniques and a thermodynamic chemical equilibrium model.

Understanding the speciation and mobilization mechanisms of potentially toxic metals in sediments is critical to aquatic ecosystem health and contamination remediation in urban rivers. In this study, chemical sequential extraction, a thermodynamic chemical equilibrium model (Visual MINTEQ ver. 3.1), diffusive gradient in thin films (DGT), and high-resolution dialysis (HR-Peeper) techniques were integrated to identify seasonal variations in cadmium (Cd) mobility in sulfidized sediments. Acid-soluble Cd was the dominant geochemical fraction in sediments, followed by residual, oxidizable, and reducible Cd. The DGT-labile Cd concentration was associated with various geochemical processes and was independent of the total concentration and geochemical fractionation of Cd in sediments. Sulfate reduction facilitated the formation of insoluble CdS and induced low Cd concentrations in sediment porewater. Sulfide oxidation was principally responsible for lowered porewater pH and elevated Cd concentrations in summer. Strongly acidic conditions promoted release of sediment Cd but might reduce the binding efficiency of Chelex resin gel for dissolved Cd, leading to underestimation of the mobility of sediment Cd. Sediments generally functioned as a sink for Cd in winter and shifted to acting as a source in summer, releasing Cd into the overlying water mainly as Cd-S complexes with high potential to migrate downstream.

Spatial variations in diffusive methane fluxes and the role of eutrophication in a subtropical shallow lake.

Shallow lakes account for most of the diffusive CH4 emissions from global lakes, and they also suffer from eutrophication worldwide. Determining the effect of eutrophication on diffusive CH4 fluxes is fundamental to understanding CH4 emissions in shallow lakes. This study aimed to investigate the spatial variations in diffusive CH4 fluxes and explore the role of eutrophication in Lake Chaohu, a large and shallow eutrophic lake in the lower reaches of the Yangtze River. A one-year field observation was carried out to examine CH4 concentrations in the sediment and water and the diffusive fluxes of CH4 across the sediment-water interface (Fs-w) and water-air interface (Fw-a). Both Fs-w (0.306-1.56 mmol m-2 d-1) and Fw-a (0.097-0.529 mmol m-2 d-1) were upward and showed significant spatial heterogeneity and were significantly positively correlated. Parameters related to eutrophication had significant positive relationships with Fw-a, and the total phosphorus distribution in the water explained the greatest proportion of the spatial variation in Fw-a. Distance to shore and water depth were inversely correlated with Fw-a and modified the effects of eutrophication. Overall, the results provide direct evidence of the key role of eutrophication in shaping the spatial distribution of diffusive CH4 fluxes and a scientific basis for predicting changes in CH4 emissions with future eutrophication changes in shallow lakes in subtropical zones.

Release characteristics and mechanisms of sediment phosphorus in contaminated and uncontaminated rivers: A case study in South China.

Phosphorus (P) cycling present in sediments associated with iron (Fe), manganese (Mn) and sulfur (S) geochemical processes may cause secondary pollution in overlying water. Understanding the mechanisms of P release from sediments should help to restore water quality. This study used the diffusive gradients in thin film (DGT) technique to investigate the seasonal variation in the lability, remobilization mechanisms, and release characteristics of sediment P in the uncontaminated Xizhi River and the severely contaminated Danshui River, South China. P accumulation in sediments contributed to higher DGT-labile P concentrations in contaminated reaches, and the highest labile P concentrations were generally observed in summer season at each site. The significant positive relationships (p < 0.05) between labile Fe and P confirmed the Fe-P coupling release mechanism in uncontaminated sediments. Stronger relationships between labile Mn and P at contaminated sites indicated that Mn oxides played an important role in P remobilization. However, sulfate reduction associated with microbial activities (crucial genera: Desulfobulbus, Desulfomicrobium and Desulforhabdus) was considered to decouple the Fe & Mn-P cycling relationship, promoting P release at contaminated sites. The effluxes of sediment P were much higher in the Danshui River (mean 0.132 mg cm-2·d-1) than in the Xizhi River (mean 0.038 mg cm-2·d-1). And hot season led to growth in P effluxes that was much greater in contaminated river.

Diffusive flux of CH4 and N2O from agricultural river networks: Regression tree and importance analysis.

River networks in subtropical agricultural hilly region become an inconvenient greenhouse gas (GHG, methane and nitrous oxide) source because of the influence of human activities, which has caused large uncertainties for refinement of national GHG inventories and their global budget. Based on field monitoring experiments at high temporal resolution, we employed regression tree and importance analysis to identify quantitatively factors that influence the diffusive flux of GHGs to provide a scientific basis for reducing GHG emissions and controlling regional carbon and nitrogen losses. The results indicate that significant spatiotemporal variation of methane (CH4) nitrous oxide (N2O) diffusion occurs in all the four reaches (W1, W2, W3 and W4) of Tuojia river networks. Among them, W1 contributed lowest CH4 (22.55 µg C m-2 h-1) and N2O (5.00 µg N m-2 h-1) diffusive flux than the other three (P < 0.05), while W4 offered highest CH4 (166.15 µg C m-2 h-1) and N2O (30.47 µg N m-2 h-1) diffusive flux but with no statistically significant difference between W2 and W3 due to homogeneous extraneous nutrition loading into the two reaches. W4 also contributed largest cumulative flux of CH4 (14.55 kg C ha-1 yr-1) and N2O (2.69 kg N ha-1 yr-1) in Tuojia River networks (P < 0.05). Furthermore, the regression tree and importance analysis indicate that, in the anaerobic environment, dissolved oxygen saturation controlled the production and diffusion for both CH4 and N2O. The findings of this investigation highlighted that decision support tools provide an effective pathway to enhance the GHG mitigation technology research in agroecosystems and simultaneously shed light on the global campaign on refinement of national GHG inventories as well as regional nutrient management.

[Release Mechanisms of Iron and Manganese from Sediments in Jinpen Reservoir].

In response to the annual hypolimnetic anoxia in stratified reservoirs, water-lifting aerators (WLAs) were used in Jinpen Reservoir to supplement the dissolved oxygen in the bottom water and suppress the release of reduced pollutants from sediment. However, due to the influence of geomorphic characteristics at the bottom of the reservoir, there were some differences in the efficiency of artificial mixing and aeration. After the deactivation of WLAs, the dissolved oxygen in the bottom water of some deeper areas was rapidly depleted, resulting in the re-release of pollutants. To explore the release mechanisms and diffusion intensity of iron and manganese during this period, the representative samples in the main reservoir area were collected to measure the distribution of dissolved iron and manganese in the pore water and overlying water and calculate the diffusive flux of dissolved iron and manganese across the sediment-water interface. The results showed that the bottom water of the lower terrain rapidly entered the anaerobic condition after the system was deactivated, resulting in the release of a large amount of dissolved manganese into the overlying water, the maximum concentration of which was 0.42 mg·L-1. However, the bottom water of the higher terrain briefly entered a state of hypoxia, after which the dissolved oxygen concentration increased rapidly, so the dissolved manganese concentration increased moderately to 0.17 mg·L-1. The distribution of iron and manganese in the pore-water-overlying water showed that the dissolved manganese was released more easily into the overlying water than the iron under anaerobic conditions and constant accumulation in the upper sediments and overlying water. However, the release of dissolved iron was not only suppressed by dissolved oxygen but also by other oxidants such as manganese oxide. The diffusion flux of dissolved manganese declined after the system was deactivated. A mass balance calculation demonstrated that the accumulation of dissolved manganese in the anaerobic layer was not only related to the diffusion flux but also to the sedimentation flux and the thickness of the anaerobic layer. Therefore, the biogeochemical cycle of iron and manganese in the anaerobic layer requires further study.

Controlling reduced iron and manganese in a drinking water reservoir by hypolimnetic aeration and artificial destratification.

The concentrations of iron (Fe) and manganese (Mn) in the water column have extremely important effects on the water quality of drinking water reservoirs; however, reservoirs often experience problematic Fe and Mn levels during seasonal stratification and rainfall events. Water-lifting aerators (WLAs) were deployed in the Jinpen Reservoir to control these issues with Fe and Mn at the source via bottom aeration and artificial destratification. In this study, variations of Fe and Mn concentrations in the water column, porewater, and sediments, were used to characterize behaviors of reduced Fe and Mn under the conditions of hypolimnetic aeration and artificial destratification during periods of hypolimnion hypoxia and rainfall events. The results showed that replenishing aquatic oxygen levels by aeration can effectively decrease the dissolved Fe and Mn in the water column thereby increasing the sedimentation rate and the diffusive flux of Fe and Mn at the sediment-water interface (SWI). The dissolved Fe was significantly chemically oxidized and the concentration remained relatively low in the water column during WLA operations, while dissolved Mn persistently accumulated in the near-sediment regions because of its complex kinetics. Our in situ profiles of labile Fe and Mn in the sediments demonstrated that the diffusive flux of Mn (JMn) was largely increased by the increased concentration gradient at the SWI, while the diffusive flux of Fe (JFe) decreased. The sediments were observed to rapidly become anoxic and release Fe and Mn after WLA deactivation; this emphasized the importance of appropriate operations linking the artificial and natural mixing periods to prevent SWI hypoxia and the release of reduced substances.

Contrasting exchanges of nitrogen and phosphorus across the sediment-water interface during the drying and re-inundation of littoral eutrophic sediment.

High water level fluctuations (WLFs) lead to periodic drying and re-inundation of sediments in the littoral area of eutrophic lakes. In this study, a series of littoral sediment cores were dried for different periods (5-30 d) and rewetted for 48 h. The sediment cores that dried for 30 d were then re-inundated for 90 d. The exchanges of nitrogen (N) and phosphorus (P) across the sediment-water interface (SWI) and the mechanisms were studied. The results showed that ammonium nitrogen (NH4+-N) fluxes increased after 5-25 d of drying, which was followed by an obvious decrease after 30 d of drying. The decreased NH4+-N fluxes remained at low levels during the 90 d re-inundation period. The soluble reactive P (SRP) fluxes decreased significantly after 15 d of drying. However, further re-inundation increased the SRP fluxes to their initial levels. The decreased water content and porosity, the oxidation of the sediment during drying, and the associated transformations of the N and P fractions in the sediment from drying to re-inundation influenced the exchanges of NH4+-N and SRP across the SWI. The decrease of labile NH4+-N in the sediment during drying was non-reversible, while the transformations between redox sensitive P (Fe-P) and aluminum-bound P were more likely to be reversible from drying to re-inundation. The increase of Fe-P during drying and dissolution of Fe-P during the re-inundation were responsible for the development of SRP fluxes from drying to re-inundation. Therefore, the periodic drying and re-inundation of the littoral eutrophic sediments reduced the release of NH4+-N but accelerated the release of SRP from the sediment. This should be given more consideration for the remediation and management of eutrophication in the lake and other similar lakes with high WLFs.

[Diffusive CO2 Flux Across the Water-air Interface of Reclaimed Shrimp Ponds in the Minjiang River Estuary Based on the TBL Model].

Freshwater aquatic ecosystems are important sources of greenhouse gases, such as CO2. However, few studies have presented data on the greenhouse gas flux from coastal aquaculture ponds. Diffusion models are important tools for estimating the CO2 exchange flux across the water-air interface of aquatic ecosystems. Several different parameterized means were selected to estimate the CO2 gas exchange rate (kx) and CO2 diffusive flux across the water-air interface of shrimp ponds in the Minjiang River Estuary. The results indicated that:① the CO2 gas exchange rate and diffusive flux over the culture period all presented significant temporal variation. This variation showed a dynamic trend:October > September > November > July > August and November > July > August > September > October. ② Wind speed, kx, CO2 concentration, pH, DOC concentration, and Chl-a concentration were important factors affecting the temporal variation of CO2 diffusive flux. ③ There were differences in the estimated value of CO2 diffusive flux across the water-air interface of the culture ponds in the Minjiang River Estuary among different parameterized approaches (P<0.01). This indicates that the model method has some uncertainties in estimating the CO2 diffusive flux in culture ponds. Our results suggest that the models RC01 and CW03 are more suitable methods for estimating the CO2 diffusive flux at the water-air interface of estuarine reclaimed aquaculture ponds in the Minjiang River Estuary, after comprehensive analysis of the water environment and the different estimation results.