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. Surface water concentrations of CO2 , CH4 , and N2O have rarely been measured simultaneously in river systemsmodified by human activities, contributing to large uncertainties inestimating global riverine emissions of greenhouse gases (GHGs). Basin-widesurveys of the three GHGs were combined with a small number of measurementsof C isotope ratios in dissolved organic matter (DOM), CO2 , and CH4 in the Han River basin, South Korea, to examine howlongitudinal patterns of the three gases and DOM are affected by four cascadedams along a middle section of the North Han River (hereafter termed “middlereach”) and treated wastewater discharged to the lower Han River (“lowerreach”) traversing the Seoul metropolitan area. Monthly monitoring andtwo-season comparison were conducted at 6 and 15 sites, respectively, tomeasure surface water gas concentrations and ancillary water qualityparameters including concentrations of dissolved organic carbon (DOC) andoptical properties of DOM. The basin-wide surveys were complemented with asampling cruise along the lower reach and synoptic samplings along an urbantributary delivering effluents from a large wastewater treatment plant (WWTP)to the lower reach. The levels of pCO2 were relatively lowin the middle reach (51–2465  µ atm), particularly at the four damsites (51–761  µ atm), compared with those found in the largelyforested upper basin with scattered patches of croplands(163–2539  µ atm), the lower reach (78–11298  µ atm), andthree urban tributaries (2120–11970  µ atm). The upper and middlereaches displayed generally low concentration ranges of CH4 and N2O , with some local peaks influenced by agricultural runoff andimpoundments. 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 asDO and PO 4 3 - for CH4 and NH 4 + and NO 3 - for N2O . Downriver increases in the levels of DOCand optical properties such as fluorescence index (FI) and protein-likefluorescence indicated an increasing DOM fraction of anthropogenic andmicrobial origin. The concentrations of the three GHGs and DOC were similarin magnitude and temporal variation at a WWTP discharge and the receivingtributary, indicating a disproportionate contribution of the WWTP effluentsto the tributary gas and DOC exports to the lower reach. The values of δ13C in surface water CO2 and CH4 measuredduring 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 DOMdelivered by the urban tributaries. The downstream enrichment of 13C in CO2 and CH4 suggested that the spatialdistribution of these gases across the eutrophic lower reach may also beconstrained by multiple concomitant processes including outgassing,photosynthesis, and CH4 oxidation. The overall results suggest thatdams and urban wastewater may create longitudinal discontinuities in riverinemetabolic processes leading to large spatial variations in the three GHGscorrelating with different combinations of DOM properties and nutrients.Further research is required to evaluate the relative contributions ofanthropogenic and in-stream sources of the three gases and DOM in eutrophicurbanized river systems and constrain key factors for the contrastingimpoundment effects such as autotrophy-driven decreases in pCO2 and in-lake production of CH4 and N2O .

Longitudinal discontinuities in riverine greenhouse gas dynamics generated by dams and urban wastewater