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Water pollution disrupts the ecological integrity of urbanized river systems, but its impacts on riverine metabolic processes and carbon fluxes are poorly studied in developing countries. Three seasonal field surveys were combined with two high-resolution measurements and an in situ incubation experiment to investigate the effects of untreated wastewater on organic matter biodegradation and the partial pressure of CO2 (pCO2) along the Mekong–Tonle Sap network around Phnom Penh. High-resolution measurements during the dry-season survey exhibited large downstream increases in pCO2 along the Mekong reaches receiving Tonle Sap inflows carrying urban sewage, contrasting with little spatial variation during a monsoon survey when the Mekong floodwater reversed the Tonle Sap flow. The monsoonal and dry-season surveys revealed flooding-induced homogenization and large spatial divergences in dissolved organic carbon (DOC) concentration and its δ13C and Δ14C between the Tonle Sap and connected Mekong reaches. During the 3-day incubation of Mekong waters, alone or mixed with sewage, a large initial nocturnal increase in pCO2 in sewage-supplemented river water exceeded the subsequent daytime CO2 uptake by phytoplankton photosynthesis varying with light exposure. This, combined with the preferential consumption of labile DOC components displaying protein-like fluorescence, implies sewage-enhanced biodegradation of riverine organic matter. These results suggest that neglecting wastewater-enhanced CO2 production in urbanized river basins during long dry periods can result in a significant underestimation of riverine CO2 emissions. [ABSTRACT FROM AUTHOR]

Despite growing research on greenhouse gas (GHG) emissions from inland waters, few systematic efforts have been made to assess the regional‐scale GHG emissions from Asian rivers under increasing anthropogenic stress. We examined factors controlling longitudinal and seasonal variations in the partial pressure of CO2 (pCO2), and CH4 and N2O concentrations in the Ganges, Mekong, and Yellow River by simultaneously measuring gas concentrations and stable C isotopes, and optical properties of dissolved organic matter (DOM) from 2016 to 2019. The levels of pCO2 and CH4 were distinctively higher in polluted tributaries and affected reaches of the Ganges and Mekong than in the Yellow River. The highest levels of N2O were found in the Ganges, followed by the Yellow River and Mekong. Across these basins, dry‐season mean concentrations of CO2, CH4, and N2O were 1.6, 2, and 7 times higher than those measured in the monsoon season, respectively. This seasonality was consistent with that of δ13C‐CO2, while δ13C‐CH4 showed an opposite pattern. GHG concentrations exhibited significant positive relationships with DOM concentrations and optical properties including fluorescence index and protein‐like fluorescence, implying the contribution of anthropogenic, labile DOM to production of GHGs in the polluted reaches. Graphical mixing models of δ13C‐CO2 and δ13C‐CH4 support the stronger impact of wastewater on the Ganges and Mekong than on the Yellow River. The overall results suggest that neglecting localized pollution impacts on GHG emissions from increasingly urbanized river basins can result in a substantial underestimation of global riverine GHG emissions. Plain Language Summary: Rapid urbanization and poor wastewater infrastructure are turning many large rivers in Asia into significant sources of greenhouse gases (GHGs). We examined controlling factors for the concentrations of CO2, CH4, and N2O in the Ganges, Mekong, and Yellow River by simultaneously measuring gas concentrations and stable carbon isotopes, and optical properties of dissolved organic matter (DOM). The concentrations of CO2 and CH4 were distinctively higher in polluted tributaries and affected sections of the Ganges and Mekong than in the Yellow River. N2O concentration was highest in the Ganges, followed by the Yellow River and Mekong. Across these basins, the concentrations of the three gases were generally higher during dry periods than in the monsoon season. GHG concentrations were positively related to DOM optical properties indicating the contribution of easily degradable DOM from pollution sources. The stable carbon isotope ratios of CO2 and CH4 support the stronger impact of wastewater on the Ganges and Mekong than on the Yellow River. The overall results suggest that neglecting local pollution impacts on GHG emissions from increasingly urbanized river basins can result in a substantial underestimation of global riverine GHG emissions. Key Points: High CO2, CH4, and N2O concentrations in urbanized river reaches reflect influences from local tributaries carrying wastewaterSeasonality in stable C isotopes and dissolved organic matter properties corresponds to higher dry‐season greenhouse gas concentrations than in the monsoonStable C isotopes and their relationships with CO2 and CH4 concentrations support localized wastewater effects on urbanized rivers [ABSTRACT FROM AUTHOR]

Despite increasing recognition of storm-induced organic carbon (C) export as a significant loss from the terrestrial C balance, little is known about the biodegradation and chemical transformation of particulate organic carbon (POC) in mountainous river systems. We combined analyses of C isotopes, solution-state H NMR, and lipid biomarkers with biodegradable dissolved organic C (BDOC) measurements to investigate downstream changes of POC composition and biodegradability at a mountainous, mixed land-use watershed in South Korea. Water and suspended sediment (SS) samples were collected in a forested headwater stream, a downstream agricultural stream, and two downstream rivers during peak flow periods of four storm events, using either sequential grab sampling from the headwater stream to the most downstream river within a few hours around the peak flow or sediment samplers deployed during the whole storm event. DOC concentrations exhibited relatively small changes across sites, whereas POC concentrations were highest in the agricultural stream, and tapered along downstream reaches. The δC and δN of SS in the agricultural stream were distinct from up- and downstream signatures and similar to those for erosion source soils and lake bottom sediment, although increases in radiocarbon age indicated continuous compositional changes toward the lake. H NMR spectra of SS and deposited sediment exhibited downstream decreases in carbohydrates and lignin but enrichment of organic structures related to microbial proteins and plant wax. The downstream sediments had more microbial n-alkanes and lipid markers indicating anthropogenic origin such as coprostanol compared to the forest soil n-alkanes dominated by plant wax. While the BDOC concentrations of filtered waters differed little between sites, the BDOC concentrations and protein- to humic-like fluorescence ratios of DOC leached from SS during a 13-day incubation were higher in downstream rivers, pointing to contribution of labile POC components to the enhanced biodegradation. Overall, inputs of microbial and anthropogenic origin, in interplay with deposition and mineralization, appear to substantially alter POC composition and biodegradability during downstream transport, raising a question on the conventional view of mountainous river systems as a passive conduit of storm pulses of POC. [ABSTRACT FROM AUTHOR]

Abstract: An overview is provided of the potential effects of climate change on the watershed biogeochemical processes and surface water quality in mountainous watersheds of Northeast (NE) Asia that provide drinking water supplies for large populations. We address major ‘local’ issues with the case studies conducted at three watersheds along a latitudinal gradient going from northern Japan through the central Korean Peninsula and ending in southern China. Winter snow regimes and ground snowpack dynamics play a crucial role in many ecological and biogeochemical processes in the mountainous watersheds across northern Japan. A warmer winter with less snowfall, as has been projected for northern Japan, will alter the accumulation and melting of snowpacks and affect hydro-biogeochemical processes linking soil processes to surface water quality. Soils on steep hillslopes and rich in base cations have been shown to have distinct patterns in buffering acidic inputs during snowmelt. Alteration of soil microbial processes in response to more frequent freeze–thaw cycles under thinner snowpacks may increase nutrient leaching to stream waters. The amount and intensity of summer monsoon rainfalls have been increasing in Korea over recent decades. More frequent extreme rainfall events have resulted in large watershed export of sediments and nutrients from agricultural lands on steep hillslopes converted from forests. Surface water siltation caused by terrestrial export of sediments from these steep hillslopes is emerging as a new challenge for water quality management due to detrimental effects on water quality. Climatic predictions in upcoming decades for southern China include lower precipitation with large year-to-year variations. The results from a four-year intensive study at a forested watershed in Chongquing province showed that acidity and the concentrations of sulfate and nitrate in soil and surface waters were generally lower in the years with lower precipitation, suggesting year-to-year variations in precipitation as a key factor in modulating the effects of acid deposition on soil and surface water quality of this region. Results from these case studies suggest that spatially variable patterns of snow or summer precipitation associated with regional climate change across NE Asia will have significant impacts on watershed biogeochemical processes and surface water quality, in interactions with local topography, land use change, or acid deposition. [Copyright &y& Elsevier]

Biogeochemical responses to changing climate and atmospheric deposition were investigated using nitrogen (N) and sulfur (S) mass balances, including dry deposition and organic solutes in the Arbutus Lake watershed in the Adirondack Mountains, New York State. Long-term monitoring of wet-only precipitation (NADP/NTN, 1983–2001) and dry deposition (AIRMoN, 1990–2001) at sites adjacent to the watershed showed that concentrations of SO42− in precipitation, SO42− in particles,and SO2 vapor all declined substantially ( P<0.005) in contrast to no marked temporal changes observed for most N constituents (NH4+ in precipitation, HNO3 vapor, and particulate NO3−), except for NO3− in precipitation, which showed a small decrease in the late 1990s. From 1983 to 2001, concentrations of SO42− in the lake outlet significantly decreased (−2.1 μeq L−1 yr−1, P<0.0001), whereas NO3− and dissolved organic N (DON) concentrations showed no consistent temporal trends. With the inclusion of dry deposition and DON fluxes into the mass balance, the retained portion of atmospheric N inputs within the main subcatchment increased from 37% to 60%. Sulfur outputs greatly exceeded inputs even with the inclusion of dry S deposition, while organic S flux represented another source of S output, implying substantial internal S sources. A significant relationship between the annual mean concentrations of SO42− in lake discharge and wet deposition over the last two decades ( r=0.64, P<0.01) suggested a considerable influence of declining S deposition on surface water SO42− concentrations, despite substantial internal S sources. By contrast, interannual variations in both NO3− concentrations and fluxes in lake discharge were significantly related to year-to-year changes in air temperature and runoff. Snowmelt responses to winter temperature fluctuations were crucial in explaining large portions of interannual variations in watershed NO3− export during the months preceding spring snowmelt (especially, January–March). Distinctive response patterns of monthly mean concentrations of NO3− and DON in the major lake inlet to seasonal changes in air temperature also suggested climatic regulation of seasonal patterns in watershed release of both N forms. The sensitive response of N drainage losses to climatic variability might explain the synchronous patterns of decadal variations in watershed NO3− export across the northeastern USA. [ABSTRACT FROM AUTHOR]

Despite the rising interest in understanding how climate change could affect the emissions of greenhouse gases (GHGs) from river systems, including floodplains, we still lack a mechanistic understanding of how changing environmental conditions, such as moisture and nutrient availability, limit the temperature responses of GHG production in floodplain sediments. To examine the environmental co-limitations on the temperature responses of three major GHGs (CO2, CH4, and N2O) produced in floodplain sediments, sediments from a constructed wetland on the floodplain of the lower Han River were incubated for 24 d at four temperatures spanning 4–28 ℃, under three conditions (closed, open/wetting, and open/drying). The net production of all three GHGs exhibited nonlinear temperature responses with gas-specific patterns and magnitudes of response varying over the incubation period. During the later incubation phase, positive temperature responses were weakened for the net production of CO2 and CH4 in the dried treatments, whereas a similar weakening occurred for N2O production in the wet treatments. This, combined with incubation-induced changes in dissolved organic carbon and its fluorescence components, indicated the lack or excess of moisture and associated changes in O2 and organic carbon availability as critical co-limiting factors for the temperature responses of GHG production. Warming decreased δ13C in the CH4 emitted from wet and hypoxic sediments, implying a stronger warming effect on CH4 production over oxidation. Unlike many studies assuming a consistent relationship between temperature and GHG production in sediments irrespective of other environmental conditions, our results suggest that warming effects on the GHG emissions from floodplain sediments would depend on the balance between gas production and consumption under the prevailing constraints of moisture, O2, and labile carbon availability. [ABSTRACT FROM AUTHOR]

Continuous underway measurements were combined with a basin-scale survey to examine human impacts on CO2 outgassing in a highly urbanized river system in Korea. While the partial pressure of CO2 (pCO2) was measured at 15 sites using syringe equilibration, 3 cruises employing an equilibrator were done along a 30 km transect in the Seoul metropolitan area. The basin-scale survey revealed longitudinal increases in surface water pCO2 and dissolved organic carbon (DOC) in the downstream reach. Downstream increases in pCO2, DOC, fluorescence index, and inorganic N and P reflected disproportionately large contributions from wastewater treatment plant (WWTP) effluents carried by major urban tributaries. Cruise transects exhibited strong localized peaks of pCO2 up to 13 000 µatm and 13CO2 enrichment along the confluences of tributaries at an average flow, whereas CO2 pulses were dampened by increased flow during the monsoon period. Fluctuations in pCO2 along the eutrophic reach downstream of the confluences reflected environmental controls on the balance between photosynthesis, biodegradation, and outgassing. The results underscore WWTP effluents as an anthropogenic source of nutrients, DOC, and CO2and their influences on algal blooms and associated C dynamics in eutrophic urbanized river systems, warranting further research on urbanization-induced perturbations to riverine metabolic processes and carbon fluxes.. [ABSTRACT FROM AUTHOR]

Highlights: [•] DOC dynamics in 1st order forested streams controlled by monsoonal climate. [•] DOC dynamics in higher order agricultural streams driven by rice paddy water inputs. [•] Monsoonal climate controls river-aquifer exchange dynamics. [•] Frequent hydraulic gradient reversals are specific to the monsoonal climate. [ABSTRACT FROM AUTHOR]

• N 2 O concentrations and isotopes reveal winter surges in wastewater and affected river • Low temperature lowers performance of wastewater N removal producing more N 2 O • Low temperature enhances N 2 O production via denitrification in hypoxic sediment Although eutrophic urban rivers receiving loads of wastewater represent an important anthropogenic source of N 2 O, little is known as to how temperature and other environmental factors affect temporal variations in N 2 O emissions from wastewater treatment plants (WWTPs) and downstream rivers. Two-year monitoring at a WWTP and five river sites was complemented with available water quality data, laboratory incubations, and stable isotopes in N 2 O and NO 3 − to explore how wastewater effluents interact with seasonal changes in environmental conditions to affect downstream metabolic processes and N 2 O emissions from the lower Han River traversing the megacity Seoul. Water quality data from four WWTPs revealed significant inverse relationships between water temperature and the concentrations or fluxes of total N (TN) in effluents. Increased TN fluxes at low temperatures concurred with N 2 O surges in WWTP effluents and downstream rivers, counteracting the long-term decline in TN fluxes resulting from enhanced wastewater treatments. Incubation experiments with river water and sediment, in isolation or combined, implied the hypoxic winter sediment as a large source of N 2 O, whereas the anoxic summer sediment produced a smaller amount of N 2 O only when it was added with oxic water. For both WWTP effluents and downstream rivers, bulk isotope ratios and intramolecular distribution of 15N in N 2 O distinctly differed between summer and winter, indicating incomplete denitrification in the hypoxic sediment at low temperatures as a primary downstream source adding to WWTP-derived N 2 O. Winter surges in wastewater TN and sediment N 2 O release highlight temperature variability as an underappreciated control over anthropogenic N 2 O emissions from increasingly urbanized river systems worldwide. Image, graphical abstract [ABSTRACT FROM AUTHOR]