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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.)

High‐frequency, paired measurements of the partial pressure of CO2 (pCO2) and dissolved O2 were combined with dissolved organic matter (DOM) characterization to investigate diurnal (7:00–19:00) and seasonal variations in CO2 dynamics in the lower Ganges River. Diurnal variations in pCO2 shifted seasonally, concurring with changes in DOM optical properties, biodegradable DOM, and O2 departures from atmospheric equilibrium (ΔO2). Increased pCO2 and CO2 flux (FCO2) in August–September corresponded to monsoonal floods carrying more terrestrial materials. Lower dry‐season pCO2 and FCO2 concurred with higher ΔO2, and exhibited strong relationships with DO and pH, indicating a dominant influence of phytoplankton production. Despite large variations in pCO2, the year‐long positive FCO2 implied that the lower Ganges acted as a CO2 source, reaching 122 mmol m−2 d−1. Increasing hydrologic variability and pollution in the Ganges Basin can amplify pCO2 variations linked to seasonal shifts in terrestrial inputs and phytoplankton production. [ABSTRACT FROM AUTHOR]

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]

Surface water concentrations of CO2, CH4, and N2O have rarely been measured simultaneously in river systems modified by human activities, contributing to large uncertainties in estimating global riverine emissions of greenhouse gases (GHGs). Basin-wide surveys of the three GHGs were combined with a small number of measurements of C isotope ratios in dissolved organic matter (DOM), CO2, and CH4 in the Han River basin, South Korea, to examine how longitudinal patterns of the three gases and DOM are affected by four cascade dams along a middle section of the North Han River (hereafter termed "middle reach") and treated wastewater discharged to the lower Han River ("lower reach") traversing the Seoul metropolitan area. Monthly monitoring and two-season comparison were conducted at 6 and 15 sites, respectively, to measure surface water gas concentrations and ancillary water quality parameters including concentrations of dissolved organic carbon (DOC) and optical properties of DOM. The basin-wide surveys were complemented with a sampling cruise along the lower reach and synoptic samplings along an urban tributary delivering effluents from a large wastewater treatment plant (WWTP) to the lower reach. The levels of pCO2 were relatively low in the middle reach (51-2465 μatm), particularly at the four dam sites (51-761 μatm), compared with those found in the largely forested upper basin with scattered patches of croplands (163-2539 μatm), the lower reach (78-11298 μatm), and three urban tributaries (2120-11970 μatm). The upper and middle reaches displayed generally low concentration ranges of CH4 and N2O, with some local peaks influenced by agricultural runoff and impoundments. By comparison, the lower reach exhibited exceptionally high concentrations of CH4 (1.2-15766 nmol L-1/ and N2O (7.5-1396 nmol L-1/, which were significantly correlated with different sets of variables such as DO and PO3- 4 for CH4 and NHC 4 and NO-3 for N2O. Downriver increases in the levels of DOC and optical properties such as fluorescence index (FI) and protein-like fluorescence indicated an increasing DOM fraction of anthropogenic and microbial origin. The concentrations of the three GHGs and DOC were similar in magnitude and temporal variation at a WWTP discharge and the receiving tributary, indicating a disproportionate contribution of the WWTP effluents to the tributary gas and DOC exports to the lower reach. The values of δ13C in surface water CO2 and CH4 measured during the sampling cruise along the lower reach, combined with &948;13C andΔ14 in DOM sampled across the basin, implied a strong influence of the wastewater-derived gases and aged DOM delivered by the urban tributaries. The downstream enrichment of 13C in CO2 and CH4 suggested that the spatial distribution of these gases across the eutrophic lower reach may also be constrained by multiple concomitant processes including outgassing, photosynthesis, and CH4 oxidation. The overall results suggest that dams and urban wastewater may create longitudinal discontinuities in riverine metabolic processes leading to large spatial variations in the three GHGs correlating with different combinations of DOM properties and nutrients. Further research is required to evaluate the relative contributions of anthropogenic and in-stream sources of the three gases and DOM in eutrophic urbanized river systems and constrain key factors for the contrasting impoundment effects such as autotrophydriven decreases in pCO2 and in-lake production of CH4 and N2 O. [ABSTRACT FROM AUTHOR]

Abstract: The objective of this study was to investigate the effect of dissolved organic ligands, such as EDTA, humic acids, hydrophobic and hydrophilic fractions of dissolved organic matter (DOM), on the growth of Pseudokirchneriella subcapitata in cultured media and natural lake waters. The growth of algae was gradually increased on the addition of dissolved organic ligands, but markedly declined at high concentrations, due to reduced bioavailable iron concentrations (e.g., Fe3+ and Fe′—inorganic ferric iron). The results demonstrated that the high growth rates of P. subcapitata were correlated with the hydrophobic, but not hydrophilic, DOM from five lakes, spiked under the nutrients-controlled conditions. This was attributed to the role of DOM in controlling the bioavailable iron due to complexation of Fe(III) with –COOH and –OH functional groups on the hydrophobic DOM. Therefore, the hydrophobic DOM, as chelating agent, is a biologically important component in the lake waters, which affects the algal growth by interacting with bioavailable iron in the lake water and can change the results of bioassay experiments. [Copyright &y& Elsevier]

The forest floor in temperate forests has become recognized for its importance in the retention of elevated inputs of dissolved inorganic nitrogen (DIN) and as a source of dissolved organic matter (DOM). A laboratory leaching experiment was conducted over the period of 98 d to examine the origin of dissolved organic carbon (DOC) and nitrogen (DON) in a deciduous forest floor, and the effect of resource availability and microbial activity on the production mechanisms involved. The experiment was composed of different types of treatments: exclusion of specific forest floor layers (no Oi, no Oe) and addition of carbon sources (glucose, cellulose, leaf, wood) and NH4NO3 (nitrogen). The cumulative amount of CO2 evolution was positively related to the availability of C sources at each treatment: glucose>leaf=wood=cellulose>control=no Oe=nitrogen>no Oi. DOC release was related to the amount of C sources but showed no clear correlation with CO2 evolution. An increase in C availability generally led to a reduction in the release of DON as well as DIN. In contrast, the amendment of NH4NO3 reduced the cumulative DOC release but enhanced the release of both DON and DIN. Fresh leaf litter was a more important DOC source than labile substrates (glucose and cellulose) as well as more stable substrates (forest floor materials and wood). Among forest floor layers, more humified horizons (Oe and Oa) were the primary source of DIN and made a similar contribution to DOM release as the Oi layer. The changes in DOM composition detected by a humification index of the leachates, in combination with a shift in the final microbial biomass C, suggested that DOM released from the soluble pools of added litter or the Oi layer contained a substantial amount of microbially processed organic matter. Our study demonstrated the importance of C availability in regulating microbial activity and immobilization of dissolved N in an N-enriched forest floor. However, the discrepancy between substrate lability and DOC production, in combination with a rapid microbial processing of DOC released from labile C pools, illustrated the complicated nature of microbial production and consumption of DOC in the forest floor. [Copyright &y& Elsevier]

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]

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]

• 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]