CO2 outgassing from an urbanized river system fueled by wastewater

Aug 22, 2017 - Cruise transects exhibited strong localized peaks of pCO2 up to 13,000 μatm and 13CO2 enrichment along the confluences of tributaries ...
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CO2 outgassing from an urbanized river system fueled by wastewater treatment plant effluents Tae Kyung Yoon, Hyojin Jin, Most Shirina Begum, Namgoo Kang, and Ji-Hyung Park Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02344 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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Environmental Science & Technology

CO2 outgassing from an urbanized river system fueled by wastewater treatment plant effluents Tae Kyung Yoon†,‡, Hyojin Jin†, Most Shirina Begum†, Namgoo Kang§,∥, Ji-Hyung Park†,*



Department of Environmental Science and Engineering, Ewha Womans University, Seoul 03760,

Republic of Korea §

Center for Gas Analysis, Korea Research Institute of Standards and Science, Daejeon 34113,

Republic of Korea

∥Science

of Measurement, University of Science and Technology, KRISS Campus, Daejeon

34113, Republic Korea

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ABSTRACT

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Continuous underway measurements were combined with a basin-scale survey to examine

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human impacts on CO2 outgassing in a highly urbanized river system in Korea. While the partial

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pressure of CO2 (pCO2) was measured at 15 sites using syringe equilibration, three cruises

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employing an equilibrator were done along a 30-km transect in the Seoul metropolitan area. The

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basin-scale survey revealed longitudinal increases in surface water pCO2 and dissolved organic

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carbon (DOC) in the downstream reach. Downstream increases in pCO2, DOC, fluorescence

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index, and inorganic N and P reflected disproportionately large contributions from wastewater

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treatment plant (WWTP) effluents carried by major urban tributaries. Cruise transects exhibited

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strong localized peaks of pCO2 up to 13,000 µatm and 13CO2 enrichment along the confluences

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of tributaries at average flow, whereas CO2 pulses were dampened by increased flow during the

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monsoon period. Fluctuations in pCO2 along the eutrophic reach downstream of the confluences

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reflected environmental controls on the balance between photosynthesis, biodegradation, and

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outgassing. The results underscore WWTP effluents as an anthropogenic source of nutrients,

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DOC, and CO2 and their influences on algal blooms and associated C dynamics in eutrophic

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urbanized river systems, warranting further research on urbanization-induced perturbations to

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riverine metabolic processes and carbon fluxes.

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INTRODUCTION

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As the terrestrial biosphere “breathes”,1 streams, rivers, and lakes release carbon dioxide (CO2)

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to the atmosphere during transit of organic and inorganic carbon (C) to the oceans, constituting

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an important link in the global carbon cycle.2-8 Recent global estimates of CO2 outgassing from

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inland waters range from < 1 to 3.8 Pg C y-1, which can result in a significant loss of the annual

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terrestrial sink (~2.5 Pg C y-1) of CO2 emitted from anthropogenic sources.4-7, 9 Large

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uncertainties in current estimates result from a combination of factors including sparse spatial

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coverage of direct measurements of the partial pressure of CO2 (pCO2) in inland waters and

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difficulties in estimating gas transfer velocities.6, 7, 9 Although direct measurements of pCO2 have

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identified previously unrecognized inland water C sources such as wetlands and estuarine

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marshes,10-12 pCO2 data are still sparse for anthropogenically modified river systems worldwide,

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except for well-studied rivers in North America13, 14 and Europe.2, 15 Particularly sparse are in-

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situ measurements in rapidly urbanizing watersheds across Asia, so obtaining pCO2 data in Asian

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rivers has recently been suggested as a top priority in reducing large uncertainty in estimating the

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global riverine CO2 outgassing.16

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Anthropogenic perturbations such as dams, river channeling, and water pollution can alter

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inland water C fluxes substantially.7 Although anthropogenic changes in water quality and

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biological communities have been studied extensively,17, 18 few studies have investigated altered

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C fluxes in urbanized river systems from the perspective of human impacts on aquatic ecosystem

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processes.19 Some recent studies conducted in urbanized watersheds have identified river

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impoundment and pollution as key controls on the quantity and quality of dissolved organic

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matter (DOM).20-22 However, only a small number of studies have linked the altered flux and

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composition of DOM to direct measurements of pCO2 in urban rivers.14, 23, 24 Large spatial

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heterogeneity in pCO2 and DOM in urbanized river systems has been attributed to infrastructures

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such as wastewater treatment plants (WWTPs), dams, weirs, channels, and urban stormwater

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facilities that may represent discontinuities in the river continuum, altering natural riverine

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processes and ecosystem resilience.23, 25, 26 Discontinuities in the river continuum can either

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decrease or increase the rate of riverine CO2 outgassing via enhanced photosynthesis27 or organic

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matter biodegradation.21 Few studies have addressed these discontinuities and associated large

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spatial variations in the riverine C fluxes in urbanized watersheds using approaches with

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adequate spatial resolution.

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The Han River basin is a highly modified river system draining the middle of the Korean

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Peninsula including the densely populated Seoul metropolitan area with a population > 25

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million along the lower reach (Figure 1). Longitudinal variations in dominant land use, along

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with a cascade of dams along the middle reach, make the Han River basin an ideal venue to

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study anthropogenic perturbations to hydrologic and biogeochemical flows. A basin-scale survey

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of pCO2, DOM, and nutrients from the forested headwater stream to the estuary of the Han River

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was combined with continuous underway measurements of pCO2 employing an automated

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equilibrator system8 along the downstream reach in the Seoul metropolitan area to explore how

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river impoundment and pollution alter riverine CO2 outgassing and organic matter transport in

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the highly urbanized watershed. We hypothesized that urban tributaries delivering high loads of

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CO2, DOM, and nutrients derived from WWTP effluents to the downstream river could not only

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enhance CO2 outgassing in the receiving water but also alter the rates of the primary production

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and organic matter biodegradation along the downstream reach. We expected that enhanced

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primary production, particularly during algal blooms, could significantly lower the level of pCO2

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via planktonic uptake of CO2, resulting in large longitudinal variations in pCO2 along the

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eutrophic downstream reach.

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Figure 1 Study sites in in the Han River basin: (a) a basin-scale transect (~300 km); (b) an

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underway transect (30 km) and an urban tributary transect (JN; ~35 km) in the Seoul

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metropolitan area. Three major urban streams (AY: Anyang River, JN: Jungnang River, TC: Tan

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River), four wastewater treatment plants (WWTPs), and two submerged weirs are marked on the

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Seoul metropolitan area map (b).

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MATERIALS AND METHODS Site description. The 515-km long Han River consists of the North Han and South Han

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branches and the lower Han River. The river drains an area of 26,142 km2 along the middle of

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the Korean Peninsula and flows westward into the Yellow Sea (Figure 1; more detailed

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descriptions of the study site and methods provided in SI).28 The dominant land cover in the Han

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River basin varies longitudinally from the highly forested upstream reach to increasingly

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urbanized areas along the mid- to downstream reaches. The Han River flowing through the Seoul 5

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metropolitan area is channelized with two submerged weirs at the east (RKM 39) and west end

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(RKM 70) of Seoul (Figure 1b). Three major urban tributaries feed into the lower reach along the

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city of Seoul: the Tan River (TC), Jungnang River (JN), and the Anyang River (AY) enter the

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mainstem at RKM 68, 64, and 49, respectively (Figure 1b). Three WWTPs discharge at the rate

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of 1.5 million m3 d-1 to JN, with the bulk (1.3 million m3 d-1) discharged from the WWTP located

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near the mouth. Four WWTPs located within Seoul release effluents from tertiary treatments

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including modified Ludzack Ettinger (MLE) and anaerobic-anoxic/oxic process (A2O) at the rate

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of 4.3 million m3 d-1.29

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Basin-scale survey. A basin-scale survey along the North Han branch and the lower reach

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was conducted at 15 locations on 4–6 May 2015 at average river flow (183 m3 s-1 at RKM 57;

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Figure S1). The sites were selected at intervals from a forested headwater stream (RKM 299)

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through the middle and lower reaches impounded by dams or weirs to the estuarine reach (RKM

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23) (Figure 1a, Table S1). A 35-km transect of the urban tributary JN, together with WWTP

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effluents near the confluence, was surveyed at eight sampling points from the headwater to the

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confluence on 13 November 2015 and 12–13 May 2016. In addition to air temperature and

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barometric pressure measured with a portable sensor (Watchdog 1650 Micro Station, Spectrum

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Technologies, USA), we measured in situ water temperature, pH, electrical conductivity, and DO

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using a portable pH meter (Orion 5-Star Portable, Thermo Scientific, USA). The surface water

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pCO2 at 20 cm depth was measured by a manual headspace equilibration method.8, 30, 31 The

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equilibrated headspace air sample, as well as an ambient air sample collected for equilibration,

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was injected into a GC (7890A, Agilent, USA) fitted with a Supelco Hayesep Q 12 ft 1/8”

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column for the measurement of CO2 concentration. Grab water samples collected 20 cm below

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the surface were filtered (GF/F, Whatman) at the laboratory and then analyzed for dissolved

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organic C (DOC; TOC-VCPH, Shimadzu, Japan), fluorescence excitation-emission matrices

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(EEMs; F7000, Hitachi, Japan), UV absorbance (8453, Agilent, USA), major ions (883 Basic IC

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plus, Metrohm, Switzerland), and chlorophyll a (Chl-a).32 Fluorescence and UV absorbance data

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were used to calculate fluorescence index (FI), as an optical index that can distinguish the origin

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of DOM between terrestrial/allochthonous sources (1.3–1.5) and microbial/autochthonous

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sources (1.7–2.0).33-35

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Continuous underway measurements. Continuous underway measurements of pCO2 were

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performed on three cruises along the lower reach (RKM 70 to 39; Fig. 1), twice at average flow

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(212 m3 s-1 on both 11 May 2015 and 10 June 2016,) and once at high flow (757 m3 s-1 on 28

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July 2015). The boat speed was maintained at ∼10 km h-1 based on the prior tests and other

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studies.8, 36 To compare spatial differences in pCO2 during a selected day period between three

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cruise transects, the measurements were conducted within four hours in the afternoon when the

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mean pCO2 is close to the daily mean value based on our year-long continuous pCO2

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measurements at RKM 53 (unpublished data). pCO2 was continuously measured using a spray-

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type equilibrator8, 10, 37, 38 connected to an infrared gas analyzer (IRGA; LI820, Li-Cor, USA).

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The equilibrated headspace air in the equilibrator chamber was circulated through a channel

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including an air filter, a desiccant (Drierite) column, and the IRGA. The data were logged every

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1 s on a laptop. Basic water quality parameters were measured in the water from an additional

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water pump channel onboard, using a portable multiparameter meter (Orion 5-Star Portable,

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Thermo Scientific, USA or 6820 V2, YSI Inc., USA). Air temperature and barometric pressure

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were recorded in a micro-logger (Watchdog 1650 Micro Station, Spectrum Technologies, USA).

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The cruises were tracked by a GPS tracking unit (Montana 650, Garmin Ltd., USA). The gas

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samples of the manual headspace and the water samples were collected onboard and at the urban

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streams near the confluences, and later analyzed in the laboratory with the same methods used

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for the basin-scale field survey. In the June 2016 transect, stable C isotope ratios of CO2 (δ13CO2)

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in the gas samples were analyzed at the UC Davis Stable Isotope Facility using a GasBench-

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IRMS (ThermoScientific, Bremen, Germany). The actual water saturated pCO2 in the

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equilibrator was calculated from the pCO2 measured in dry air, barometric pressure, water

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temperature, and equilibrium water vapor (refer to the supporting information; Eq. S1).39

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Aqueous pCO2 in the equilibrator was then corrected to in situ temperature using a temperature

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effect coefficient. To compensate the time lags of the measured pCO2 resulting from the

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residence time of the water circuit and the response time of the sensors, the logged values were

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corrected according to Eq. S2.36

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CO2 outgassing estimation and downstream mass balance of CO2 and DOC. The CO2

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flux at the water-air interface (fCO2) was estimated for the mainstem section along the city of

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Seoul based on the differences in measured pCO2 between water and air and an estimated gas

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transfer velocity (K). Our observations and available data were insufficient to apply empirical K

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models,40 so a K value was adopted from a study conducted in the Hudson River (5 cm h-1)41

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based on the similarities in river hydrology and climatic conditions. The downstream evolution

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of CO2 and DOC was simulated using a simple mass balance model adapted from the modeling

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approach used for estimating CO2 fluxes from the Amazon river and wetlands.12 The model

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consists of inputs of CO2 and DOC from the upper reach (RKM 69) and from the urban

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tributaries (TC and JN); and CO2 outgassing from the section from RKM 69 to 48 (Eq. S3, S4).

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The proportion (termed “residual”) of the total C measured at RKM 62 and 48 that was not

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accounted for by the upstream and/or tributary inputs of CO2 and DOC and CO2 outgassing from

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the given section was assumed to represent either CO2 production (i.e., gain) or consumption (i.e.,

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loss) resulting from organic matter biodegradation or photosynthesis, respectively. All data

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processing, statistical analyses, and modeling were conducted using R.42

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RESULTS AND DISCUSSION

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Spatial pattern of pCO2 and DOC across the Han River basin. The basin-wide survey

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revealed distinct downstream increases in pCO2, DOC, and FI, particularly from the point where

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the river flows into the city boundary of Seoul (Figures 2a–c). The surface water pCO2 ranged

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from 100 to 730 µatm across the upper and middle reaches and began to exceed 1,500 µatm in

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the upper part of the lower reach (Figure 2a). Relatively low pCO2 values in the vicinity of the

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dams (288 µatm; N = 7) may indicate an enhanced algal uptake of CO2 in the impounded reaches.

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Compared to the upstream values, DOC and FI increased 1.5–2-fold in the highly urbanized

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lower reach (Figure 2b, c). The three major urban tributaries exhibited distinctively higher values

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of pCO2, DOC, and FI than all the mainstem sites. This finding suggests that the urban tributaries

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and WWTP effluents, in combination with in-stream metabolic processes in the downstream

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reach, can significantly alter the fluxes of CO2 and DOC in the urbanized river system.

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The transect study along the urban tributary JN revealed longitudinal increases in CO2, DOC,

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and inorganic nutrients resulting in large exports into the confluence (Figures 2e-h). The

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effluents from a WWTP located 4 km upstream of the JN confluence were highly enriched in

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CO2, DOC, and inorganic nutrients (Figures 2e-h, Table S2). pCO2, DOC, and FI measured at the

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most downstream location of the tributary were similar to those measured at the WWTP effluents,

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indicating that the WWTP effluents account for the bulk of the tributary export of carbon and

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other nutrients to the Han River. Along the tributary upstream of the WWTP, spanning from the

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upper, rural area (> 28 km upstream from the confluence) to the middle, urbanized area (4–28

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km), pCO2 ranged from 1,000–3,000 µatm. Upon receiving the WWTP effluent, the pCO2 in the

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receiving water increased 4-fold, approaching 8,000 µatm.

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Figure 2 Spatial variations in pCO2, dissolved organic carbon (DOC), and fluorescence index (FI)

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along a basin-scale transect (a–d) and an urban tributary (Jungnang River; JN on Figure 1)

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transect (e–h). RKM refers to the river km from the mouth. The measurements in the wastewater

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treatment plant (WWTP) effluents are indicated by red box plots (e-g).

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The spatial pattern of DOC along the JN transect was similar to the pCO2 pattern (Figure 2f).

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The FI values indicated a gradual shift from the dominance of terrestrial DOM (FI < 1.5) in the

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upper reach (Figure 2g) to the greater contribution of autochthonous DOM (FI > 1.7) in the

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WWTP effluent and its downstream reach.21, 34 High loads of CO2 and DOC in the WWTP

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effluents likely originate from the wastewater treatment processes including microbial oxidation

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and biomass respiration that transform organic compounds to CO2 and DOM.43, 44 The WWTP

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effluents strongly influenced other water quality variables, especially inorganic nutrients (Table

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S2).

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Continuous underway measurements of pCO2 along the lower reach. Three cruise

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surveys along the lower reach of the Han River exhibited unique spatial patterns of pCO2, DOC,

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and inorganic nutrients that reflect the material inputs from tributaries and WWTPs (Figures 3, 4;

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Table S3). Mean pCO2 values of 1,066 µatm, 2,518 µatm, and 2,566 µatm, for May 2015, July

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2015, and June 2016 transects, respectively, were all above the atmospheric saturation level (409

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µatm), pointing to the role of the lower reach as a net source of CO2. The levels of

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supersaturation are similar to or higher than the reported values for some temperate rivers,14, 45

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but lower than those for subtropical and tropical rivers.23, 46, 47 Two transects at average flow

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(May 2015 and June 2016) showed strong localized pulses of pCO2 along the tributary JN

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confluence (Figures 3a,c) and noticeable downstream increases in DOC, FI, and inorganic N and

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P concentrations (Figure 4; Table S3). pCO2 increased from < 100 µatm at a location upstream of

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the TC confluence (RKM 69) to ~3,000 µatm in the downstream reach of the JN confluence

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(RKM 62). Strong localized pulses of pCO2 up to ~13,000 µatm were observed in the JN stream

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plume mixing with the mainstem along RKM 62–65, reflecting large inputs of CO2 delivered by

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the tributary (Figures 3, S2). The observed localized pCO2 peaks along the tributary confluences

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have rarely been observed by the previous continuous underway measurements conducted in

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urbanized rivers and estuaries14, 45 or systems with lower proportions of urban land use.46-48 In a

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rare case, Zhai et al.49 found exceptionally high pCO2 values approaching 5,000 µatm along an

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upper reach of the Pearl River estuary with a polluted tributary.

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Figure 3 Underway measurements of pCO2 along the 30-km lower reach of the Han River

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dissecting the Seoul metropolitan area. The gray lines indicate the measurements along the

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confluences receiving the urban tributary discharge (a, c). The May 2015 measurements along

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the mainstem were modified from a prior publication.8

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Figure 4 Urban tributary impacts on downstream water quality: pCO2 (a), dissolved organic

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carbon (DOC; b), fluorescence index (FI; c), chlorophyll-a (Chl-a; d), inorganic nutrients (e–g),

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and dissolved oxygen (DO; h). The upstream (76–68 km; N = 6) and downstream (62–48 km; N

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= 12) reaches were compared with two urban streams (TC; N = 2–3) and (JN; N = 5–8). Asterisks

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(*) and daggers (†) indicate significant (* P < 0.05, ** P < 0.01, and *** P < 0.001) differences

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in space (before and after the confluence; α) and time (sampling season; β), respectively (Yijk = µ

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+ αi + βj + (αβ)ij + εijk).

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DOC concentrations increased by 45% and 8% across the transition from the up- to the

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downstream location of the TC and JN confluences, respectively (Figure 4b; Table S3). FI,

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inorganic N and P, EC, and Chl-a also showed general longitudinal increases, consistent with the

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basin-scale survey. The longitudinal increases along the reach downstream of the two tributaries

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TC and JN were greater at average flow than at high flow, indicating a homogenizing effect of

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fast and high river flow during the monsoon period. In the May 2015 transect, high

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concentrations of PO43- and NH4+ peaked at RKM 53 near the JN confluence and rapidly

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decreased along the downstream reach, likely due to a combination of dilution and rapid algal

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uptake of labile nutrients during the algal bloom.

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Observed longitudinal patterns can be explained by the mixing of the mainstem and tributary

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waters, as indicated by downstream changes associated with distinctively higher pCO2, DOM,

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and inorganic nutrients and lower DO and Chl-a in the tributaries (Figure 4). For example, pCO2

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at TC (6240 ± 794 µatm) and JN (9699 ± 1442 µatm) was 80- and 124-fold higher, respectively,

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than the value (78 µatm) measured at an upstream location (RKM 69). DOC concentrations were

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2‒3 times higher at the two urban tributaries than the upstream value. The May 2015

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measurements had particularly high values of pCO2 along the JN confluence and gradually

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decreased moving downstream from ~3,000 µatm at RKM 62 to ~260 µatm at RKM 53.

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The measurements of δ13CO2 during the June 2016 cruise exhibited a gradual longitudinal

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enrichment of 13C along the reach downstream of two tributaries TC and JN (Figure 3c). δ13C

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increased from −20.9 ‰ at RKM 76 to −16.7 ‰ at RKM 49. The observed values of δ13C fall in

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the lower range of δ13CO2 and δ13DIC measured for various inland waters (−25 – 0‰).23, 46, 50, 51

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Interpreting the observed variations in δ13C is challenging because δ13CO2 reflects the interplay

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between the complex processes involving gas exchange, organic matter decomposition, and

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dissolution of carbonates of either natural or anthropogenic origin.23, 50 The observed range is

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similar to those for CO2 originating from the decomposition of organic matter (−25 – −15‰),

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implying the dominant role of biogenic processes in the river or anthropogenic sources such as

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WWTPs. 13C enrichment along the reach downstream of the tributaries also suggests that there

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was a contribution from urban tributaries enriched in 13C (TC: –18.3 ‰, JN: –18.2 ‰, AY: –14.7

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‰) relative to the upstream value (–20.9 ‰). As suggested by Zeng et al.23, 13C enrichment in

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DIC can result from a combination of processes including the preferential algal uptake of CO2

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depleted in 13C, organic matter degradation, and the enhanced air-water gas exchange in the

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impounded, slow-moving water facilitating the invasion of atmospheric CO2 enriched in 13C

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(δ13C: –8 ‰) as observed at a location upstream of the TC confluence (RKM 69 and 78), in

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which pCO2 was undersaturated.

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In the July 2015 transect during the summer monsoon period, spatial variations in pCO2,

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DOM, and inorganic nutrients were less pronounced than the measurements during the other

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transects at lower flows (Figure 3b; Table S3). The higher river flow might have weakened the

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effects of urban tributary inputs and the spatial variability along the mainstem. Most

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measurements except pCO2 were lower in magnitude in the monsoon season than at lower flows

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(Table S3). The lower FI values observed during the monsoon transect (1.41) than those obtained

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during the low-flow transects (1.55 in 2015 and 1.52 in 2016) can be explained by the greater

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contribution of DOM from terrestrial sources in the upper reaches. The decrease in FI resulting

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from an increasing contribution of terrestrial DOM has been observed during the monsoon

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season across major rivers in Korea.52

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Downstream transformation of DOC and CO2. The mass balance model consisting of the

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measurements of pCO2 and DOC in the mainstem and the two urban tributaries (TC and JN) and

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the estimated rate of CO2 outgassing were used to track changes in major C components along

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the sections from RKM 69 (upstream of TC confluence) through RKM 62 (downstream of JN

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confluence) to RKM 48 (upstream of AY confluence) (Figure 5). In May 2015, two tributaries

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TC and JN accounted for 108% and 47% of pCO2 and DOC concentrations measured at RKM 62,

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respectively, with the WWTP effluent near the JN confluence alone contributing to 15

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approximately one-half of CO2 and one-fifth of DOC carried by the two tributaries. Although we

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cannot rule out potential overestimation resulting from cumulative errors in estimating the

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upstream and tributary contributions and CO2 evasion downstream of the confluence, the

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extremely high tributary pCO2 relative to the very low upstream value indicates the dominant

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role of the tributary carrying WWTP effluents in increasing pCO2 along the downstream reach at

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relatively low flow (May 2015 and June 2016) compared to the monsoon period (July 2015). In

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the monsoon season (July 2015), however, the contribution of the same WWTP effluents was

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much smaller than at lower flows because of the dilution effect of high river flow. Outgassing

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from the 14-km downstream transect was estimated to release 38% and 12% of CO2 measured at

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RKM 62 at average and high flow, respectively. When the rate of outgassing is estimated for the

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lower reach up to the weir preventing the tidal water intrusion (RKM 39), 55% and 23% of the

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CO2 inflow from the WWTP near the JN confluence may be released within the 23-km

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downstream transect (RKM 62 to 39) at average and high flow, respectively.

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When the fraction associated with CO2 outgassing (fCO2) from the RKM 62-48 section was

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subtracted from the total C concentration measured at RKM 62, the difference (DOCRKM62 +

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CO2RKM62 – fCO2RKM48–62) was either higher (May 2015) or lower (July 2015 and June 2016) than

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the combined concentration of DOC and CO2 measured at RKM 48 (DOCRKM48 + pCO2RKM48).

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The residual found in May 2015 indicates the consumption of CO2 via planktonic uptake in the

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14-km transect. In contrast, the increased C concentration at RKM 48 observed in July 2015 and

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June 2016 can be explained by the production of CO2 in the same section. At high flow (July

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2015) the concentration of Chl-a varied little across the transect, but increased at average flow

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between RKM 62 and 48 by 31.4 and 11.8 µg L-1 in May 2015 and June 2016, respectively

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(Table S3).

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Figure 5 Changes in CO2, dissolved organic carbon (DOC), chlorophyll-a (Chl-a) from a

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mainstem location (RKM 69) upstream of the first tributary (TC) through a downstream location

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(RKM 62) of the second tributary (JN) to a location (RKM 48) upstream of the third tributary

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(AY). Estimates of the tributary input and CO2 outgassing, and residuals are indicated between

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the measurements at three locations. Note that fCO2 from RKM 69 to RKM 62 was too low (