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Biodegradable dissolved organic carbon (BDOC) constitutes the most labile fraction of dissolved organic matter (DOM), which also functions as a source of CO 2 emissions from inland waters. However, no systematic review is available on DOM indicators of BDOC and CO 2 production potential. Optical and molecular indices can be used to track small changes in DOM composition during biodegradation. In this review, we identified four different methods for measuring BDOC together with their strengths and limitations. In addition, we discuss the potential of using documented optical indices based on absorption and fluorescence spectroscopy and molecular indices based on Fourier transform ion cyclotron mass spectrometry as proxies for estimating BDOC and biodegradation-induced CO 2 production based on previously reported relationships in the literature. Many absorbance- and fluorescence-based indices showed inconsistent relationships with BDOC depending on watershed characteristics, hydrology, and anthropogenic impacts. Nevertheless, several indices, including specific UV absorbance at 254 nm (SUVA 254 ), humification index (HIX), and terrestrial humic-like fluorescent DOM (FDOM) components, tended to have negative relationships with BDOC in tropical and temperate watersheds under baseflow or drought periods. Protein-like FDOM exhibited the strongest correlation with BDOC in different systems, except during storms and flood events. Despite the limited number of studies, DOM molecular indices exhibited consistent relationships with BDOC, suggesting that the relative abundance of aliphatic formulas and the molecular lability index could act as reliable proxies. The DOM optical indices explain up to 96% and 78% variability in BDOC and CO 2 , respectively; nonetheless, there were limited studies on molecular indices, which explain up to 74% variability in BDOC. Based on literature survey, we recommend several sensitive indices such as SUVA 254 , HIX, and terrestrial humic- and protein-like FDOM, which could be useful indicators of BDOC and dissolved CO 2 in inland water. Future research should incorporate a wider range of geographic regions with various land use, hydrology, and anthropogenic disturbances to develop system- or condition-specific DOM optical or molecular proxies for better prediction of BDOC and CO 2 emissions., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Anthropogenic perturbations are increasing uncertainties in estimating CO 2 emissions via air-water CO 2 flux (F CO2 ) from large rivers of the Indian subcontinent. This study aimed to provide an improved estimate of the total F CO2 from the subcontinental rivers by combining calculations of the partial pressure of CO 2 (pCO 2 ) in eight major rivers with new measurements in the Ganges and Godavari. The average pCO 2 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 pCO 2 exceeded 20,000 μatm, ~40 times the background levels of the headwaters originating in the carbonate-rich Himalayas. The high pCO 2 above 28,000 μatm in the lower reach of the Godavari was seven times the moderate levels of pCO 2 in the headwaters of the volcanic Deccan Traps, indicating enhanced CO 2 production in soils and anthropogenic sources under favorable conditions for organic matter degradation. Across the northern rivers, pCO 2 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 pCO 2 suggests that CO 2 emissions from hypoxic, eutrophic reaches can greatly exceed phytoplanktonic CO 2 uptake. Spatially resolved pCO 2 data, combined with three gas transfer velocity estimates, provided a higher range of F CO2 from the subcontinental rivers (100.9-130.2 Tg CO 2 yr -1 ) than the previous estimates (7.5-61.2 Tg CO 2 yr -1 ). The revised estimates representing 2-5% of the global riverine F CO2 illustrate the importance of the Indian subcontinental rivers under increasing anthropogenic pressures in constraining global inland waters F CO2 ., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

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]

• A 25-year time series shows recent increases in N/P driven by decreasing P levels. • Harmful algal blooms concur with short-term changes in P, N/P, and temperature. • Cyanobacterial dominance lowers phytoplankton P use efficiency at low N/P. • CO 2 relationship with N/P or Chl a indicates phytoplankton biomass as a CO 2 source. Long-term trends in riverine nutrient availability have rarely been linked to both phytoplankton composition and functioning. To explore how the changing availability of N and P affects not only phytoplankton abundance and composition but also the resource use efficiency of N, P, and CO 2 , a 25-year time series of water quality in the lower Han River, Korea, was combined with additional measurements of riverine dissolved organic carbon (DOC) and CO 2. Despite persistent eutrophication, recent decreases in P relative to N have been steep in the lowest reach, increasing the annual mean mass ratio of N to P (N/P) from 24 (1994–2015) to 65 (2016–2018). While Chl a and cyanobacterial abundance exhibited overall positive and inverse relationships with P concentrations and N/P, respectively, severe harmful algal blooms (HABs) concurred with short-term increases in P and temperature. Microcystis often dominated HABs at low N/P that usually favors N-fixing cyanobacteria such as Anabaena. In the middle and lower reaches, phytoplanktonic P use efficiency was typically lower at low N/P. V-shaped relationships between N/P and CO 2 concentrations, together with longitudinal upward shifts in the inverse relationship between Chl a and CO 2 , implied that eutrophication-enhanced phytoplankton biomass could turn into a significant source of CO 2. after passing a threshold. The combined results suggest that cyanobacterial dominance co-limited by P availability and temperature can lower planktonic P use efficiency, while enhancing riverine CO 2 emissions at low N/P ratios. [Display omitted] [ABSTRACT FROM AUTHOR]