Abstract. High-frequency continuous measurements of the partial pressure of CO2 (pCO2) are crucial for constraining the spatiotemporal dynamics of CO2 emissions from inland water systems. However, direct measurements of pCO2 are scarce, and no systematic comparisons have been conducted on the suitability of the widely used measurement systems for continuous underway or long-term deployment in various field conditions. We compared spray- and marble-type equilibrators and a membrane-enclosed CO2 sensor to assess their suitability for continuous long-term or underway pCO2 measurements in an urbanized river system in Korea. Both equilibrators had a shorter response time compared with the membrane-enclosed sensor, and could capture large spatial variations of pCO2 during a transect study along a highly urbanized river reach. The membrane-enclosed sensor based on passive equilibration provided comparable underway measurements along the river sections where pCO2 varied within the sensor detection range. When deployed in a eutrophic river site, the membrane-enclosed sensor was able to detect large diel variations in pCO2. However, biofouling on the membrane could reduce the accuracy of the measurement during long deployments exceeding several days. The overall results suggest that the fast response of the equilibrator systems facilitates capturing large spatial variations in pCO2 during short underway measurements. However, the attendant technical challenges of these systems, such as clogging and desiccant maintenance, have to be addressed carefully to enable their long-term deployment. The membrane-enclosed sensor would be suitable as an alternative tool for long-term continuous measurements if membrane biofouling could be overcome by appropriate antifouling measures such as copper mesh coverings.

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

Abstract. 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 PO43- for CH4 and NH4+ and NO3- 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 δ13C and Δ14C 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 autotrophy-driven decreases in pCO2 and in-lake production of CH4 and N2O.

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Abstract.Riverine emissions of CO 2 and other greenhouse gases (GHGs) represent crucial, but poorly constrained components of the global GHG budgets. Three major GHGs -CO 2 , CH 4 , and N 2 O -have rarely been measured simultaneously in river systems modified by human activities, adding uncertainties to the estimates of global riverine GHG emissions. Measurements of C isotopes in dissolved organic carbon (DOC), CO 2 , and CH 4 were combined with basin-wide surveys of three GHGs in the Han 15 River, South Korea to investigate the effects of dams and urban water pollution as primary controls on GHG dynamics in the highly human-impacted river basin with a population >25 million. Monthly monitoring and two-season comparison were conducted at 6 and 15 sites, respectively, to measure surface water concentrations of three GHGs, along with DOC and its optical properties. The basin-wide surveys were complemented with a boat cruise along the lower reach and synoptic samplings along a polluted tributary delivering effluents from a large wastewater treatment plant (WWTP) to the lower reach. The basin-20 wide surveys of three GHGs revealed distinct increases in the concentrations of three gases along the lower reach receiving urban tributaries enriched in GHGs and DOC. Compared to the spatial patterns of GHGs observed in the upper and lower reaches, the levels of pCO 2 were consistently lower across the impounded middle reach, whereas concentrations of CH 4 and N 2 O were relatively high in some impoundment-affected sites. Similar levels and temporal variations in three GHGs at the WWTP effluents and the receiving tributary indicated a disproportionate contribution of the WWTP to the tributary exports of 25 DOC and GHGs. Measurements of δ 13 C in surface water CO 2 and CH 4 sampled during the cruise along the lower reach, combined with δ 13 C and Δ 14 C in dissolved organic matter (DOM) sampled across the basin, implied that longitudinal decreases in Δ 14 C in DOM mighte be associated with wastewater-derived, old DOM in urban tributaries, which, together with enhanced photosynthesis and CH 4 oxidation in the eutrophic lower reach, appear to constrain downstream changes in δ 13 C in CO 2 and CH 4 . The overall results suggest that dams and urban wastewater may create longitudinal discontinuities in riverine metabolic 30 processes leading to large spatial variations in three GHGs. Further research is required to evaluate the relative contributions Biogeosciences Discuss., https://doi