UBC Faculty Research and Publications

River Inflow Dominates Methane Emissions in an Arctic Coastal System Manning, Cara C.; Preston, Victoria L.; Jones, Samantha F.; Michel, Anna P. M.; Nicholson, David P.; Duke, Patrick J.; Ahmed, Mohamed M. M.; Manganini, Kevin; Else, Brent G. T.; Tortell, Philippe Daniel, 1972- 2020-04-23

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River Inflow Dominates Methane Emissionsin an Arctic Coastal SystemCara C. Manning1 , Victoria L. Preston2,3 , Samantha F. Jones4 , Anna P. M. Michel2 ,David P. Nicholson5 , Patrick J. Duke4,6 , Mohamed M. M. Ahmed4 , Kevin Manganini2,Brent G. T. Else4 , and Philippe D. Tortell1,71Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC, Canada,2Applied Ocean Physics and Engineering Department, Woods Hole, MA, USA, 3Department of Aeronautics andAstronautics, Massachusetts Institute of Technology, Cambridge, MA, USA, 4Department of Geography, University ofCalgary, Calgary, AB, Canada, 5Marine Chemistry and Geochemistry Department, Woods Hole OceanographicInstitution, Woods Hole, MA, USA, 6Now at School of Earth and Ocean Sciences, University of Victoria, Victoria, BC,Canada, 7Department of Botany, University of British Columbia, Vancouver, BC, CanadaAbstract We present a year‐round time series of dissolved methane (CH4), along with targetedobservations during ice melt of CH4 and carbon dioxide (CO2) in a river and estuary adjacent toCambridge Bay, Nunavut, Canada. During the freshet, CH4 concentrations in the river and ice‐coveredestuary were up to 240,000% saturation and 19,000% saturation, respectively, but quickly dropped by>100‐fold following ice melt. Observations with a robotic kayak revealed that river‐derived CH4 and CO2were transported to the estuary and rapidly ventilated to the atmosphere once ice cover retreated. Weestimate that river discharge accounts for >95% of annual CH4 sea‐to‐air emissions from the estuary. Theseresults demonstrate the importance of resolving seasonal dynamics in order to estimate greenhouse gasemissions from polar systems.Plain Language Summary The primary cause of recent global climate change is increasingconcentrations of heat‐trapping greenhouse gases in the atmosphere. Ongoing rapid Arctic climatechange is affecting the annual cycle of sea ice formation and retreat; however, most published studies ofgreenhouse gases in Arctic waters have been conducted during ice‐free, summertime conditions. In order tocharacterize seasonal variability in greenhouse gas distributions, we collected year‐round measurementsof the greenhouse gas methane (CH4) in a coastal Arctic system near Cambridge Bay, Nunavut, Canada. Wefound that during the ice melt season, river water contains methane concentrations up to 2,000 times higherthan the wintertime methane concentrations in the coastal ocean. We utilized a novel robotic kayak toconduct high‐resolution mapping of greenhouse gas distributions during ice melt. From these data, wedemonstrate that the river water containing elevated levels of methane and carbon dioxide (CO2) flowed intothe coastal ocean, and when ice cover melted, these greenhouse gases were rapidly emitted into theatmosphere. We estimate that in this system, more than 95% of all annual methane emissions from theestuary are driven by river inflow.1. IntroductionMethane (CH4) emissions from Arctic waters and sediments may accelerate in the future as part of posi-tive feedback from ongoing climate change (Biastoch et al., 2011; James et al., 2016; Shakhovaet al., 2010). Landscapes that were once permanently frozen are now seasonally thawing, and theice‐free season is lengthening in freshwater and marine systems (Magnuson, 2000; Stroeve et al., 2012;Zona et al., 2016). Thawing can result in the mobilization of labile organic matter and emissions ofgreenhouse gases such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O; Karlssonet al., 2013; Kvenvolden et al., 1993; Lamarche‐Gagnon et al., 2019; Voigt et al., 2017; Zona et al., 2016).Studies of terrestrial and freshwater Arctic systems have demonstrated strong temporal variability ingreenhouse gas emissions in these environments (Denfeld et al., 2018; Karlsson et al., 2013;Lamarche‐Gagnon et al., 2019; Phelps et al., 1998; Voigt et al., 2017; Zona et al., 2016), yet publishedmeasurements in Arctic marine and estuarine waters are strongly biased toward summertime, low‐iceconditions (Fenwick et al., 2017; Shakhova et al., 2010), establishing a need for long‐term studies thatcharacterize the full range of seasonal variability.©2020. American Geophysical Union.All Rights Reserved.RESEARCH LETTER10.1029/2020GL087669Key Points:• Methane concentrations in an Arcticestuary show strong seasonality;river inflow at the start of the freshetdrives elevated concentrations• Observations with a novel robotickayak demonstrate that methaneand carbon dioxide in the estuaryare rapidly ventilated following icemelt• River discharge is estimated toaccount for >95% of annualmethane emissions from the estuarySupporting Information:• Supporting InformationCorrespondence to:C. C. Manning, A. P. M. Michel,and D. P. Nicholson,cmanning@alum.mit.edu;amichel@whoi.edu;dnicholson@whoi.eduCitation:Manning, C. C., Preston, V. L., Jones, S.F., Michel, A. P. M., Nicholson, D. P.,Duke, P. J., et al. (2020). River inflowdominates methane emissions in anArctic coastal system. GeophysicalResearch Letters, 47, e2020GL087669.https://doi.org/10.1029/2020GL087669Received 24 FEB 2020Accepted 14 APR 2020Accepted article online 23 APR 2020MANNING ET AL. 1 of 10Ice acts as a barrier to gas exchange, sustaining strong disequilibria in gas concentrations between the atmo-sphere and ice‐covered waters (Butterworth & Miller, 2016; Denfeld et al., 2018; Karlsson et al., 2013; Wandet al., 2006). Rapid reequilibration of the mixed layer can occur following ice melt. Quantifying the impactsof sea ice loss on Arctic greenhouse gas emissions requires seasonally resolved measurements; yet, few mea-surements of dissolved CH4 or other greenhouse gases are available in ice‐covered or recently ice‐liberatedArctic Ocean waters and connected estuaries. Here, we present new observations that address this criticalobservational gap, demonstrating that the vast majority of annual CH4 release in an Arctic estuary occursduring the ice melt period.2. Observations, Results, and Discussion2.1. Field ObservationsTo quantify the annual sea‐air emissions of greenhouse gases in a coastal Arctic system, we collected mea-surements in a well‐sheltered bay with two inlets (west arm and east arm, Figure 1a) adjacent to the town ofCambridge Bay (Iqaluktuuttiaq), Nunavut, Canada. Surface waters are seasonally ice covered, and the domi-nant freshwater source is Freshwater Creek, which discharges water into the east arm of Cambridge Bayfrom Greiner Lake and the associated watershed (mean annual discharge of 1.4 × 108 m3 year−1 from1970 to 2017). Terrestrial snowmelt in this region typically begins in late May (Tedesco et al., 2009), and,as a result, Freshwater Creek begins to flow before significant sea ice melt has occurred. This freshwaterFigure 1. Map of study site and time‐series data. (a) Map of eastern Canadian Arctic showing the location of CambridgeBay. (b) Satellite image of study area on 21 June 2017 (obtained by Google, DigitalGlobe). Pink circles indicate thelocations of the main sampling stations FWC (Freshwater Creek) and B1, as well as ONC (ocean networks Canadaobservatory with ice profiler). The approximate region where the ChemYak was deployed is shown with a green outline.Time‐series of CH4 concentrations in Cambridge Bay estuary and Freshwater Creek in (c) 2017 and (d) 2018. Surfacesamples in Cambridge Bay were collected at 2 m depth below the ice surface, or 0.75 m below the open water surface. Thedashed horizontal line represents atmospheric equilibrium and the dashed vertical line indicates when sampling stationB1 became ice‐free. Error bars reflect the standard deviation of duplicate measurements.10.1029/2020GL087669Geophysical Research LettersMANNING ET AL. 2 of 10(f)Figure 2. Spatial observations made by the ChemYak vehicle. (a) The study site with ChemYak trajectories from each day overlaid. The mouth of FreshwaterCreek (69.1257°N, 105.0042°W) is marked with a star, and concentric rings at increments of 500 m centered at the mouth are provided for scale. Northeast ofthe red dashed line lies Freshwater Creek (red arrow and box) and a small embayment (blue label and box) which receives input from a much smaller river. (b–g)Observations made by the ChemYak for two representative days, 29 June and 2 July, are plotted by depth versus distance from the Freshwater Creek mouth.Negative distances (to the right of the axis) represent points northeast of the mouth (a small embayment), and positive distance (to the left of the axis) representpoints southwest of the mouth (downstream). As indicated by the salinity plots (f, g), the mixed layer depth is <2 m throughout the study area, and the freshsurface layer was generally higher in both CH4 and CO2 concentration than layers deeper than 2 m. The gas concentrations decreased over the multidaymeasurement campaign. Equivalent plots and temperature data for the other measurement days are shown in Figure S5.10.1029/2020GL087669Geophysical Research LettersMANNING ET AL. 3 of 10discharge causes the rapid melt of sea ice along the east arm, creating open water in June (Figure 1b), withthe rest of the bay typically becoming ice‐free 2–3 weeks later, in late June to early July. During 2017 and2018, we collected a time‐series of dissolved CH4 and N2O measurements in the estuary and river(Figures 1 and S1 in the supporting information). Additionally, in 2018, we used a remotely operatedrobotic kayak, the ChemYak (Kimball et al., 2014; Nicholson et al., 2018), to characterize fine‐scalespatiotemporal changes in dissolved CH4 and CO2 in the estuary during peak river inflow (Figures 2 and3) and collected water samples from Greiner Lake. Methodological details for bottle samples and theChemYak are provided in the Supporting Information. The datasets collected by the authors have beenarchived with PANGAEA (Manning et al., 2019).Figure 3. Spatial and temporal trends observed by the ChemYak. (a, b) Each day of the measurement campaign is marked with a unique color, and samplescollected are binned into 0.25 m increments from the surface to 6 m. Both CH4 (a) and CO2 (b) exhibit decreasing trends for each subsequent day, and thereis strong stratification between the surface layer and water below 2 m. Error bars represent the standard deviation of measurements for each depth bin. (c–j)Temperature‐salinity plots showing changes in CH4 (c–f) and CO2 (g–j) concentrations and water mass distributions over the time‐series.10.1029/2020GL087669Geophysical Research LettersMANNING ET AL. 4 of 10During winter and spring (January–May) in 2017 and 2018, CH4 concentrations throughout the estuarywater column (station B1 in Figure 1b) were closely distributed around the atmospheric equilibrium of4 nM (range 3–10 nM). In early June, river discharge from the spring thaw began to enter the estuary,and elevated CH4 concentrations up to 860 nM (19,000% saturation) were measured in near‐surface watersof Cambridge Bay (2 m below the surface of the ice). A water mass analysis using salinity and water isotopedata from station B1 confirmed that the elevated CH4 concentrations were associated with river runoffrather than ice melt (Figure S2 and Text S1). Ice‐free summer surface waters sampled in July 2017 and2018 had much lower CH4 concentrations, ranging from 4–65 nM.In contrast to CH4, N2O concentrations throughout 2017 and 2018 at station B1 and Freshwater Creek dis-played limited seasonal variability and were close to equilibrium (Figure S1 and Text S2).From 28 June to 2 July 2018, we used the ChemYak for high‐resolution spatial mapping and verticalprofiling of CH4, CO2, salinity, and temperature distributions in an ~1 km2 open water area betweenFigure 4. Observations and model‐derived output of CH4 delivery to the estuary mixed layer. (a) historical riverdischarge data from Freshwater Creek. (b) Modeled and measured CH4 concentration in the river (Freshwater Creek)and the Cambridge Bay estuary mixed layer (model regions 1 and 2), based on data and the model. The range of modeledvalues across the estuary is shown with gray shading and the range of values measured with the ChemYak is shown withblue symbols (error bars represent the standard deviation of daily measurements). The red symbols represent bottlemeasurements at station B1, and the black outlined symbols represent measurements at Freshwater Creek. The graydotted line shows the equilibrium concentration in the estuary. (c, d) Modeled daily (c) and cumulative (d) CH4 deliveryto the estuary, from river inflow, ice melt, and gas exchange (positive delivery represents an input to the mixed layer).The CH4 delivery caused by ice melt is negligible relative to the other terms.10.1029/2020GL087669Geophysical Research LettersMANNING ET AL. 5 of 10the river mouth and the ice edge during the dynamic melt period (Figures 2, 3, and S3–S6). TheseChemYak measurements confirmed elevated greenhouse gas concentrations in river‐derived estuarywater. The river‐derived water occurred throughout the study area as a shallow, fresh surface mixedlayer (<2 m depth), separated from deeper waters by a sharp pycnocline (Figures 2f and 2g). Duringthe ChemYak measurement period, CH4 concentrations in the surface water decreased each day andas the water flowed from the river toward the coastal ocean, suggesting a rapid ventilation within theestuary. For example, the CH4 concentration in Freshwater Creek decreased from 560 ± 10 nM on27 June to 290 ± 20 nM by 3 July, whereas CH4 at station B1 was 130 ± 10 nM on 3 July(Figure 4b). In the ChemYak sampling area (between Freshwater Creek and station B1), on 28 June,CH4 and CO2 concentrations in the upper 1 m of the water column were up to 470 nM and1,470 μatm, respectively (mean 410 ± 20 nM and 1340 ± 40 μatm). Concentrations in the upper 1 mdecreased over the campaign to 150 ± 70 nM CH4 and 600 ± 150 μatm CO2 by 2 July (Figures 3and S6).In the estuary, at depths below the mixed layer (>2 m depth), CH4 and CO2 concentrations decreasedfrom 29 June to 2 July. Elevated wind speeds (up to 10 m s−1) appear to have enhanced mixing acrossthe sharp pycnocline on 30 June (Figures 3d, 3h, S5, and S6). On 30 June, the depth of the pycnoclineshoaled and CH4 and CO2 concentrations below the mixed layer increased near the ice edge and theriver mouth. Over the following days, lower wind speeds (3.7 ± 1.1 m s−1), coupled with decreasingriver inflow concentrations and restratification of the water column, led to decreased gas concentrationsthroughout the water column by 2 July. Changes in the observed temperature‐salinity properties of thewater suggest that mixing reduced the vertical salinity gradient over the measurement period. Themixed layer near the river mouth showed significant warming between 28 June and 2 July(Figures 3c–3j, S5, and S6).To evaluate the importance of atmospheric ventilation to the CH4 budget in the estuary, we performedsea‐air flux calculations (Wanninkhof, 2014) using observed wind speeds. In the absence of lateral transportand river discharge, CH4 concentrations in the estuary over our sampling period would be expected todecrease to ~70 nM. In actuality, we observed a mean surface CH4 concentration of ~150 nM at the end ofthe sampling period, suggesting that the continued inflow of high‐CH4 river water into the ChemYak sam-pling region contributed to maintaining elevated CH4 concentrations following ice melt (Figure 4a). Basedon the observed river discharge of ~40 m3 s−1, we estimate that the residence time of water in theChemYak measurement region was ~0.6 days.In addition to measuring the estuary downstream of the Freshwater Creek mouth, the ChemYak was alsoused to collect observations in a small embayment at the outlet of a much smaller river on 29 June, 1July, and 2 July (Figures 2, 3, S4, and S5). We present the results from this embayment to highlight the com-plexities of quantifying greenhouse gas fluxes from estuarine systems. This embayment generally exhibitedhigher temperatures and lower CH4 and CO2 concentrations relative to adjacent waters. For example, on 29June, the mean CH4 concentration in the upper 2 m was 242 ± 41 nM in the embayment (upstream of theFreshwater Creek mouth), in contrast to 417 ± 31 nM within 100 m downstream of the Freshwater Creekmouth (Figure 2b). For CO2, the mean concentration was 790 ± 140 μatm in the embayment and1,400 ± 110 μatm downstream of the Freshwater Creek mouth. In late June to early July 2018, we collectedbottle samples at the head of the embayment in this smaller river and found that CH4 concentrations inFreshwater Creek were two times higher than in the smaller river. The CH4 and CO2 levels in the embay-ment may therefore reflect lower inflowing CH4 and CO2 from the smaller river and/or a longer residencetime for river‐derived surface water to exchange with the atmosphere in the embayment. The observed dif-ferences between the smaller river and embayment compared to Freshwater Creek and the rest of the estu-ary demonstrate the need to conduct studies in a diverse range of Arctic coastal systems to better understandthe complex hydrological controls on the magnitude and location of greenhouse gas emissions.Overall, we conclude that the declining CH4 and CO2 concentrations throughout the water column between28 June and 2 July, and along the spatial gradient from Freshwater Creek to station B1, primarily reflect acombination of decreasing gas concentrations in the river water (Figure 1c), loss due to gas exchange withinthe ice‐liberated area and oceanward lateral advective export. Below, we present a physical model for theestuarine mixed layer CH4 budget and discuss potential impacts of microbial processes on the CH4 budget.10.1029/2020GL087669Geophysical Research LettersMANNING ET AL. 6 of 102.2. Mixed Layer ModelTo quantify the fate of river‐derived CH4 over the entire river inflow season, we developed a mixed layermodel for the Cambridge Bay estuary (Figures 4 and S7). The model was constrained with measured CH4concentrations in Freshwater Creek (bottle samples), river discharge and wind speed measurements, andice thickness records from the Ocean Networks Canada (ONC) cabled observatory. The model is describedin detail in Figure S7 and Text S3, and the model code, including all input data, is available on GitHub(Manning & Preston, 2020). The results of this analysis suggest that the annual CH4 cycle in the estuary isdriven by river inflow, with sea ice melt contributing a comparatively negligible amount of CH4 to the mixedlayer (Figure 4d). This conclusion is consistent with a water mass property (salinity/water isotope) analysisshowing an insignificant impact of sea ice melt on the CH4 budget (Figure S2 and Text S1).Using the model, we estimate that ~730 kg CH4 was released from Freshwater Creek into the estuary in 2018(volume‐weighted mean concentration in river water of 360 nM), with 24% (177 kg) ventilated to the atmo-sphere from the estuary following ice cover retreat (Figure 4d). The remaining 76% of the river‐derived CH4was laterally transported across the estuary beneath sea ice to the coastal ocean, where it was likely venti-lated to the atmosphere following ice melt in mid‐late July. Indeed, a model run with a larger spatial foot-print (including 60 km2 of coastal ocean surrounding the 5 km2 Cambridge Bay estuary) yielded acumulative annual sea‐air emission of 548 kg CH4 from the coastal ocean derived from river discharge,which occurs rapidly following ice melt (Figure 4d). To estimate the CH4 emissions from the estuary inthe absence of river inflow, we prescribe a fixed CH4 concentration of 6 nM (~150% saturation), the typicalsurface concentration before and after the river inflow period. Under this scenario, with no river discharge,we derived annual estuarine CH4 emissions of 7.4 kg, 24 times lower than the emissions derived from the fullmodel including riverine CH4 inputs. Given that our model predicts that 76% of the riverine CH4 is venti-lated beyond the estuary in the adjacent coastal ocean, small river systems such as Freshwater Creek maybe of primary importance to annual CH4 budgets through much of the coastal Arctic Ocean. Accurate cal-culation of such short‐lived, high magnitude CH4 emissions following ice melt, requires a more extensiveunder‐ice sampling program, including melt‐season measurements in multiple river‐influenced Arctic estu-aries and coastal systems.The extent of microbial CH4 oxidation in river‐derived water discharged from Freshwater Creek is currentlyunknown. The early‐season river discharge containing >1,000 nM CH4 may remain under the ice for~1 month before being exposed to the atmosphere, during which time microbial oxidation could potentiallydecrease CH4 levels. Recent incubation studies measuring CH4 oxidation rates and rate constants in Arcticwaters and ice‐covered lakes have reported a wide range of values (Bastviken et al., 2002; Bussmannet al., 2017; Gentz et al., 2014; Mau et al., 2013; Ricão Canelhas et al., 2016; Uhlig & Loose, 2017). For exam-ple, in an Arctic fjord, Mau et al. (2013) reported first‐order CH4 oxidation rate constants ranging over threeorders of magnitude, from ~0.0001 to 0.1 day−1. To determine the maximum possible impact of CH4 oxida-tion on the CH4 budget, we tested a model run incorporating CH4 oxidation with a rate constant of 0.1 day−1(Text S3 and Figure S9). With this high rate of CH4 oxidation, the modeled annual CH4 emissions within theestuary decreased only slightly (from 177 to 164 kg), but CH4 emissions from the adjacent coastal oceandecreased significantly (from 540 to 130 kg). An oxidation rate of 0.01 day−1 yielded emissions of 175 and480 kg for the estuary and coastal ocean respectively (Figure S9). Overall, these results suggest that microbialoxidation could potentially contribute to a significant reduction in the fraction of laterally exported CH4 thatultimately is emitted to the atmosphere, but likely has a small impact on CH4 emissions within theCambridge Bay estuary. In the future, we hope to measure CH4 concentration in the coastal ocean adjacentto Cambridge Bay estuary, and CH4 oxidation rates in Cambridge Bay and adjacent waters, and use thesedata to improve the model.3. Conclusions and Future WorkOur results, derived from a year‐round times series of CH4 measurements and dense spatio‐temporal obser-vations from a remotely operated robotic kayak, show that CH4 discharge via Freshwater Creek drivesintense CH4 emissions immediately following ice melt in the Cambridge Bay estuary and surroundingwaters. River discharge also acts as a significant seasonal source of CO2 to the estuary. This study10.1029/2020GL087669Geophysical Research LettersMANNING ET AL. 7 of 10demonstrates the importance of fully resolving seasonal processes in interconnected marine and freshwaterArctic environments to accurately quantify greenhouse gas emissions. We have also demonstrated theadvantages of using new sensing technologies to study heterogeneous and dynamic systems. Similar seaso-nal variability in CH4 emissions likely occurs in some other river‐influenced, seasonally ice‐covered Arcticestuaries, which receive ~10% of global river discharge (Dai & Trenberth, 2002). More field studies in a widerange of Arctic river and coastal systems are needed to determine the impact of ongoing and projected futureincreases in Arctic river discharge (Macdonald et al., 2015) on greenhouse gas emissions. For example, moreextensive measurements of CH4 concentration and isotopic composition across the land‐ocean continuumwould assist in determining CH4 and CO2 sources and sinks. Radiocarbon measurements would demon-strate whether ancient CH4 sources such as thawing permafrost are significant (Sparrow et al., 2018).Such studies will provide critical information to characterize current and future Arctic greenhouse gascycling, improving quantitative estimates of changes in CH4 and CO2 emissions.The low and stable CH4 concentrations observed below the mixed layer in the Cambridge Bay estuary indi-cate that sedimentary CH4 sources within this estuary are negligible relative to river‐derived inputs, in con-trast to published studies in some other Arctic coastal and shelf systems where significant sedimentarysources are observed (Gentz et al., 2014; Shakhova et al., 2010). Therefore, more research is needed in a widerange of Arctic coastal waters to more accurately characterize the relative importance of terrestrial sourcesversus subseafloor deposits.Freshwater input to the Canadian Arctic Archipelago is dominated by small river systems such FreshwaterCreekwith a collective discharge on the same order as large rivers such as theMackenzie (Alkire et al., 2017),yet these small rivers are rarely sampled due to challenging conditions. The CH4 concentrations we mea-sured in Freshwater Creek (ranging from 10–11,000 nM, volume‐weighted mean 360 nM) are similar toother river systems in the Arctic and worldwide. For example, mean CH4 concentrations observed in theYukon River, Lena Delta, and Leverett Glacier runoff range from 70–750 nM (Bussmann, 2013;Lamarche‐Gagnon et al., 2019; Striegl et al., 2012). Furthermore, a data compilation of over 900 rivers andstreams worldwide reported a mean CH4 concentration of 1,400 ± 5,200 nM and median of 250 nM(Stanley et al., 2016). The peak CH4 concentrations observed in Cambridge Bay estuary (up to 900 nM duringthe freshet, prior to ice melt) are similar to maximum values measured in other Arctic coastal waters(Bussmann et al., 2017; Shakhova et al., 2010).The results of this study motivate future coastal Arctic field campaigns at other sites with measurement tech-nologies capable of high spatial and temporal resolution mapping immediately before and during ice melt.Such studies will provide critical information to characterize current and future Arctic greenhouse gas emis-sion, improving quantitative estimates of changes in CH4 and CO2 emissions across the rapidly changingArctic environment.ReferencesAlkire, M. B., Jacobson, A. D., Lehn, G. O., Macdonald, R. W., & Rossi, M. W. (2017). 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