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Continuous underway measurements were combined with a basin-scale survey to examine human impacts on CO 2 outgassing in a highly urbanized river system in Korea. While the partial pressure of CO 2 (pCO 2 ) 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 pCO 2 and dissolved organic carbon (DOC) in the downstream reach. Downstream increases in pCO 2 , 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 pCO 2 up to 13 000 μatm and 13 CO 2 enrichment along the confluences of tributaries at an average flow, whereas CO 2 pulses were dampened by increased flow during the monsoon period. Fluctuations in pCO 2 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 CO 2 and 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.

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]

Dams and wastewater may greatly perturb riverine fluxes of dissolved organic matter (DOM) and CO 2 , yet little is known about the relationships between altered DOM quality and CO 2 emission in eutrophic impounded river systems. A basin-wide field survey of surface water CO 2 and dissolved organic carbon (DOC) was combined with laboratory incubations to examine how dams and urban tributaries delivering treated wastewater influence longitudinal patterns in DOM properties and CO 2 along the impounded Han River traversing Seoul metropolitan area. Fluorescent DOM indices including parallel factor analysis (PARAFAC) components were used to characterize DOM in relation to biodegradable DOC (BDOC). Compared with distinct downstream increases in DOC and CO 2 , BDOC concentration and its proportion in DOC (%BDOC) were highly variable along the mainstem and peaked at urban tributaries. Longitudinal increases in fluorescence index (FI), biological index (BIX), and two PARAFAC components (C2 and C3) contrasted with general decreases in humification index (HIX) and C1, reflecting increasing downstream inputs of anthropogenic DOM. During a 5-day incubation employing continuous CO 2 measurements, the cumulative production of CO 2 in the mainstem water mixed with urban tributary water was significantly higher than the level expected for conservative mixing of the two samples, indicating a synergistic enhancement of DOM biodegradation. Molecular formulas identified by Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) revealed more consumed molecules in the mainstem water and more newly produced molecules in the tributary water over the 5-day incubation, implying abundant labile components in the mainstem water discharged from the upstream dam and highly processed tributary DOM limited in immediately biodegradable organic materials. Downstream increases in CO 2 and DOC along the Han River, combined with the synergistic effect observed in the mixed water, suggest that mixing wastewater-derived DOM with labile autochthonous DOM can enhance CO 2 production in the river system perturbed by impoundment and wastewater. Unlabelled Image • Field survey was combined with lab incubations to study human impact on river carbon. • Dissolved organic matter and CO 2 vary along impounded river with wastewater influence. • Urban tributary mixing enhances organic matter degradation and CO 2 in impounded river. • FT-ICR-MS detects organic matter degradation in impounded river and urban tributary. [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]