Different effects of seasonal impoundment and land use change on microbiome in a tributary sediment of the three gorgers reservoir.

Anthropogenic activities significantly impact river ecosystem nutrient fluxes and microbial metabolism. Here, we examined the seasonal and spatial variation of sediments physicochemical parameters and the associated microbiome in the Pengxi river, a representative tributary of Three Gorges Reservoir, in response to seasonal impoundment and land use change by human activities. Results revealed that seasonal impoundment and land use change enhanced total organic carbon (TOC), total nitrogen (TN) and ammonium nitrogen (NH4+-N) concentration in the sediment, but have different effects on sediment microbiome. Sediment microbiota showed higher similarity during the seasonal high-water level (HWL) in consecutive two years. The abundant phyla Acidobacteria, Gemmatimonadetes, Cyanobacteria, Actinobacteria and Planctomycetes significantly increased as water level increased. Along the changes in bacterial taxa, we also observed changes in predicted carbon fixation functions and nitrogen-related functions, including the significantly higher levels of Calvin cycle, 4HB/3HP cycle, 3HP cycle and assimilatory nitrate reduction, while significantly lower level of denitrification. Though land use change significantly increased TOC, TN and NH4+-N concentration, its effects on spatial variation of bacterial community composition and predicted functions was not significant. The finding indicates that TGR hydrologic changes and land use change have different influences on the carbon and nitrogen fluxes and their associated microbiome in TGR sediments. A focus of future research will be on assessing on carbon and nitrogen flux balance and the associated carbon and nitrogen microbial cycling in TGR sediment.

Characterizing the impact of reservoir storage and discharge on nitrogen dynamics in an upstream wetland using a δ15N and δ18O dual-isotope approach.

The identification of nitrate sources in reservoir water is important for watershed-scale surface pollution management. Significant fluctuations in river water levels arising from reservoir storage and discharge influence nitrate sources and transport processes. The Sanmenxia Reservoir, in the middle reaches of the Yellow River in China, undergoes significant water level changes (290-316 m), altering the composition of the nitrogen sources. This study employed a δ15N and δ18O dual-isotope method and MixSIAR modeling to quantify the contributions of nitrate sources. This reveals the impact of reservoir water impoundment and discharge on nitrogen dynamics in the upstream region of the wetland and the model sensitivity for each nitrate source. The results showed that the average concentrations of nitrate­nitrogen (NO- 3-N) were elevated during the impoundment period compared to the discharge period. Nitrogen sources exhibited varying proportions in surface water, groundwater, and soil water during both the impoundment and discharge periods. The predominant sources include manure and sewage (MS), with a maximum proportion of 57.4 % in surface water. Soil nitrogen (SN) accounted for 25.8 % of groundwater nitrogen and 32.1 % of soil water nitrogen during the impoundment period, whereas, during the discharge period, soil nitrogen made up 41.4 % of surface water nitrogen, manure and sewage contributed 44.8 % of groundwater nitrogen, and manure and sewage dominated with 56.7 % of soil water nitrogen. Sensitivity analysis of the MixSIAR model revealed that the isotopic composition of the manure and sewage primary source most significantly influenced the apportionment results of the riverine nitrate source. Reservoir discharge facilitates the dissimilatory nitrate reduction to ammonium (DNRA). The migration of NO- 3 from surface water to soil water and groundwater occurred from the impoundment period to the discharge period.

Influence of a hydroelectric dam on fish mercury contamination along the Sinnamary River (French Guiana).

The Petit Saut hydroelectric dam and the upstream and downstream areas of the Sinnamary River in French Guiana (Amazon basin) have been studied from 1993 to 2020. The nearly thirty-years-long study of the monitoring of total mercury concentration in fish and the physicochemical survey of the environment made it possible to demonstrate the impact of the flooding of the forest and the role of the hydroelectric dam on the methylation of mercury. Results show that the physicochemical modifications generated by the construction of the dam led to a significant production of methylmercury (MeHg) in the anoxic part of the reservoir and downstream of the river leading to a strong spatio-temporal impact of the dam. Seven species of fishes are studied and their mercury concentrations vary according to many parameters: fish diet, position in the water column, site, lake oxycline level and time.

Assessment of combustion residual leachate volume, composition, and treatment costs.

Combustion residuals and the resulting leachate from storage sites represent a large volume of wastewater in the United States (U.S.) that has not been quantified. This work estimates the constituents present, volume of wastewater, and costs of treatment for both combustion residual landfill leachate and the leachate from surface impoundment closures. Combustion residual landfill leachate produced from contact with bituminous coal combustion byproducts is generally predicted to be higher in lithium and manganese, whereas landfill leachate produced from contact with subbituminous coal combustion byproducts is generally predicted to be higher in mercury and vanadium. The annual volume of a single landfill with combustion residual leachate can reach more than 800,000 cubic meters. This leachate represents an annual volume of 26.8-42.8 million cubic meters nationally. Closing surface impoundments can yield between 830 and 1040 cubic meters of leachate nationally for a three-year closure period. Costs as low as $1.5/m3 or as high as $95/m3 are observed. Treatment trains will need to remove 72% of total suspended solids (TSS), 87% of arsenic, and 64% of mercury from landfill leachate. When applied to impoundments, these treatment trains would need to remove 97% of arsenic.

Effect of land use conversion on heavy metals and magnetic minerals on water reservoir riparian soils.

Spatial hydrological alterations seem to influence the variability of soil-bound magnetic particles and heavy metals in water reservoir riparian wetlands (RW). To date, applying geochemical analysis with magnetic techniques to assess heavy metal pollution is rarely practiced in RW soils. We studied the magnetic properties and heavy metals, including Cu, Cr, and Zn, of topsoils in RW and the adjacent upland regions (UR, as control) in the Three Gorges Reservoir, China. Potentially elevated low-frequency mass magnetic susceptibility (χLF), anhysteretic remanent magnetization susceptibility (χARM), isothermal remanent magnetism, and all selected heavy metals were found in RW. The grain size of the magnetic minerals was coarser in RW than that in UR. The pollution load index (PLI) of the studied samples was 1.18 ± 0.12 and 1.04 ± 0.21 in RW and UR, respectively. PLI and concentrations of Cu, Cr, and Zn were positively correlated with χLF, χARM, and isothermal remanent magnetism in RW, whereas no clear linkages were observed between PLI and isothermal remanent magnetism in UR. This finding reveals that hydrological alterations increased the magnetic enhancement and heavy metal enrichment in RW. We find that magnetic proxies of soils could trace the concentration of selected anthropogenic heavy metals and their pollution level in RW.

Basin-specific pollution and impoundment effects on greenhouse gas distributions in three rivers and estuaries.

Large uncertainties exist regarding the combined effects of pollution and impoundment on riverine greenhouse gas (GHG) emissions. It has also been debated whether river eutrophication can transform downstream estuaries into carbon sinks. To assess human impacts on the riverine and estuarine distributions of CO2, CH4, and N2O, two source-to-estuary surveys along three impounded rivers in Korea were combined with multiple samplings at five or six estuarine sites. The basin-wide surveys revealed predominant pollution effects generating localized hotspots of riverine GHGs along metropolitan areas. The localized pollution effect was pronounced in the lower Han River and estuary adjacent to Seoul, while the highest GHG levels in the upper Yeongsan traversing Gwangju were not carried over into the faraway estuary. CH4 levels were elevated across the eutrophic middle Nakdong reaches regulated by eight cascade weirs in contrast to undersaturated CO2 indicating enhanced phytoplankton production. The levels of all three GHGs tended to be higher in the Han estuary across seasons. Higher summer-time δ13C-CH4 values at some Nakdong and Yeongsan estuarine sites implied that temperature-enhanced CH4 production may have been dampened by increased CH4 oxidation. Our results suggest that the location and magnitude of pollution sources and impoundments control basin-specific longitudinal GHG distributions and estuarine carryover effects, warning against simple generalizations of eutrophic rivers and estuaries as carbon sinks.

Field investigation on the change process of microbial community structure in large-deep reservoir during the initial impoundment.

During the initial impoundment of large-deep reservoir, the aquatic environment changed dramatically in various aspects such as water level, hydrological regime, and pollutants, which could alter microorganisms' community structure, break the balance of the aquatic ecosystem and even endanger the aquatic ecosystem. However, the interaction of microbial communities and water environment during the initial impoundment process of a large-deep reservoir remained unclear. To this end, in-situ monitoring and sampling analysis on water quality and microbial communities during the initial impoundment process of a typical large-deep reservoir named Baihetan were conducted so as to explore the response of microbial community structure to the changes of water environmental factors during the initial impoundment of large deep reservoir and reveal the key driving factors affecting microbial community structure. The spatio-temporal variation in water quality was analyzed, and the microbial community structure in the reservoir was investigated based on high-throughput sequencing. The results showed that the COD of each section increased slightly, and the water quality after impoundment was slightly poorer than that before the impoundment. Water temperature and pH were proved to be the key factors affecting the structure of bacterial and eukaryotic communities respectively during the initial impoundment. The research results revealed the role of microorganisms and their interaction with biogeochemical processes in the large-deep reservoir ecosystem, which was crucial for later operation and management of the reservoir and the protection of the reservoir water environment.

Evaluation of re-depositing hydropower trapped sediments at large rivers to improve flood protection and aquatic habitats - Case study Danube/Austria.

Sedimentation in impoundments of run-of-river hydropower plants is an ongoing and progressing management issue for hydropower operators because of its consequences for e.g., the maintenance of flood protection and waterway parameters. Current practices in sediment management are often unsustainable, associated with high costs, and can pose risks for downstream biota (e.g. during flushing). The present study elaborated a conceptual model of a novel sediment management strategy for impounded river sections consisting of a current-state and deficit analysis, and the application of a novel sediment management practice, which was practically implemented at the study site at the Austrian Danube River. This novel practice consists of (i) local dredging of gravel at locations in the impoundment, which are problematic in terms of flood protection and waterway maintenance, and (ii) the re-deposition of dredged sediments by artificial placement of gravel structures. The present study included morphological analyses of the impounded section at the study site by applying the channel profile budget technique. The knowledge of the long-term morphological development served as a basis for the evaluation of the sediment management measures and for the elaboration of the conceptual model. The combination of the morphodynamic characteristics in the impoundment with the implementation of the novel sediment management practice helped to derive generalized statements for the potential implementation of the presented conceptual model in large rivers with similar impoundment characteristics. We further defined several aspects, which are related to sediment dynamics in impounded river sections, flood protection, waterway demands, and ecological criteria that require consideration for an efficient realization of the proposed conceptual model.

Submergence in the Dry Season Alters Microbial Nitrogen Transformations in the Root Zone of Carex cinerascens: A Mesocosm Study in One Floodplain Lake.

The increasing demand for water resources has triggered a series of water level regulation (WLR) projects, which exerts considerable effects on local hydrologic conditions. In particular, artificial impoundments, which may occur during the dry season in wetlands, increase the periods of waterlogging. However, little is known about their potential effects on biogeochemical cycling. To evaluate how impoundments affect nitrogen (N) cycling in the floodplain ecosystem, we conducted a mesocosm experiment to investigate N dynamics and the potential N-gene changes in the root-zone soil of the dominant plant in one large floodplain lake (Poyang Lake, China). The results indicated that, compared with the control, the 12 cm submergence treatment (SP12) caused NH4 +-N accumulation in the root-zone soil on day 14 and day 41. On the contrary, NO3 --N levels in SP12 were statistically lower than those in the control from day 7 to day 28. The curve of organic N had a tendency of declining as a whole. Changes in N-gene abundances revealed that SP12 significantly inhibited nitrification and enhanced denitrification in root-zone soil. Moreover, SP12 enhanced the links and complexity of the N-gene network, reflecting the increased correlations among the N transformations under flooding stress. Considering the increasing demand for WLR worldwide, the study about the effects of anti-seasonal submergence on biogeochemical cycling in floodplains provides insight into the ecological impacts of anthropogenic activities. Supplementary Information: The online version contains supplementary material available at 10.1007/s13157-022-01656-1.

Reservoirs regulated by small dams have a similar warming effect than lakes on the summer thermal regime of streams.

Small dams account for the majority of reservoirs throughout the world, yet little is known about their effects on stream temperatures. Given that water temperature is vital for maintaining the integrity of aquatic ecosystems, studying the effects of small dams is important. This study aims to understand the effect of small dams on summer stream temperatures in a protected area in southern Quebec, Canada. We assessed the effect of small surface-release dams on four attributes of the thermal regimes (magnitude, frequency and duration of warm events, and rate of change) of streams by comparing water temperature measured in the main tributary upstream and in the main outlet downstream of the reservoir. We also compared the thermal effects of reservoirs to those of natural lakes of similar size. Using a generalized additive model, we identified key determinants of stream temperature to assess the influence of reservoir and natural lake characteristics on the thermal regime of streams. In August 2020, we observed an average warming of 3.7 °C downstream of reservoirs regulated by small dams compared to conditions upstream of the reservoir. During this period, the warming effect of reservoirs was not significantly different from the warming effect of natural lakes (3.4 °C). In addition to the drainage area, distance to an upstream water body, and the proportion of the watershed occupied by water bodies were the primary determinants of stream temperature in August, demonstrating the importance of nearby water bodies on stream thermal regimes. Given their warming effect, small waterbodies may limit the available habitat for species that are sensitive to warm temperatures. As the construction of small dams is accelerating at the global scale, a clear understanding of the cumulative effects of small lakes and reservoirs on stream temperature is required to ensure the sound management of aquatic ecosystems.

Adaptation measures under the impacts of climate and land-use/land-cover changes using HSPF model simulation: Application to Gongola river basin, Nigeria.

Recently, there is an upsurge in flood emergencies in Nigeria, in which their frequencies and impacts are expected to exacerbate in the future due to land-use/land cover (LULC) and climate change stressors. The separate and combined forces of these stressors on the Gongola river basin is feebly understood and the probable future impacts are not clear. Accordingly, this study uses a process-based watershed modelling approach - the Hydrological Simulation Program FORTRAN (HSPF) (i) to understand the basin's current and future hydrological fluxes and (ii) to quantify the effectiveness of five management options as adaptation measures for the impacts of the stressors. The ensemble means of the three models derived from the Coupled Model Intercomparison Project Phase 5 (CMIP5) are employed for generating future climate scenarios, considering three distinct radiative forcing peculiar to the study area. Also, the historical and future LULC (developed from the hybrid of Cellular Automata and Markov Chain model) are used to produce the LULC scenarios for the basin. The effective calibration, uncertainty and sensitivity analyses are used for optimising the parameters of the model and the validated result implies a plausible model with efficiency of up to 75 %. Consequently, the results of individual impacts of the stressors yield amplification of the peak flows, with more profound impacts from climate stressor than the LULC. Therefore, the climate impact may trigger a marked peak discharge that is 48 % higher as compared to the historical peak flows which are equivalent to 10,000-year flood event. Whilst the combine impacts may further amplify this value by 27 % depending on the scenario. The proposed management interventions such as planned reforestation and reservoir at Dindima should attenuate the disastrous peak discharges by almost 36 %. Furthermore, the land management option should promote the carbon-sequestering project of the Paris agreement ratified by Nigeria. While the reservoir would serve secondary functions of energy production; employment opportunities, aside other social aspects. These measures are therefore expected to mitigate feasibly the negative impacts anticipated from the stressors and the approach can be employed in other river basins in Africa confronted with similar challenges.

Freshwater fishes (Actinopterygii) of Kenyir Reservoir, Peninsular Malaysia: Updated checklist, taxonomic concerns and alien species.

Background: A total of 87 freshwater fish species from 30 families were recorded from the Kenyir Reservoir, Peninsular Malaysia, where 75 are native and 12 are introduced species. Few species still have unstable taxonomy identities which urge further studies. Most of the species were categorised as Least Concern (LC) and two were threatened species; Endangered and Critically Endangered (EN and CR). One introduced species, Gambusiaaffinis is widespread in the human-associated area, while other introduced fish species can be considered low in numbers. New information: Twenty five fish species are recorded for the first time in the Kenyir Reservoir.

Soil carbon consequences of historic hydrologic impairment and recent restoration in coastal wetlands.

Coastal wetlands provide key ecosystem services, including substantial long-term storage of atmospheric CO2 in soil organic carbon pools. This accumulation of soil organic matter is a vital component of elevation gain in coastal wetlands responding to sea-level rise. Anthropogenic activities that alter coastal wetland function through disruption of tidal exchange and wetland water levels are ubiquitous. This study assesses soil vertical accretion and organic carbon accretion across five coastal wetlands that experienced over a century of impounded hydrology, followed by restoration of tidal exchange 5 to 14 years prior to sampling. Nearby marshes that never experienced tidal impoundment served as controls with natural hydrology to assess the impact of impoundment and restoration. Dated soil cores indicate that elevation gain and carbon storage were suppressed 30-70 % during impoundment, accounting for the majority of elevation deficit between impacted and natural sites. Only one site had substantial subsidence, likely due to oxidation of soil organic matter. Vertical and carbon accretion gains were achieved at all restored sites, with carbon burial increasing from 96 ± 33 to 197 ± 64 g C m-2 y-1. The site with subsidence was able to accrete at double the rate (13 ± 5.6 mm y-1) of the natural complement, due predominantly to organic matter accumulation rather than mineral deposition, indicating these ecosystems are capable of large dynamic responses to restoration when conditions are optimized for vegetation growth. Hydrologic restoration enhanced elevation resilience and climate benefits of these coastal wetlands.

Downstream Transport of Geosmin Based on Harmful Cyanobacterial Outbreak Upstream in a Reservoir Cascade.

Understanding water quality events in a multiple-impoundment series is important but seldom presented comprehensively. Therefore, this study was conducted to systematically understand the explosion event of geosmin (GSM) in the North Han River (Chuncheon, Soyang, Euiam, and Cheongpyeong Reservoirs) and Han River (Paldang Reservoir), which consists of a cascade reservoir series, the largest drinking water source system in South Korea. We investigated the spatiotemporal relationship of harmful cyanobacterial blooms in the upstream reservoir (Euiam) with the water quality incident event caused by the GSM in the downstream reservoir (Paldang) from January to December 2011. The harmful cyanobacterial bloom occurred during August−September under a high water temperature (>20 °C) after a heavy-rainfall-based flood runoff event. The high chlorophyll-a (Chl-a) concentration in the upper Euiam Reservoir was prolonged for two months with a maximum concentration of 1150.5 mg m−3, in which the filamentous Dolichospermum circinale Kütz dominated the algal community at a rate of >99%. These parameters remarkably decreased (17.3 mg Chl-a m−3) in October 2011 when the water temperature decreased (5 °C) and soluble reactive phosphorus was depleted. However, high and unprecedented GSM concentrations, with a maximum value of 1640 ng L−1, were detected in the downstream reservoirs (Cheongpyeong and Paldang); the level was 11 times higher than the value (10 ng L−1) recommended by the World Health Organization. The concentrations of GSM gradually decreased and had an adverse effect on the drinking water quality until the end of December 2011. Our study indicated that the time lag between the summer−fall cyanobacterial outbreak in the upstream reservoir and winter GSM explosion events in the downstream reservoirs could be attributed to the transport and release of GSM through the effluent from hydroelectric power generation in this multiple-reservoir system. Therefore, we suggest that a structural understanding of the reservoir cascade be considered during water quality management of drinking water sources to avoid such incidents in the future.

Fencing farm dams to exclude livestock halves methane emissions and improves water quality.

Agricultural practices have created tens of millions of small artificial water bodies ("farm dams" or "agricultural ponds") to provide water for domestic livestock worldwide. Among freshwater ecosystems, farm dams have some of the highest greenhouse gas (GHG) emissions per m2 due to fertilizer and manure run-off boosting methane production-an extremely potent GHG. However, management strategies to mitigate the substantial emissions from millions of farm dams remain unexplored. We tested the hypothesis that installing fences to exclude livestock could reduce nutrients, improve water quality, and lower aquatic GHG emissions. We established a large-scale experiment spanning 400 km across south-eastern Australia where we compared unfenced (N = 33) and fenced farm dams (N = 31) within 17 livestock farms. Fenced farm dams recorded 32% less dissolved nitrogen, 39% less dissolved phosphorus, 22% more dissolved oxygen, and produced 56% less diffusive methane emissions than unfenced dams. We found no effect of farm dam management on diffusive carbon dioxide emissions and on the organic carbon in the soil. Dissolved oxygen was the most important variable explaining changes in carbon fluxes across dams, whereby doubling dissolved oxygen from 5 to 10 mg L-1 led to a 74% decrease in methane fluxes, a 124% decrease in carbon dioxide fluxes, and a 96% decrease in CO2 -eq (CH4 + CO2 ) fluxes. Dams with very high dissolved oxygen (>10 mg L-1 ) showed a switch from positive to negative CO2 -eq. (CO2 + CH4 ) fluxes (i.e., negative radiative balance), indicating a positive contribution to reduce atmospheric warming. Our results demonstrate that simple management actions can dramatically improve water quality and decrease methane emissions while contributing to more productive and sustainable farming.

Impoundment increases methane emissions in Phragmites-invaded coastal wetlands.

Saline tidal wetlands are important sites of carbon sequestration and produce negligible methane (CH4 ) emissions due to regular inundation with sulfate-rich seawater. Yet, widespread management of coastal hydrology has restricted tidal exchange in vast areas of coastal wetlands. These ecosystems often undergo impoundment and freshening, which in turn cause vegetation shifts like invasion by Phragmites, that affect ecosystem carbon balance. Understanding controls and scaling of carbon exchange in these understudied ecosystems is critical for informing climate consequences of blue carbon restoration and/or management interventions. Here, we (1) examine how carbon fluxes vary across a salinity gradient (4-25 psu) in impounded and natural, tidally unrestricted Phragmites wetlands using static chambers and (2) probe drivers of carbon fluxes within an impounded coastal wetland using eddy covariance at the Herring River in Wellfleet, MA, United States. Freshening across the salinity gradient led to a 50-fold increase in CH4 emissions, but effects on carbon dioxide (CO2 ) were less pronounced with uptake generally enhanced in the fresher, impounded sites. The impounded wetland experienced little variation in water-table depth or salinity during the growing season and was a strong CO2 sink of -352 g CO2 -C m-2  year-1 offset by CH4 emission of 11.4 g CH4 -C m-2  year-1 . Growing season CH4 flux was driven primarily by temperature. Methane flux exhibited a diurnal cycle with a night-time minimum that was not reflected in opaque chamber measurements. Therefore, we suggest accounting for the diurnal cycle of CH4 in Phragmites, for example by applying a scaling factor developed here of ~0.6 to mid-day chamber measurements. Taken together, these results suggest that although freshened, impounded wetlands can be strong carbon sinks, enhanced CH4 emission with freshening reduces net radiative balance. Restoration of tidal flow to impounded ecosystems could limit CH4 production and enhance their climate regulating benefits.

[Effects of the Three Gorges Reservoir Operation on Vertical Distribution of Chlorophyll a and Environmental Factors in Tributaries].

The hydrodynamics and environmental factors in the Xiangxi River (XXR) and Shennong River (SNR), which are tributaries of the Three Gorges Reservoir (TGR), were monitored from July to August (the low water level period) and in October (the impoundment period) in 2018. The vertical distribution characteristics of chlorophyll a and other indicators of the two tributaries were analyzed during the different operation periods, and the factors that affected the vertical distribution in each period were discussed. The results showed that the vertical distribution of dissolved oxygen, water temperature, pH value, and chlorophyll a of the XXR and SNR during the low water level period was relatively consistent. The indexes 0-10 m (0-5 m for chlorophyll a) from the surface of the XXR and SNR, respectively, showed significant stratification and decreased with increasing water depth; the stability index of thermal stratification (RWCS/H) was 13.71-29.07 m-1, which was stable. After the water depth reached 10 m (5 m for chlorophyll a), the indexes tended to be stable along the water depth. During the impoundment period, there was no obvious stratification for each index; the stability index of thermal stratification was 0-0.5 m-1, the stability of the water body was weak, and the vertical variation of each index was relatively stable. The comprehensive trophic state index (TLI) of the XXR and SNR were 55 and 53 during the low water level period, respectively, indicating that they were in a slightly eutrophic state, and 39 and 46 during the impoundment period, respectively, indicating a mesotrophic state. Linear regression analysis showed that chlorophyll a, dissolved oxygen, water temperature, and pH in the two tributaries were significantly correlated in the vertical direction in the low water level period, indicating that dissolved oxygen, water temperature stratification, and pH were important factors affecting the vertical distribution of chlorophyll a. During the impoundment period, a large amount of backflow from the Yangtze River, a large fluctuation in tributary water level, and the decrease in RWCS/H were the important factors that affected the small vertical change in the water body. The enhancement of vertical mixing and the decrease in Zeu/Zmix were the key factors affecting the nutritional status of the water.

Seasonal impoundment management reduces nitrogen cycling but not resilience to surface fire in a tidal wetland.

Hydrology and salinity regimes of many impounded wetlands are manipulated to provide seasonal habitats for migratory waterfowl, with little-known consequences for ecosystem structure and function. Managed hydrology can alter ecosystems by directly changing soil properties and processes and by influencing plant community dynamics. Additionally, management history may influence ecosystem response to disturbance, including fires. To better understand how wetland management regime influences ecosystem response to disturbance, we quantified elevation, soil nitrogen concentrations and process rates, and plant community structure and diversity in a natural experiment following the 2018 Branscombe Fire. We measured paired burned-unburned patches in both tidally-influenced and managed, seasonally-impounded wetlands in Suisun Marsh, California, USA. Unburned ecosystem structure and nutrient cycling differed by wetland management history; unburned impounded wetlands were ∼1 m lower in elevation and plant community composition was dominated by succulents whereas the unburned tidal wetland was dominated by graminoids. Unburned impounded wetland soil nitrogen cycling (potential nitrification and denitrification) rates were <28% of those measured in unburned tidal wetland soils and soil extractable nitrate, ammonium, and dissolved inorganic phosphorus concentrations were also substantially lower in unburned impounded than unburned tidal wetlands. Despite these differences in pre-disturbance (i.e., unburned) conditions, all soil processes recovered to baseline levels within 6 months after surface fire, and we found no evidence of plant community change 1 year after fire in either wetland management type. Overall, water management history exerted stronger control on ecosystem processes and structure than surface fire disturbance. Low extractable soil nitrate and potential denitrification rates may indicate limitation of soil nitrogen removal in impounded wetlands, with implications for downstream environmental quality and eutrophication across managed landscapes.

Temporal-spatial variations, source apportionment, and ecological risk of trace elements in sediments of water-level-fluctuation zone in the Three Gorges Reservoir, China.

The Three Gorges Reservoir (TGR) plays a crucial role in providing electricity for mega-cities across China. However, since the impoundment was completed in 2006, attention to environmental concerns has also been intensive. In order to determine the distribution, sources, and pollution status of trace elements in the water fluctuation zone of the TGR following ten years of repeated "submergence" and "exposure", we systematically collected 16 paired surface sediment samples (n = 32) covering the entire main body of the TGR in March 2018 (following 6 months of submergence) and September 2018 (after 6 months of exposure), and quantitatively analyzed 13 elements (e.g., Mn, Fe, V, Cr, Ni, Cu, Zn, As, Sr, Y, Zr, Ba, and Pb) using X-ray fluorescence spectrophotometry (XRF). The results showed that, except for Sr, concentrations of trace metals following submergence were generally higher than those after exposure due to the less settling of suspended solids at the faster flow velocity during the drawdown period. Assessment using enrichment factors (EFs) and a geo-accumulation index (Igeo) both characterized a relatively serious anthropogenic pollution status of metals in the upper reaches of the TGR with respect to the middle-lower reaches. Source apportionment by positive matrix factorization (PMF) analysis indicated that agricultural activities (24.8 and 24.3%, respectively) and industrial emissions (24.5 and 22.9%, respectively) were the two major sources in these two periods, followed by natural sources, domestic sewage, and ore mining. Ecological risk assessment showed that metalloid arsenic (As) could be the main potential issue of risk to aquatic organisms and human health. A new source-specific risk assessment method (pRI) combined with PMF revealed that agricultural activities could be the major source of potential ecological risk and should be prioritized as the focus of metal/metalloid risk management in the TGR.

Reassessing riverine carbon dioxide emissions from the Indian subcontinent.

Anthropogenic perturbations are increasing uncertainties in estimating CO2 emissions via air-water CO2 flux (FCO2) from large rivers of the Indian subcontinent. This study aimed to provide an improved estimate of the total FCO2 from the subcontinental rivers by combining calculations of the partial pressure of CO2 (pCO2) in eight major rivers with new measurements in the Ganges and Godavari. The average pCO2 in the two newly surveyed rivers, including tributaries, wastewater drains, and impoundments, were 3-6 times greater than the previously reported values. In some highly polluted urban tributaries and middle reaches of the Ganges that drain metropolitan areas, the measured pCO2 exceeded 20,000 µatm, ~40 times the background levels of the headwaters originating in the carbonate-rich Himalayas. The high pCO2 above 28,000 µatm in the lower reach of the Godavari was seven times the moderate levels of pCO2 in the headwaters of the volcanic Deccan Traps, indicating enhanced CO2 production in soils and anthropogenic sources under favorable conditions for organic matter degradation. Across the northern rivers, pCO2 exhibited a significant negative relationship with dissolved oxygen, but a positive relationship with inorganic N or P concentrations. The strong influence of water pollution on riverine pCO2 suggests that CO2 emissions from hypoxic, eutrophic reaches can greatly exceed phytoplanktonic CO2 uptake. Spatially resolved pCO2 data, combined with three gas transfer velocity estimates, provided a higher range of FCO2 from the subcontinental rivers (100.9-130.2 Tg CO2 yr-1) than the previous estimates (7.5-61.2 Tg CO2 yr-1). The revised estimates representing 2-5% of the global riverine FCO2 illustrate the importance of the Indian subcontinental rivers under increasing anthropogenic pressures in constraining global inland waters FCO2.