Patterns and Drivers of Carbon Dioxide Concentrations in Aquatic

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boreal freshwaters in particular, there is often a close association between pCO2 and DOC concentrations (Algesten et al., 2003; Sobek et al., 2003; Teodoru et al., 2009). Thus, small and shallow Arctic freshwater ecosystems may play a key, but rarely quantiﬁed, role in the movement and transformation of terrestrial carbon

into CO2. There is thus a need for a comprehensive comparison of the magnitude and drivers of terrestrially derived carbon as it ﬂows through multiple freshwater landscape components (i.e., rivers, ponds, and lakes) on a path from land to sea (Aufdenkampe et al., 2011), especially in watersheds dominated by permafrost (McClelland et al., 2007, 2014; McGuire et al., 2010; Vonk & Gustafsson, 2013; Wrona et al., 2016). Most existing studies focused on aquatic ecosystems are limited to the comparison to two ecosystem types, or

along a gradient of sizes. For example, streams were found to release more CO2 per unit area than subarctic lakes (Lundin et al., 2013). Smaller waterbodies (i.e., streams or ponds) can be a stronger source of carbon to the atmosphere and nearshore waters than can larger rivers or lakes (Denfeld et al., 2013; Shirokova et al., 2013; Teodoru et al., 2009). This is a trend that holds for both lotic and lentic sites globally (Butman & Raymond, 2011; Holgerson & Raymond, 2016). The goal of this study was to (1) examine patterns of carbon ﬂux among multiple Arctic aquatic ecosystem

types (pond, lake, river, lagoon, and ocean), (2) identify drivers of daily peak season pCO2 concentrations in tundra ponds, and (3) model how recent warming may have affected carbon release from Arctic tundra ponds.

2. Materials and Methods The ACP is a large expanse of Arctic tundra with low relief and high soil organic carbon located north of the Brooks Range and its northern foothills. The entire ACP is underlain by continuous permafrost. It is bor- dered by the eroding coastlines of the Chukchi Sea to the west and the Beaufort Sea to the north and east (Arp et al., 2010; Jones et al., 2009; Jorgenson & Brown, 2004; Tweedie et al., 2012). Nearly one half of the land cover of the ACP is composed of thaw lakes or drained thaw lake basins (Frohn et al., 2005), many con- taining numerous ponds (Hinkel et al., 2003). Across the Arctic permafrost lowlands, 17% of the landscape is covered by ponds and lakes (Muster et al., 2017). In spring, these ponds and lakes typically drain into small meandering rivers that ﬂow into coastal lagoons. This study focused on ponds, streams, lakes, coastal lagoon, and nearshore ocean near Utqiaġvik, AK (formerly Barrow, AK) (Figure 1). 2.1. Data Collection

pCO2 (partial pressure of carbon dioxide) was monitored in late summer (late July to mid‐August) over a period of 4 years (2013, 2015, 2017, and 2018) from open water areas of a variety of aquatic habitats. Habitats included the following: tundra ponds (henceforth ponds), which are those formed through the seasonal thaw of the active layer within low‐centered polygons, and thermokarst (TK) ponds, which are typically formed in more discrete locations among polygon troughs and caused by abrupt thawing of ice‐rich permafrost, and subsequent land subsidence and slumping. Freshwater river sites were river reaches not influ- enced by intrusion of water from the lagoon and, on average, had pH <6.2 and specific conductance <1 mS/cm. Conversely, brackish river sites, on average, had pH >7.3 and specific conductance >19 mS/cm; brackish water was likely pushed upstream by nontidal (i.e., wind) movement. Lakes ranged in size from a maximum length of 0.4 to 4.4 km (average = 2 km). Marine sites included Elson Lagoon and the nearshore Chukchi Sea. For nonmarine sites, a total of 20 tundra ponds, 6 thermokarst ponds, 6 rivers (freshwater and brackish), and 6 lakes were sampled at least once. Of these sites, 40% were sampled on more than one occasion; 47% were

monitored for at least one continuous 24 hr period. For each sampling effort, pCO2 was monitored for a minimum of 30 min, to allow measurements to stabilize, up to a maximum of nearly 3 weeks of diel monitoring, depending on the site. In particular, a small tundra pond (IBP‐C) was monitored each of the 4 years of sampling on a diel basis for 10–18 days, depending on the year. For the purposes of this paper, data for all sites except the IBP‐C diel study (see below) and marine sites represent weekly averages of all available data (Table 1).

At sites monitored on a diel basis, aquatic CO2 was logged midwater column every minute using an non‐- dispersive infrared (NDIR) CO2 sensor (Vaisala GMM 222) enclosed in a Polytetraﬂuoroethylene (PTFE)

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Figure 1. Site locations near Utqiaġvik, AK. Named pond, lake, and river sites were sampled continuously for at least 1 week.

membrane (sensu Bass et al., 2012, Hari et al., 2008, Johnson et al., 2010) linked to a Campbell Scientiﬁc

CR1000 datalogger. At sites where continuous stations were not installed, a portable NDIR CO2 sensor (Vaisala GMT220) was allowed to stabilize for at least 30 min before a value was recorded. Data were

converted to pCO2 using water temperature and pressure logged with a HOBO U20 logger. Our two marine regions, Elson Lagoon and the nearshore Chukchi Sea near Utqiaġvik (Figure 1), were

sampled over a 1 week period in late summer 2015 along preplanned survey lines. pCO2 was logged continually over a combined 350 km of boat‐based surveys using a Vaisala GMM 222 towed approximately

40 cm below the surface. In Elson Lagoon, the data were used to map spatial patterns in pCO2 along 307 km of transects using the Topo to Raster tool in ArcMap 10.3.1. The average depth surveyed in Elson Lagoon was 1.76 m. Depth was determined by continual logging at 1 Hz with a SonarMite Echo Sounder (SeaFloor Systems). Along these marine survey lines, grab sample sites were premapped approximately 0.5–1 km apart (n = 25 for Chukchi; n = 107 for Elson). In 2018, we also sampled Elson Lagoon at three sites

in late July. Because CO2 data were not collected from individual sites, but rather continually along transects, and given that each day covered a relatively regionalized but large spatial extent, data from the marine sites were averaged per day, as opposed to weekly. In other words, for comparison among site types, we averaged all data from marine sites visited on a given day.

To complement pCO2 data, grab samples for DOC were collected during Table 1 site visits, when other autonomous data were downloaded. Samples were Samples Sizes (N) for all Site Types Visited in a Given Year ﬁltered through precombusted Whatman GF/F ﬁlters (0.7 μm) and stored Year (date) Ponds (N) Rivers (N) Lakes (N) Marine (N) in precombusted amber glass bottles at 4 °C until analysis. DOC was mea- 2013 (8/4 to 8/14) 1 —— —sured on either a Lachat IL‐550 or a Shimadzu TOC‐V analyzer. DOC was 2015 (7/24 to 8/19) 10 6 5 5 days measured as nonpurgeable organic carbon (NPOC) on the Shimadzu 2017 (7/27 to 8/8) 23 1 2 — instrument; data from the Lachat were converted to NPOC based on 86 2018 (7/24 to 8/3) 3 3 — 1 day samples run on both machines (range 1.9 to 68 mg/L NPOC; r2 = 0.98). Note. Samples sizes (N) represent the number of sites visited in a given year, except for the marine sites, where data collected continuously along Downward total solar irradiance data were retrieved from the Barrow transects were averaged per day. “—” indicates no data were collected. Atmospheric Baseline Observatory (National Oceanic and Atmospheric Dates are formatted as month/day. Administration [NOAA] Earth System Research Laboratory [ESRL])

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prior to 2017. Post 2017, these data were acquired from the National Ecological Observatory Network's Barrow Environmental Observatory Site (National Ecological Observatory Network, 2018). A model relating radiation measured at both sites in 2017 was used to make the data comparable among years (r2 = 0.97). Air temperature and precipitation amounts were collected from a HOBO weather station installed near pond IBP‐C up until 2013. Starting in 2015, rain data were retrieved from the National

Weather Service, Barrow Airport; air temperatures and atmospheric CO2 were from NOAA ESRL. The 95th percentile of each of these environmental variables was calculated and plotted for the study period— July and August in the four study years (2013, 2015, 2017, and 2018).

2.2. Data Analysis

CO2 ﬂux was calculated using wind‐derived gas transfer coefﬁcients (k) for lagoon and ocean sites (Wanninkhof, 2014) and lakes (Cole & Caraco, 1998). Wind data were retrieved from the Barrow

Atmospheric Baseline Observatory (NOAA ESRL). For streams, we estimated kCO2 based on a relationship with stream slope, velocity, and depth (Raymond et al., 2012). Slope, velocity, and depth were measured during a period of fairly typical ﬂow in July 2019; thus, they do not represent the exact days of sampling from this study. We are aware of no published relationships for small ponds, such as those studied here.

Therefore, at a small subset of pond (n = 6) sites/dates, ecosystem‐specific gas transfer coefficients (kCO2) were empirically derived from floating chamber measurements using published techniques (Campeau et al., 2014; Crawford et al., 2013; Striegl et al., 2012). In brief, within a closed floating chamber 3 (~0.05 m ), submerged by 2–5 cm into the water, CO2 concentration was measured continually for 20 min using an INNOVA photoacoustic analyzer. The coefficient kCO2 was calculated based on change in concentration through time, chamber height, and difference between the concentration of CO2 in water and air. These empirically derived kCO2 represented conservative choices for kCO2 relative to published calculations used for lakes, as described above. In all cases the Schmidt number (Sc) was calculated based on published relationships for freshwater and seawater (Wanninkhof, 2014). We acknowledge that our flux values are merely estimates and further work is required to establish ecosystem‐specific gas transfer coefficients in this landscape.

Daily averages for pCO2 and select environmental factors (water temperature, rain, and solar radiation) were used to identify drivers of pCO2 concentrations in pond IBP‐C using simple linear regressions. Only days with complete data sets (i.e., entire 24 hr of data) were used. Running 2, 3, and 4 day averages and maxima (temperature and radiation) and sum through time (rain) were also examined. Prior to statistical analysis, data were checked for normality and log transformed, where applicable. All statistical analyses were per- formed in JMP PRO v11. To allow for comparison of historical and modern data, DOC data from 1971 to 1973 were compiled from historical sources (Barsdate & Prentki, 1973). While 1971 was the only historical year where DOC data were collected from IBP‐C, DOC was collected from IBP‐B in 1971–1973. Therefore, data from IBP‐B, located 25 m away from IBP‐C, were included in the analysis to allow for estimation of variability. In 2009–2013, IBP pond DOC samples were collected on a weekly basis from mid‐June to mid‐August (n = 12 weeks); however, sampling during the 1970s was more irregular, ranging from biweekly to twice weekly. Because historical water temperatures were not collected from these particular ponds in the corresponding years, water temperature was estimated based on available historical air temperature using a regression established between air and water temperatures collected simultaneously (U.S. Tundra Biome, 1974). For the purposes of this paper, data from ponds IBP‐C and B from 1971 to 1973 or 2009 to 2013 were compared using a paired t test on weekly averages.

To estimate pCO2 concentrations historically, we created a multiple regression model relating pCO2 to previously identiﬁed important environmental variables: DOC, water temperature, and total amount of rainfall received over 4 days. This analysis assumed that these variables are (and have historically been) the primary

drivers of CO2 ﬂux, even early in the season, a period from which we have no pCO2 data. To create the model, data from all the freshwater sites (including nonponds) sampled in 2015, 2017, and 2018 were combined. While our goal was to create a model for tundra ponds, without data from lakes and rivers, the range of DOC available in the data set did not overlap those DOC concentrations observed in the 1970s. Data were entered into a stepwise regression, with a forward selection procedure; the best model was selected based on

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the lowest Bayesian information criterion (JMP version 11.0). The strongest model was then applied to modern and historical datasets.

3. Results

3.1. Differences in CO2 Among Aquatic Habitats

We recorded substantial differences in DOC and pCO2 among a variety of ecosystems on the ACP, with the highest aquatic‐atmospheric carbon ﬂuxes and levels of DOC and pCO2 observed in the smallest ecosystems in the landscape, including thermokarst (TK) ponds, ponds, and freshwater rivers (Figure 2; Table 2). DOC

and pCO2 levels were lowest in saline and brackish systems, including the ocean, lagoon, and brackish rivers. Among all site types, weekly average pCO2 could be partially explained by DOC concentration, with the slope of the line for freshwater sites (equation 1; r2 = 0.56, p < 0.0001), more than 4 times as steep as that for saline/brackish sites (equation 2; r2 = 0. 68, p = 0.00062). These models were, at least in part, likely driven by differences among habitat types and may need further sampling to increase overlap among site types in the relationship.

log pCO2 ¼ 1:95 þ 0:92*log DOC (1)

log pCO2 ¼ 2:22 þ 0:22*log DOC (2)

Conversely, the saline/brackish systems were undersaturated with CO2 (Figures 2 and 3) and represented carbon sinks (Table 2). In the spatial survey of Elson Lagoon in 2015, the highest pCO2 levels were seen in the outlet of Avak Creek (Figure 3, bottom right) and along the mainland coast.

3.2. Patterns and Drivers of Variation in pCO2 in a Tundra Pond

We recorded substantial diel variability in pCO2 in pond IBP‐C over a 4 year period (Figure 4); each year included at least one relatively extreme peak in pCO2. Average pCO2 ranged from 943 μatm (2015) to 1,366 μatm (2018). Maximum observed pCO2 was 1.7 to 2 times higher in 2017 (4,106 μatm) and 2018 (4,818 μatm) compared to the other years (<2,500 μatm). Maximum air and water temperatures were also noticeably higher in these years, with air temperatures often exceeding the 95th percentile of all temperatures recorded during the study period.

There was not a significant relationship between daily average DOC and pCO2 using solely IBP‐C data, suggesting that DOC was not a driver of daily variation in pCO2 levels. Therefore, to find the drivers of daily variation in pCO2 in tundra ponds, daily averages for pCO2 were related to select environmental factors, including running 2, 3, and 4 day averages. Mean water temperature was the strongest driver of both mean 2 2 (Figure 5; r = 0.61) and maximum daily pCO2 (r = 0.68; not shown). In particular, very high temperatures in 2017 and 2018, defined as those dates where air temperature exceeded the 95th percentile of 11.1 °C, also

resulted in peak pCO2 concentrations (Figure 4). Of the remaining nontemperature variables, the next strongest relationship was a moderate one between pCO2 and the total amount of rainfall received over 4 days (Figure 5; r2 = 0.39). Total 4‐day rainfall amounts exceeding 10 mm, in particular, were observed preceding

pCO2 peaks in 2013 (11 mm of rain), 2017 (19 mm), and 2018 (16 mm). While there was also a moderately strong relationship between pCO2 and solar radiation, particularly the average radiation received over a 2 day period (r2 = 0.26, p = 0.0009; not shown), this effect of solar radiation became insigniﬁcant once the effect of temperature was accounted for.

3.3. Modeling and Long‐Term Changes in pCO2 DOC has increased signiﬁcantly in IBP tundra ponds since the 1970s (paired t test on weekly averages; p = 0.0004; Figure 6a), with differences most notable later in the season. Given our observation that DOC

was a driver of pCO2 at broad spatial and temporal scales, while rainfall and temperature were the strongest drivers of daily pCO2 in tundra ponds, a multiple regression was created to describe daily average pCO2 in pond IBP‐C (equation 3; r2 = 0.59; p < 0.0001). Although other studies using water temperature data from different IBP ponds and years have found differences in temperature among decades (Lougheed et al., 2011), the data we used for these sites and dates showed no differences among eras for either mean weekly temperature (p = 0.41) or 4 day rain (p = 0.55).

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Figure 2. Comparison of carbon compounds among site types sampled in 2015, 2017, and 2018. (a) pCO2 among site types. Gray bars are brackish/saline sites (BR = brackish); white bars are freshwater (FW = fresh). Different letters indicate statistical differences (analysis of variance, p < 0.05). (b) Relationship between pCO2 (μatm) and DOC (mg/L) in brackish/saline sites (solid line) and freshwater sites (dashed line). For sites sampled more than once per week, these data represent weekly averages in any given year.

Table 2 Mean (± SE) of pCO2,kCO2, and Carbon Flux (F) for All Habitats Oceana Lagoona Brackish riverb Lakec Freshwater riverb Pondd Thermokarst pondd

N 2 4 4 12 9 42 11 pCO2 (μatm) 191.7 ± 5.8 181.4 ± 3.1 262.9 ± 9.3 339.5 ± 21.3 1,395.5 ± 183.5 1,333.6 ± 114.6 4,882.8 ± 693.4 k (m/day) 2.45 ± 0.01 1.61 ± 0.06 1.51 ± 0.06 4.48 ± 0.53 1.67 ± 0.09 1.96 ± 0.67 1.96 ± 0.67 F (g/m2/day) −1.25 ± 0.04 −0.79 ± 0.06 −0.43 ± 0.04 −0.62 ± 0.30 4.59 ± 0.62 4.23 ± 0.59 18.41 ± 2.45 F (mmol/m2/day) −28.41 ± 0.85 −18.02 ± 1.29 −9.78 ± 0.81 −14.16 ± 6.74 104.3 ± 14.07 96.19 ± 13.41 418.60 ± 55.76 Note. Data were averaged from 2015, 2017, and 2018. For freshwater sites sampled more than once per week, N represents the total number of weekly averages that were included in the calculations. Marine sites represent daily averages of data collected along transects. ak (m/day) were calculated based on Wanninkhof (2014). bk (m/day) were calculated based on Raymond et al. (2012). ck (m/day) were calculated based on Cole and Caraco (1998). dk (m/day) were empirically derived (see section 2).

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Figure 3. Spatial variability in pCO2 in Elson Lagoon in August 2015. Map was created in ArcMap (v. 10.4) using the Topo to Raster function (an iterative ﬁnite difference interpolation technique) on 307 km of pCO2 data taken throughout the lagoon (see Figure 1 for survey lines).

log pCO2 ¼ 1:63 þ 0:92*log DOC þ 0:029*Temp þ 0:019*4 day rain (3)

Using equation 3 to model recent and past pCO2 concentrations indicates that the ponds were likely a small sink of CO2 early in the season, relative to atmospheric CO2 levels, but after mid‐June are a source of CO2, even in the 1970s (Figure 6b). On average, the difference between the two eras (1970s vs. 2010s) was 307 μatm, with a maximum difference as high as 1,004 μatm later in the summer (Week 30). Looking just at the last 5 weeks of data, which most closely matches the period from which the model data were collected (mid‐July to August), the mean difference among decades was just over 600 μatm, with recent concentrations 1.8 times higher than historical levels, on average. The 95% conﬁdence interval for the modeled mod-

ern pCO2 data overlap observed pCO2 values collected from 2013 to 2018 ( Figure 6b, gray box) at IBP‐C over the study period.

4. Discussion This paper is among the ﬁrst to describe carbon ﬂux from contemporaneous sampling in multiple aquatic ecosystems, ranging from small freshwater systems to nearshore marine waters in the Arctic coastal landscape. On a per‐area basis, these small aquatic ecosystems of the northernmost Alaskan ACP are greater

sources of carbon than are other sampled waterbodies. In small ponds, CO2 concentrations were driven by factors expected to increase in inﬂuence with warming, including temperature, rainfall, and DOC concentrations.

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Figure 4. Trends in pCO2, light, temperature, and rainfall in July–August 2013, 2015, 2017, and 2018 for pond IBP‐C. Dashed 95th percentiles for irradiance and temperature were calculated for the months of July and August during the study years only.

4.1. Differences in pCO2 Among Aquatic Habitats The greatest ﬂux to the atmosphere was observed from the smallest waterbodies: thermokarst ponds, fol- lowed by tundra ponds and small freshwater rivers. Our estimated river ﬂuxes (4.59 g C/m2/day) and

pCO2 (1,396 μatm) are comparable to other studies (Crawford et al., 2013; Kling et al., 1992; Lundin et al., 2013). Pond ﬂuxes were similar to those highest values reported elsewhere for the Arctic (Abnizova

et al., 2012; Hamilton et al., 1994; Kling et al., 1992) and were a moderate source of CO2 to the atmosphere (4.2 g C/m2/day; 1,334 μatm), with thermokarst ponds emitting more than quadruple that amount (18.4 g C/ m2/day; 4,883 μatm). While smaller water bodies were a substantial source of C to the atmosphere, lakes were a small sink (−0.62 g/m2/day; 340 μatm). Elsewhere in the Arctic, lakes have been described as sources of C: in studies by Kling et al. (1992) of lakes of a similar size at more southerly latitudes and for smaller lakes

elsewhere in the Arctic and subarctic (Abnizova et al., 2012; Lundin et al., 2013). However, a lesser CO2 ﬂux from lakes is not unexpected at the more northerly latitudes studied here (Holgerson & Raymond, 2016).

The strong linkage between CO2‐rich groundwater and adjacent open water is often cited as a driver of high CO2 release from smaller water bodies (Butman & Raymond, 2011; Crawford et al., 2013), as seen here. Smaller ponds and rivers generally have a shallow depth and high perimeter‐to‐volume ratio, thus receiving relatively high amounts of terrestrially derived carbon relative to their volume, and with the capacity for sediment respiration and solar radiation to affect the entire water column in these well‐mixed systems

(Abnizova et al., 2012; Holgerson & Raymond, 2016). Indeed, the close association between pCO2 and DOC observed here suggests a direct inﬂuence of the surrounding landscape on the differences in pCO2 among aquatic ecosystem types with the smallest, shallowest waterbodies receiving greater amounts of terrestrially derived carbon compounds. Several recent reviews have summarized the greater relative importance of hydrologic inputs of dissolved inorganic carbon, produced in adjacent or upstream terrestrial catchments, as opposed to in situ hetero-

trophic metabolism of terrestrially derived DOC, to the CO2 oversaturation of lakes and ponds (Maberly et al., 2012; McDonald et al., 2013). However, studies in Arctic and boreal regions have found that the domi-

nant factors controlling freshwater CO2 supersaturation vary among sites and can include the following: input of CO2 from groundwater (Teodoru et al., 2009), mineralization of allochthonous carbon by microbes (Rasilo et al., 2017; Shirokova et al., 2013), or photochemical oxidization of DOC into CO2 (Cory et al., 2014).

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The slope of our pCO2~DOC models are substantially less than those reported by others whose studies were completed at a more southerly lati- tude and on a single waterbody type (Sobek et al., 2003; Teodoru et al., 2009). Lower slopes have been associated with a greater role for

DOC mineralization, as opposed to direct CO2 inputs (Teodoru et al., 2009); however, further analysis and experimentation may be required to deter- mine the dominant processes occurring in these tundra ecosystems. Indeed, the dominant factors may differ among the ecosystem types.

The rate of pCO2 release as a function of DOC in saline/brackish water was much less than that observed for freshwaters. All saline and brackish sys-

tems were a sink of CO2, even brackish river sites that were found within a few hundred meters downstream of riverine sites with substantial CO2 efﬂux. As pH of the system increased past a threshold of 7.5, it is likely that

inﬂowing CO2 was transformed into HCO3. Nonetheless, elevated CO2 in the nearshore waters of Elson Lagoon suggests a role for terrestrial inputs of carbon, especially at the inﬂow from the largest river in the region, Avak Creek, and along the southwest edge of the lagoon in areas of actively eroding coastlines (Tweedie et al., 2012). This shallow coastal water was also warmer, was oxygen rich, and with more chlorophyll, ammonia, and total

phosphorus (not shown) than the offshore waters. Our pCO2 values were slightly lower than those reported previously along the Chukchi shoreline and the western end of Elson Lagoon (Ikawa & Oechel, 2011) but comparable to those reported in more open ocean Chukchi coastal sites (Bates et al., 2006). Sampling and seasonal differences can likely account for much of the dissimilarity; however, there remain important unknowns for the Arctic nearshore environment (McGuire et al., 2010), including understanding the relative roles of a warmer ocean in stimulating coastal

algal uptake of CO2, as opposed to increasing bacterial transformations of DOC into CO2. The linkage between elevated pCO2 along the coast and carbon inputs derived from coastal erosion is also a topic that demands further research (Tanski et al., 2016).

4.2. Drivers and Historical Change of Tundra Pond CO2 Flux

Diel continuous monitoring of pCO2 in aquatic ecosystems can reveal patterns not evident from discrete sampling. While the pioneering studies of ‐ Figure 5. Relationship between (a) log transformed mean daily pCO2 and Kling et al. (1991, 1992) based their analyses on a single sample taken at water temperature and (b) rain summed over a 4 day period for tundra ﬂ pond IBP‐C. midday, only relatively recently has continuous monitoring of CO2 ux in Arctic aquatic environments become more readily possible. Continuous monitoring can detect substantial diel variation within sites and the impact of storm events

on CO2 flux (Åberg et al., 2010; Crawford et al., 2013; Dinsmore & Billett, 2008). Using continuous monitoring, we show that pCO2 in a tundra pond increased substantially during periods of elevated temperature. Furthermore, relatively large multiday rain events likely contributed to the inflow of both CO2 and DOC from terrestrial sources, which has been observed elsewhere (Lynch et al., 2010; Macrae et al., 2004). Regression analysis confirmed that water temperature and large rainfall events were the dominant drivers

of daily CO2 efflux from the ponds, while the effect of radiation was relatively small. These results suggest a role for both inflowing terrestrially derived pCO2 and DOC mineralization in driving these patterns. However, measurement of CO2 in adjacent moist soil is rarely measured to confirm such assertions and is another area in need of further research. DOC has nearly doubled in tundra ponds over the past half century, especially later in the season. Using a

model based on DOC, temperature, and rainfall, we estimate that pCO2 concentrations in the open water areas of tundra ponds, especially later in the summer, have similarly increased 1.8 times since the 1970s. These results suggest both an effect of increased temperature on carbon release from permafrost thaw (Reyes & Lougheed, 2015) and a greater hydrological connection to the surrounding landscape,

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Figure 6. Comparison of (a) DOC ± SE and (b) modeled pCO2 ± 95% CI from samples collected from IBP‐C and B in the early 1970s relative to the late‐2000s. In (b), the gray box indicates the observed pCO2 (±95% CI) from IBP‐C (2013–2018); atmospheric CO2 levels in summer 1973 and 2018 are indicated by dashed lines. (c) The ﬁt of the regression model for pCO2.

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contributing terrestrially derived carbon into these ponds. Although productivity of the upland landscape appears to have declined over a similar time frame (Lara et al., 2012), changes in the plant community (Villarreal et al., 2012) and increased productivity in nearby wet tundra (Lara et al., 2012) are likely impor-

tant drivers of increased DOC, pCO2, and CH4 efflux (Andresen et al., 2017) in these ponds. Finally, it is worth emphasizing that these data were collected from the open water areas of tundra ponds. The deepest open water areas of the tundra ponds are surrounded by Carex aquatilis in the shallow pond margins and Arctophila fulva in moderately deep areas (Andresen & Lougheed, 2015). C flux from open water areas has rarely been compared to vegetated, flooded margins, with limited results indicating open

water in ponds may be a larger source of both CO2 and CH4 (Hamilton et al., 1994), although the opposite was found in boreal lakes (Juutinen et al., 2003). On the North Slope of Alaska, vegetated, shallow pond mar-

gins have relatively high rates of CO2 uptake compared to moist and dry sites (Lara et al., 2012; von Fischer et al., 2010). Our data show that, while vegetated aquatic margins in Utqiaġvik, AK, may have C uptake on the order of ~1–2.5 g C/m2/day (Lara et al., 2012, von Fischer et al., 2010), efflux from some open waters, in particular those found in thermokarst ponds (18.4 ± 2.5 g C/m2/day), may act to counterbalance this uptake. Recent observed changes in pond size and cover (Andresen & Lougheed, 2015) indicate expansion of flooded vegetation and concomitant loss of open water, possibly leading to reduced landscape‐level fluxes in future. Further data are required to directly compare these two habitats using similar methodologies and to account for relative cover of these habitat types in the landscape.

5. Conclusions In conclusion, the data presented here indicate that the relatively small but abundant aquatic ecosystems of

the northernmost Alaskan ACP have abundant DOC and are supersaturated with CO2, which is likely related, in part, to their strong hydrologic connection to nearby moist tundra ecosystems. Daily pCO2 in tundra ponds is driven by temperature and rainfall, suggesting that warmer temperatures lead to mineralization of carbon, in both terrestrial and aquatic environments, while large rainfall events facilitate the movement of

this carbon into the open water. A regression model suggests that pCO2 levels in tundra ponds have increased substantially in recent history and appears to be driven by factors expected to increase further in influence in the future, including temperature, rainfall, and carbon released from permafrost thaw and from adjacent terrestrial environments. Arctic coastal systems have been identified as one of the most threa- tened environments on Earth (Lantuit et al., 2011) and appear to be undergoing more dramatic climatic, physical, and biological change (Bhatt et al., 2010; Forbes, 2011; Lantuit et al., 2013; Post et al., 2013) than Acknowledgments This study was funded in part by the most other regions in the Arctic. The important role for the inflow of both dissolved inorganic carbon and NSF (ARC‐0909502 and DOC from groundwater into arctic aquatic ecosystems has important implications for aquatic carbon bud- ‐ IUSE 1612212), including the Beaufort gets with future warming, as subsurface hydrologic flow paths in the Arctic will likely increase with a dee- Lagoon Ecosystems Long Term Ecological Research program (NSF pening active layer (Frey & McClelland, 2009). OPP‐1656026), as well as the Bureau of Ocean Energy Management. Land access, permitting, and logistical support was provided by the Ukpeaġvik References Iñupiat Corporation (UIC), UMIAQ, Åberg, J., Jansson, M., & Jonsson, A. (2010). Importance of water temperature and thermal stratification dynamics for temporal variation of and the Barrow Arctic Science surface water CO2 in a boreal lake. Journal of Geophysical Research, 115, G02024. https://doi.org/10.1029/2009JG001085 Consortium (BASC). Thanks to many Abnizova, A., Siemens, J., Langer, M., & Boike, J. (2012). Small ponds with major impact: The relevance of ponds and lakes in permafrost students for help in the field and lab, landscapes to carbon dioxide emissions. Global Biogeochemical Cycles, 26, GB2041. https://doi.org/10.1029/2011GB004237 including Luis Del Val, Christina Algesten, G., Sobek, S., Bergstrom, A. K., Agren, A., Tranvik, L. J., & Jansson, M. (2003). Role of lakes for organic carbon cycling in the Hernandez, Monica Mendoza, Gesuri boreal zone. Global Change Biology, 10, 141–147. Ramirez, Frankie Reyes, Rocio Anderson, L. G., Jutterström, S., Hjalmarsson, S., Wåhlström, I., & Semiletov, I. P. (2009). Out‐gassing of CO2 from Siberian Shelf seas by Ronquillo, and Fabian Urribari. This terrestrial organic matter decomposition. Geophysical Research Letters, 36, L20601. https://doi.org/10.1029/2009GL040046 manuscript was improved thanks to the Andresen, C. G., Lara, M. J., Tweedie, C. T., & Lougheed, V. L. (2017). Rising plant‐mediated methane emissions from arctic wetlands. comments of several anonymous Global Change Biology, 23(3), 1128–1139. https://doi.org/10.1111/gcb.13469 reviewers. This work was inspired, in Andresen, C. G., & Lougheed, V. L. (2015). Disappearing Arctic tundra ponds: Fine‐scale analysis of surface hydrology in drained thaw lake part, by the pioneering limnology basins over a 65 year period (1948–2013). Journal of Geophysical Research: Biogeosciences, 120, 466–479. https://doi.org/10.1002/ research by Vera Alexander, John 2014JG002778 Hobbie, Dick Prentki, and others in Andresen, C. G., Tweedie, C. E., & Lougheed, V. L. (2018). Climate and nutrient effects on arctic wetland plant phenology. Remote Sensing Barrow, AK, during the early 1970s. of Environment, 205,46–55. The data are archived at the LTER Arp, C. D., Jones, B. M., Schmutz, J. A., Urban, F. E., & Jorgenson, M. T. (2010). Two mechanisms of aquatic and terrestrial habitat change Network Data Portal (https://doi.org/ along an Alaskan Arctic coastline. Polar Biology, 33, 1629–1640. 10.6073/pasta/ Aufdenkampe, A. K., Mayorga, E., Raymond, P. A., Melack, J. M., Doney, S. C., Alin, S. R., et al. (2011). Riverine coupling of biogeo- 045604ca1533c1f4a28eaf961cda8900). chemical cycles between land, oceans, and atmosphere. Frontiers in Ecology and the Environment, 9,53–60.

LOUGHEED ET AL. 11 of 13 Global Biogeochemical Cycles 10.1029/2020GB006552

Barsdate, R. J., & Prentki, R. T. (1973). Nutrient metabolism and water chemistry in lakes and ponds of the Arctic coastal tundra. Report 73‐27.US. Arctic Research Program, U.S. International Biological Program, U.S. Tundra Biome. Bass, A. M., Bird, M. I., Morrison, M. J., & Gordon, J. (2012). CADICA: Continuous automated dissolved inorganic carbon analyzer with application to aquatic carbon cycle science. Limnology and Oceanography‐Methods, 10,10–19. Bates, N. R., Moran, S. B., Hansell, D. A., & Mathis, J. T. (2006). An increasing CO2 sink in the Arctic Ocean due to sea‐ice loss. Geophysical Research Letters, 33, L23609. https://doi.org/10.1029/2006GL027028 Bhatt, U. S., Walker, D. A., Raynolds, M. K., Comiso, J. C., Epstein, H. E., Jia, G., et al. (2010). Circumpolar Arctic tundra vegetation change is linked to sea ice decline. Earth Interactions, 14. Billings, W. D. (1987). Carbon balance of Alaskan tundra and taiga ecosystems: Past, present and future. Quaternary Science Reviews, 6, 165–177. Butman, D., & Raymond, P. A. (2011). Significant efflux of carbon dioxide from streams and rivers in the United States. Nature Geoscience, 4, 839–842. Campeau, A., Lapierre, J., Vachon, D., & del Giorgio, P. A. (2014). Regional contribution of CO2 and CH4 fluxes from the fluvial network in a lowland boreal landscape of Québec. Global Biogeochemical Cycles, 28(1), 57–69. https://doi.org/10.1002/2013GB004685 Cole, J. J., & Caraco, N. F. (1998). Atmospheric exchange of carbon dioxide in a low‐wind oligotrophic lake measured by the addition of SF6. Limnology and Oceanography, 43,647–656. Cory, R., Ward, C., Crump, B. C., & Kling, G. W. (2014). Sunlight controls water column processing of carbon in arctic freshwaters. Science, 345, 925–928. Coyne, P. I., & Kelley, J. J. (1975). CO2 exchange over the Alaskan Arctic tundra: Metorological assessment by an aerodynamic method. Journal of Applied Ecology, 12, 587–611. Crawford, J. T., Striegl, R. G., Wickland, K. P., Dornblaser, M. M., & Stanley, E. H. (2013). Emissions of carbon dioxide and methane from a headwater stream network of interior Alaska. Journal of Geophysical Research: Biogeosciences, 118, 482–494. https://doi.org/10.1002/ jgrg.20034 Denfeld, B. A., Frey, K. E., Sobczak, W. V., Mann, P. J., & Holmes, R. M. (2013). Summer CO2 evasion from streams and rivers in the Kolyma River basin, north‐east Siberia. Polar Research, 32, 19,704. Dinsmore, K. J., & Billett, M. F. (2008). Continuous measurement and modeling of CO2 losses from a peatland stream during stormflow events. Water Resources Research, 44, W12417. https://doi.org/10.1029/2008WR007284 Forbes, D. L. (2011). State of the Arctic coast 2010—Scientific review and outlook. Geesthacht, Germany: Helmholtz‐ Zentrum. Frey, K. E., & McClelland, J. W. (2009). Impacts of permafrost degradation on arctic river biogeochemistry. Hydrological Processes, 23, 169–182. Frohn, R. C., Hinkel, K. M., & Eisner, W. R. (2005). Satellite remote sensing classification of thaw lakes and drained thaw lake basins on the North Slope of Alaska. Remote Sensing of Environment, 97,116–126. Hamilton, J. D., Kelly, C. A., Rudd, J. W. M., Hesslein, R. H., & Roulet, N. T. (1994). Flux to the atmosphere of CH4 and CO2 from wetland ponds on the Hudson Bay lowlands (HBLs). Journal of Geophysical Research, 99, 1495. Hari, P., Pumpanen, J., Huotari, J., Kolari, P., Grace, J., Vesala, T., & Ojala, A. (2008). High‐frequency measurements of productivity of planktonic algae using rugged nondispersive infrared carbon dioxide probes. Limnology and Oceanography‐Methods, 6, 347–354. Hinkel, K. M., Eisner, W. R., Bockheim, J. G., Nelson, F. E., Peterson, K. M., & Dai, X. (2003). Spatial extent, age, and carbon stocks in drained thaw lake basins on the Barrow Peninsula, Alaska. Arctic, Antarctic, and Alpine Research, 35, 291–300. Holgerson, M. A., & Raymond, P. A. (2016). Large contribution to inland water CO2 and CH4 emissions from very small ponds. Nature Geoscience, 9, 222–226. Ikawa, H., & Oechel, W. C. (2011). Air–sea CO2 exchange of beach and near‐coastal waters of the Chukchi Sea near Barrow, Alaska. Continental Shelf Research, 31, 1357–1364. Johnson, M. S., Billett, M. F., Dinsmore, K. J., Wallin, M., Dyson, K. E., & Jassal, R. S. (2010). Direct and continuous measurement of dissolved carbon dioxide in freshwater aquatic systems—method and applications. Ecohydrology, 3,68–78. Jones, B. M., Arp, C. D., Jorgenson, M. T., Hinkel, K. M., Schmutz, J. A., & Flint, P. L. (2009). Increase in the rate and uniformity of coastline erosion in Arctic Alaska. Geophysical Research Letters, 36, L03503. https://doi.org/10.1029/2008GL036205 Jorgenson, M. T., & Brown, J. (2004). Classification of the Alaskan Beaufort Sea Coast and estimation of carbon and sediment inputs from coastal erosion. Geo‐Marine Letters, 25,69–80. Juutinen, S., Alm, J., Larmola, T., Huttunen, J. T., Morero, M., Martikainen, P. J., & Silvola, J. (2003). Major implication of the littoral zone for methane release from boreal lakes. Global Biogeochemical Cycles, 17(4), 1117. https://doi.org/10.1029/2003GB002105 Kling, G. W., Kipphut, G. W., & Miller, M. C. (1991). Arctic lakes and streams as gas conduits to the atmosphere—Implications for tundra carbon budgets. Science, 251, 298–301. Kling, G. W., Kipphut, G. W., & Miller, M. C. (1992). The flux of CO2 and CH4 from lakes and rivers in Arctic Alaska. Hydrobiologia, 240, 23–36. Lantuit, H., Overduin, P. P., Couture, N., Wetterich, S., Aré, F., Atkinson, D., et al. (2011). The Arctic coastal dynamics database: A new classification scheme and statistics on Arctic permafrost coastlines. Estuaries and Coasts, 35, 383–400. Lantuit, H., Overduin, P. P., & Wetterich, S. (2013). Recent progress regarding permafrost coasts. Permafrost and Periglacial Processes, 24, 120–130. Lara, M. J., McGuire, A. D., Euskirchen, E. S., Tweedie, C. E., Hinkel, K. M., Skurikhin, A. N., et al. (2015). Polygonal tundra geomor- phological change in response to warming alters future CO2 and CH4 flux on the Barrow Peninsula. Global Change Biology, 21(4), 1634–1651. https://doi.org/10.1111/gcb.12757 Lara, M. J., Villarreal, S., Johnson, D. R., Hollister, R. D., Webber, P. J., & Tweedie, C. E. (2012). Estimated change in tundra ecosystem function near Barrow, Alaska between 1972 and 2010. Environmental Research Letters, 7, 015507. Lin, D., Johnson, D. R., Andresen, C. G., & Tweedie, C. E. (2012). High spatial resolution decade‐time scale land cover change at multiple locations in the Beringian Arctic (1948–2000s). Environmental Research Letters, 7, 025502. Lougheed, V. L., Butler, M. G., McEwen, D. C., & Hobbie, J. E. (2011). Changes in tundra pond limnology: Re‐sampling Alaskan ponds after 40 years. Ambio, 40(6), 589–599. https://doi.org/10.1007/s13280‐011‐0165‐1 Lundin, E. J., Giesler, R., Persson, A., Thompson, M. S., & Karlsson, J. (2013). Integrating carbon emissions from lakes and streams in a subarctic catchment. Journal of Geophysical Research: Biogeosciences, 118, 1200–1207. https://doi.org/10.1002/jgrg.20092 Lynch, J. K., Beatty, C. M., Seidel, M. P., Jungst, L. J., & DeGrandpre, M. D. (2010). Controls of riverine CO 2 over an annual cycle determined using direct, high temporal resolution pCO2 measurements. Journal of Geophysical Research, 115, G03016. https://doi.org/ 10.1029/2009JG001132

LOUGHEED ET AL. 12 of 13 Global Biogeochemical Cycles 10.1029/2020GB006552

Maberly, S. C., Barker, P. A., Stott, A. W., & De Ville, M. M. (2012). Catchment productivity controls CO2 emissions from lakes. Nature Climate Change, 3, 391–394. Macrae, M. L., Bello, R. L., & Moot, L. A. (2004). Long‐term carbon storage and hydrological control of CO2 exchange in tundra ponds in the Hudson Bay Lowland. Hydrological Processes, 18, 2051–2069. McClelland, J., Townsend‐Small, A., Holmes, R. M., Pan, F., Stieglitz, M., Khosh, M., & Peterson, B. (2014). River export of nutrients and organic matter from the North Slope of Alaska to the Beaufort Sea. Water Resources Research, 50, 1823–1839. https://doi.org/10.1002/ 2013WR014722 McClelland, J. W., Holmes, R. M., Dunton, K. H., & Macdonald, R. W. (2012). The Arctic Ocean estuary. Estuaries and Coasts, 35, 353–368. McClelland, J. W., Stieglitz, M., Pan, F., Holmes, R. M., & Peterson, B. J. (2007). Recent changes in nitrate and dissolved organic carbon export from the upper Kuparuk River, North Slope, Alaska. Journal of Geophysical Research, 112, G04S60. https://doi.org/10.1029/ 2006JG000371 McDonald, C. P., Stets, E. G., Striegl, R. G., & Butman, D. (2013). Inorganic carbon loading as a primary driver of dissolved carbon dioxide concentrations in the lakes and reservoirs of the contiguous United States. Global Biogeochemical Cycles, 27, 285–295. https://doi.org/ 10.1002/gbc.20032 McGuire, A., Macdonald, R. W., Schuur, E. A., Harden, J. W., Kuhry, P., Hayes, D. J., et al. (2010). The carbon budget of the northern cryosphere region. Current Opinion in Environmental Sustainability, 2, 231–236. Muster, S., Roth, K., Langer, M., Lange, S., Aleina, F. C., Bartsch, A., et al. (2017). PeRL: A circum‐Arctic permafrost region pond and lake database. Earth System Science Data,317–348. National Ecological Observatory Network. (2018). Data products: D18.BARR.DP1.00014.001. Provisional data downloaded from http:// data.neonscience.org on 03/04/2020. Battelle, Boulder, CO, USA. Post, E., Bhatt, U. S., Bitz, C. M., Brodie, J. F., Fulton, T. L., Hebblewhite, M., et al. (2013). Ecological consequences of sea‐ice decline. Science, 341, 519–524. Rasilo, T., Hutchins, R. H. S., Ruiz‐González, C., & del Giorgio, P. A. (2017). Transport and transformation of soil‐derived CO2,CH4 and DOC sustain CO2 supersaturation in small boreal streams. Science of the Total Environment, 579, 902–912. https://doi.org/10.1016/j. scitotenv.2016.10.187 Raymond, P. A., Zappa, C. J., Butman, D., Bott, T. L., Potter, J., Mulholland, P., et al. (2012). Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers. Limnology and Oceanography: Fluids and Environments, 2,41–53. Repo, M. E., Huttunen, J. T., Naumov, A. V., Chichulin, A. V., Lapshina, E. D., Bleuten, W., & Martikainen, P. J. (2007). Release of CO2 and CH4 from small wetland lakes in western Siberia. Tellus B, 59, 788–796. Reyes, F. R., & Lougheed, V. L. (2015). Rapid nutrient release from permafrost thaw in Arctic aquatic ecosystems. Source: Arctic, Antarctic, and Alpine Research Arctic, Antarctic, and Alpine Research, 47,35–48. Semiletov, I. P., Pipko, I. I., Shakhova, N. E., Dudarev, O. V., Pugach, S. P., Charkin, A. N., et al. (2011). Carbon transport by the Lena River from its headwaters to the Arctic Ocean, with emphasis on ﬂuvial input of terrestrial particulate organic carbon vs. carbon transport by coastal erosion. Biogeosciences, 8, 2407–2426. Shirokova, L. S., Pokrovsky, O. S., Kirpotin, S. N., Desmukh, C., Pokrovsky, B. G., Audry, S., & Viers, J. (2013). Biogeochemistry of organic carbon, CO2,CH4, and trace elements in thermokarst water bodies in discontinuous permafrost zones of Western Siberia. Biogeochemistry, 113, 573–593. Sobek, S., Algesten, G., Bergstrom, A. K., Jansson, M., & Tranvik, L. J. (2003). The catchment and climate regulation of pCO2 in boreal lakes. Global Change Biology, 9,630–641. Stackpoole, S. M., Butman, D. E., Clow, D. W., Verdin, K. L., Gaglioti, B. V., Genet, H., & Striegl, R. G. (2017). Inland waters and their role in the carbon cycle of Alaska. Ecological Applications, 27(5), 1403–1420. https://doi.org/10.1002/eap.1552 Striegl, R. G., Dornblaser, M. M., Aiken, G. R., Wickland, K. P., & Raymond, P. A. (2007). Carbon export and cycling by the Yukon, Tanana, and Porcupine rivers, Alaska, 2001–2005. Water Resources Research, 43, W02411. https://doi.org/10.1029/2006WR005201 Striegl, R. G., Dornblaser, M. M., McDonald, C. P., Rover, J. R., & Stets, E. G. (2012). Carbon dioxide and methane emissions from the Yukon River system. Global Biogeochemical Cycles, 26, GB0E05. https://doi.org/10.1029/2012GB004306 Tank, S. E., Lesack, L. F. W., Gareis, J. A. L., Osburn, C. L., & Hesslein, R. H. (2011). Multiple tracers demonstrate distinct sources of dissolved organic matter to lakes of the Mackenzie Delta, western Canadian Arctic. Limnology and Oceanography, 56, 1297–1309. Tanski, G., Couture, N., Lantuit, H., Eulenburg, A., & Fritz, M. (2016). Eroding permafrost coasts release low amounts of dissolved organic carbon (DOC) from ground ice into the nearshore zone of the Arctic Ocean. Global Biogeochemical Cycles, 30, 1054–1068. https://doi. org/10.1002/2015GB005337 Teodoru, C. R., del Giorgio, P. A., Prairie, Y. T., & Camire, M. (2009). Patterns in pCO2 in boreal streams and rivers of northern Quebec, Canada. Global Biogeochemical Cycles, 23, GB2012. https://doi.org/10.1029/2008GB003404 Tweedie, C. E., Aguirre, A., Vargas, C., & Brown, J. (2012). Spatial and temporal dynamics of erosion along the Elson Lagoon Coastline near Barrow, Alaska (2002–2011). In K. Hinkel (Ed.), Proceedings of the Tenth International Conference on Permafrost, Volume 1: International Contributions (pp. 425–430). Salekhard, Russia: The Northern Publisher. U.S. Tundra Biome. (1974). Daily summary of local climatological data 1970–1973. Report 74‐1.US. Arctic Research Program, U.S. International Biological Program, U.S. Tundra Biome. Villarreal, S., Hollister, R. D., Johnson, D. R., Lara, M. J., Webber, P. J., & Tweedie, C. E. (2012). Tundra vegetation change near Barrow, Alaska (1972–2010). Environmental Research Letters, 7, 015508. von Fischer, J. C., Rhew, R. C., Ames, G. M., Fosdick, B. K., & von Fischer, P. E. (2010). Vegetation height and other controls of spatial variability in methane emissions from the Arctic coastal tundra at Barrow, Alaska. Journal of Geophysical Research, 115, G00I03. https:// doi.org/10.1029/2009JG001283 Vonk, J. E., & Gustafsson, Ö. (2013). Permafrost‐carbon complexities. Nature Geoscience, 6,675–676. Walter, K. M., Zimov, S. A., Chanton, J. P., Verbyla, D., & Chapin, F. S. (2006). Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming. Nature, 443(7107), 71–75. https://doi.org/10.1038/nature05040 Wanninkhof, R. (2014). Relationship between wind speed and gas exchange over the ocean revisited. Limnology and Oceanography: Methods, 12, 351–362. Wrona, F. J., Johansson, M., Culp, J. M., Jenkins, A., Mård, J., Myers‐Smith, I. H., et al. (2016). Transitions in Arctic ecosystems: Ecological implications of a changing hydrological regime. Journal of Geophysical Research: Biogeosciences, 121, 650–674. https://doi.org/10.1002/ 2015JG003133

LOUGHEED ET AL. 13 of 13