Gravel bars are sites of increased CO2 outgassing in stream corridors

Boodoo, Kyle S.; Trauth, Nico; Schmidt, Christian; Schelker, Jakob; Battin, Tom J.

doi:10.1038/s41598-017-14439-0

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Published: 31 October 2017

Gravel bars are sites of increased CO₂ outgassing in stream corridors

Scientific Reports volume 7, Article number: 14401 (2017) Cite this article

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Abstract

Streams are significant sources of CO₂ to the atmosphere. Estimates of CO₂ evasion fluxes (f _CO2) from streams typically relate to the free flowing water but exclude geomorphological structures within the stream corridor. We found that gravel bars (GBs) are important sources of CO₂ to the atmosphere, with on average more than twice as high f _CO2 as those from the streamwater, affecting f _CO2 at the level of entire headwater networks. Vertical temperature gradients resulting from the interplay between advective heat transfer and mixing with groundwater within GBs explained the observed variation in f _CO2 from the GBs reasonably well. We propose that increased temperatures and their gradients within GBs exposed to solar radiation stimulate heterotrophic metabolism therein and facilitate the venting of CO₂ from external sources (e.g. downwelling streamwater, groundwater) within GBs. Our study shows that GB f _CO2 increased f _CO2 from stream corridors by [median, (95% confidence interval)] 16.69%, (15.85–18.49%); 30.44%, (30.40–34.68%) and 2.92%, (2.90–3.0%), for 3^rd, 4^th and 5^th order streams, respectively. These findings shed new light on regional estimates of f _CO2 from streams, and are relevant given that streamwater thermal regimes change owing to global warming and human alteration of stream corridors.

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Introduction

Streams and rivers emit large amounts of carbon dioxide (CO₂) to the atmosphere. Regional and global estimates of these evasion fluxes are being revised at rapid pace^1,2,3,4,5,6. Constraining estimates of CO₂ evasion fluxes (f _CO2) from streams and rivers is not trivial considering the difficulties associated with the quantification of their surface area, the determination of gas exchange at the interface between the water surface and the atmosphere, and the measurement of the partial pressure of CO₂ (pCO₂) in the water^1,7. Recent studies on the diurnal and seasonal variability of streamwater pCO₂ and f _CO2 are increasingly highlighting further sources of uncertainty to regional and global estimates of f _CO2 ^8,9,10. Estimates of f _CO2 are typically based on discrete or continuous streamwater samples from the active channel, thereby omitting much of the environmental heterogeneity inherent to the corridors of stream ecosystems¹¹.

Geomorphological features ranging from small ripples to bars and meanders diversify hydrodynamic exchange and residence time distributions in streams, thereby adding to their environmental heterogeneity^12,13,14. Gravel bars (GBs) induce hydrodynamic exchange where streamwater typically enters the streambed (that is, downwelling) at the head and returns (that is, upwelling) downstream of the GB tail to the streamwater^12,15,16. Owing to this enforced hydrodynamic exchange, GBs are sites of increased biogeochemical reactions, as has been shown for dissolved organic carbon (DOC)^17,18 and nitrate¹⁹. Exposed to solar radiation, GBs can absorb and store heat, which is transferred to the deeper porewater and further to the streamwater upon upwelling or to the groundwater. GBs can thereby impact the thermal regime of entire stream reaches, including their adjacent groundwater^20,21. The role for carbon dynamics, including CO₂ evasion to the atmosphere from GBs (and likely from other geomorphological features) within stream corridors remains poorly studied at present¹¹.

The aim of our study was to evaluate GBs in subalpine streams as distinct and potentially relevant sources of CO₂ to the atmosphere. We further explored temperature distribution within these GBs as a potential driver of f _CO2. Based on seasonal and spatial surveys, our findings consistently show that f _CO2 from the GBs exceed those from the streamwater. We also found that temperature gradients in the GBs, changing with season, drove f _CO2 from the GBs. Including the f _CO2 from these ubiquitous geomorphological structures within stream corridors will likely further increase current estimates of regional and global CO₂ emissions from streams.

Results

CO₂ evasion fluxes

We found the GB in OSB to be a site of increased f _CO2 to the atmosphere. Spatially averaged f _CO2 from the GB (mean ± standard deviation: 30.72 ± 16.20 mg C m² h⁻¹) was significantly (t-test: n = 60, t = 4.223, p < 0.001) higher than streamwater f _CO2 (20.02 ± 10.89 mg C m² h⁻¹) (Fig. 1). Over the study period, f _CO2 varied significantly (Kruskal-Wallis test, H = 59.99, n = 60, p < 0.001) across the GB, with highest values (median; 25–75 percentile) at the tail (37.29 mg C m⁻² h⁻¹; 30.85–44.70 mg C m⁻² h⁻¹); followed by the crest (19.12 mg C m⁻² h⁻¹; 13.66–30.55 mg C m⁻² h⁻¹) and head (14.25 mg C m⁻² h⁻¹; 8.87–30.05 mg C m⁻² h⁻¹). The crest of the GB was the only location with consistently similar fCO2 to those from the streamwater (17.48 mg C m⁻² h⁻¹; 12.08–28.50 mg C m⁻² h⁻¹) for all seasons (Wilcoxon test, Tukey HSD for multiple pairwise comparisons, p > 0.05).

Evasion fluxes of CO₂ also varied seasonally (Fig. 1). Repeated measures ANOVA revealed significant seasonal differences in average f _CO2 from the GB (F = 40.37, p < 0.001, n = 19) and the streamwater (F = 6.16, p < 0.01, n = 18). Pairwise comparisons (Tukey HSD) showed overall GB f _CO2 to be significantly higher (mean ± standard deviation) in summer (48.78 ± 16.90 mg C m⁻² h⁻¹) than in autumn (22.85 ± 6.85 mg C m⁻² h⁻¹) and winter (21.89 ± 4.32 mg C m⁻² h⁻¹), while they did not significantly differ between autumn and winter. Streamwater f _CO2 were significantly lower in winter (13.89 ± 6.51 mg C m⁻² h⁻¹) than in summer (24.06 ± 10.62 mg C m⁻² h⁻¹) and autumn (22.83 ± 12.28 mg C m⁻² h⁻¹).

Average streamwater f _CO2 exhibited pronounced diurnal changes, with values (mean ± standard deviation) being highest in the morning (5 am: 27.65 ± 10.01 mg C m⁻² h⁻¹) and in the evening (8 pm: 23.29 ± 7.48 mg C m⁻² h⁻¹) and lowest in the early afternoon (2 pm: 10.96 ± 7.25 mg C m⁻² h⁻¹). Unexpectedly, average f _CO2 from the GB did not significantly vary diurnally within or across seasons (Kruskal-Wallis H test, H = 0.423, p = 0.66, n = 18).

Groundwater can deliver CO₂ from soil respiration to the stream²². Our mixing model, based on time series of electrical conductivity, revealed only minor contributions (<14%) of groundwater to the GB in OSB, which did not correlate (Pearson’s product moment correlation: p > 0.05 for all locations and seasons, except p < 0.05 for the crest in summer) with f _CO2 from the GB (see Supplementary Fig. S6). We therefore argue that groundwater likely does not substantially contribute to the pool of CO₂ and its dynamics in the GB of our study stream.

DOC dynamics

Streamwater DOC concentration did not vary substantially (Kruskal-Wallis H-test, H = 0.779, p = 0.677, n = 19) across seasons (median; 25–75 percentile): summer (1.23 mg C L⁻¹; 1.15–1.36 mg C L⁻¹); autumn (1.27 mg C L⁻¹; 1.19–1.31 mg C L⁻¹) and winter (1.21 mg C L⁻¹; 1.16–1.27 mg C L⁻¹). Depth averaged (0.75 m and 1.25 m) DOC concentrations along the GB (at the GB head, crest and tail) and in the streamwater throughout the year, significantly varied (Kruskal-Wallis H test, H = 35.60, p < 0.001, n = 53). Throughout the year and across seasons, DOC concentrations at the GB head (1.36 mg C L⁻¹; 1.27–1.44 mg C L⁻¹) and tail (1.37 mg C L⁻¹; 1.26–1.49 mg C L⁻¹) were statistically indistinguishable, and were significantly higher than at the GB crest (1.27 mg C L⁻¹; 1.21–1.36 mg C L⁻¹) and streamwater (1.23 mg C L⁻¹; 1.16–1.30 mg C L⁻¹). Seasonally, GB depth averaged DOC concentrations (mean ± standard deviation) peaked in autumn (head: 1.39 ± 0.18 mg C L⁻¹, crest: 1.42 ± 0.32 mg C L⁻¹ and tail: 1.55 ± 0.41 mg C L⁻¹) and were lowest in winter (head: 1.33 ± 0.10 mg C L⁻¹, crest: 1.29 ± 0.16 mg C L⁻¹ and tail: 1.28 ± 0.09 mg C L⁻¹), and at intermediate levels in summer (head: 1.41 ± 0.28 mg C L⁻¹, crest: 1.28 ± 0.19 mg C L⁻¹ and tail: 1.55 ± 0.40 mg C L⁻¹). DOC concentration and its seasonal variability (coefficient of variation) were typically higher within the upper section (0.75 m) of the GB (see Supplementary Table S1) than at the lower section (1.25 m). Overall, depth and spatially averaged GB porewater DOC concentration (that is the average of all GB locations and depths, per sampling time) did not correlate with GB f _CO2 (r = 0.06, p = 0.661, n = 52) across seasons.

Thermal regime in the OSB gravel bar

Average streamwater temperature in OSB was 9.64 ± 0.74 °C, 7.47 ± 0.51 °C and 6.53 ± 0.64 °C during the summer, autumn and winter sampling, respectively. Corresponding temperatures of the adjacent groundwater averaged 12.16 ± 0.14 °C, 8.60 ± 0.07 °C and 4. 82 ± 0.90 °C (Fig. 2). During spring and summer, the temperature of the GB surface followed distinct diurnal fluctuations and was consistently higher than both stream and groundwater temperature (Fig. 2, see Supplementary Fig. S4). During these seasons, particularly at baseflow, temperatures within the saturated sediment (that is, ca. 0.7 to 1.0 m below surface) were relatively stable, exceededing the temperature of the streamwater and groundwater during almost 20% of the year (see Supplementary Fig. S4). Storm events transiently collapsed vertical gradients, which recovered rapidly during flow recession (see Supplementary Fig. S4).

Temperature within the GB varied with depth and exhibited marked seasonal and diurnal patterns that also changed over the GB from its head to the crest and tail (Fig. 2, see Supplementary Fig. S4). Depth gradients (mean ± standard deviation) were pronounced in winter (−0.025 ± 0.009 °C cm⁻¹), with lower temperatures close to the GB surface than in the streamwater and groundwater, a difference that became alleviated with depth. These pronounced gradients were broken and reversed in summer when porewater below the GB surface became warmer than the streamwater (see Supplementary Fig. S4). Depth gradients in summer were most pronounced at the head (0.017 ± 0.016 °C cm⁻¹) and crest (0.019 ± 0.008 °C cm⁻¹) of the GB (see Supplementary Fig. S4). With the onset of autumn, the depth gradients collapsed and temperature patterns in the GB became more homogenous and comparable to streamwater temperature.

Temperature as a driver of CO₂ evasion fluxes

Because ecosystem metabolism contributions to f _CO2 and the solubility and release of CO₂ depend on temperature²³, we explored the effect of temperature within the GB on f _CO2 to the atmosphere. We found that the average temperature of the saturated GB sediments explained 62% (n = 56, p < 0.001) of the variation in f _CO2 across all three seasons (Fig. 3A). Discharge, an important driver of pCO₂ in streams⁹, did not increase the predictive explanatory power of this simple model (see Supplementary Fig. S5). As GBs are exposed to the atmosphere and solar radiation, and can store thermal energy, which in turn may be transferred to deeper sediment layers^20,21, we further tested if vertical temperature gradient, rather than absolute temperature, affect f _CO2 from the GB in OSB. We found that vertical temperature gradient explained 54% (p < 0.001) of the variation in f _CO2 from the GB across seasons (Fig. 3B).

Our spatial survey on the ancillary GBs (n = 13) provided a similar relationship (r² = 0.73, p < 0.001) between vertical temperature gradients and average diurnal f _CO2 (Fig. 3C) as we found from the temporal variation on the GB in OSB (Fig. 3B). Furthermore, this extended analysis confirmed that f _CO2 from the GBs (mean ± standard deviation: 14.56 ± 7.24 mg C m⁻² h⁻¹) were significantly (t-test, t = 3.40, n = 13, p < 0.01) higher than average f _CO2 from their respective streamwater (6.85 ± 3.81 mg C m⁻² h⁻¹). During this 10-day survey, early morning (evening) streamwater and GB surface temperature averaged 11.7 ± 1.4 °C (14.4 ± 1.8 °C) and 14.4 ± 1.4 °C (20.6 ± 2.8 °C), respectively, across all sites (see Supplementary Table S2, Supplementary Methods). Given these small spatial temperature ranges, we exclude absolute temperature differences between sites as a possible driver of the observed relationship (Fig. 3C).

Extrapolation of f _CO2 from gravel bars

We extrapolated our findings from individual GBs to the scale of the Ybbs River system using Monte-Carlo simulation and resampling techniques, and based on the ratio of GB f _CO2 to streamwater f _CO2, GB area coverage and previously published median streamwater f _CO2 data from the same system (3^rd-order streams: 46.0 mg C m⁻² h⁻¹; 4^th-order streams: 43.5 mg C m⁻² h⁻¹; 5^th-order streams: 50.4 mg C m⁻² h⁻¹)¹⁰. Average relative coverage of GBs per stream order varied from 25.97% ± 5.52% to 16.12% ± 2.69% and 5.39% ± 0.73% for 3^rd, 4^th and 5^th-order streams, respectively; we surveyed four 3^rd, three 4^th and one 5^th-order stream for this analysis. Similarly, ratios of GB f _CO2 to streamwater f _CO2 varied from 1.64 ± 0.78, 2.99 ± 2.03 and 1.54 ± 0.13 for 3^rd, 4^th and 5^th-order streams. We estimated that the inclusion of f _CO2 from GBs increases stream corridor f _CO2 [median, (95%. confidence interval)] within the Ybbs River network by 16.69%, (15.85–18.49%); 30.44%, (30.40–34.68%) and 2.92%, (2.90–3.0%) for 3^rd, 4^th and 5^th-order streams, respectively at baseflow (Supplementary Table S3). Our estimates of f _CO2 from the stream corridor showed a higher sensitivity to GB f _CO2: streamwater f _CO2 ratios (correlation = 0.98, 0.97 and 0.85, re-sampled n = 1000) than % GB coverage (correlation = 0.16, 0.21 and 0.53, re-sampled n = 1000) for 3^rd, 4^th and 5^th-order streams in the Ybbs network, respectively.

Discussion

Gravel bars are important geomorphological features within stream corridors. Our findings from a broad range of GBs across several stream orders, show that mean diurnal f _CO2 from GBs were on average 2.19 ± 1.43 times higher than the f _CO2 from streamwater. Thereby, these results reveal GBs as hitherto potentially significant sources of CO₂ to the atmosphere and link these critical carbon fluxes to the thermal regime in GBs. Overall, our chamber measurements produced fluxes closely bracketed by long-term data from OSB⁹ and spatial surveys in the same catchment¹⁰ as inferred from CO₂ partial pressure in the streamwater and the atmosphere, and from gas exchange velocity.

The stream corridor, including its GBs, is a transition zone between the active stream channel and the adjacent terrestrial environment²⁴. As such, GBs and the terrestrial environment share physical properties relevant for gas exchange, such as elevated gas diffusivity^{25,26,27,28,29,30}. A major difference between GBs and the soil environment is that GBs contain a continuously saturated near-surface zone, subject to hydrodynamic exchange with the streamwater, and that the thermal regime within GBs is driven by a suite of hydrodynamic and geomorphological processes^20,31,32.

Exposed to solar radiation, the GB surface accumulates heat that can be advected downwards to deeper sediment layers. During extended baseflow in summer, heat advection can lead to strong temperature-depth gradients where cooler groundwater entering OSB from the left hillslope and mixing with porewater in the GB (see Supplementary Fig. S3) depresses the advective effect in deeper layers. We also noticed that storm events can transiently erode established temperature gradients in summer. Whereas in autumn and winter, owing to reduced solar radiation and low air temperature diurnality and unsteady flow, vertical hydraulic gradients within the GB do not establish, or even periodically inverse (see Supplementary Fig. S4). We argue that these thermal dynamics and their effects on stream biogeochemistry is comparable to the ecosystem impacts of the thermal stratification and mixing in lakes³³.

We consider vertical temperature gradients within the GB as a direct measure of the heating (or cooling) from the GB surface toward the subsurface and driver of temperature mediated processes within GBs. Temperature distribution in GBs played a major role for f _CO2 from a broad range of GBs differing in size, geometry and positioning within the stream corridor. This is likely due to the catalyzing effect of absolute temperature on heterotrophic metabolism^23,34, but also to the effect of temperature gradients on porewater CO₂ solubility³⁵ and the resulting diffusive flux. The observed vertical temperature gradients and fact that permanent water table temperatures exceeded the temperature of the streamwater and groundwater during almost 20% of the year suggest that the GB surface heats up in response to the sum diurnal fluctuations in air temperature and solar radiation, particularly in spring and summer. Downwelling streamwater can transport heat absorbed from the warmer GB surface downwards, while extended travel times within the GB should facilitate the warming of streamwater near the heated GB surface and the advective transfer of this heat to deeper layers.

Our notion of solar radiation and air temperature generating the thermal gradients in the GB is also supported by the diurnal patterns of temperature gradients during summer. In summer the heat transfer reached down to 1m depth to become alleviated by mixing with cooler groundwater (Fig. 2, see Supplementary Fig. S4). These temperature gradients explained (>70%) of the variation in f _CO2 from the various GBs across the study catchments. Because streams have distinctive temperature regimes, we argue that temperature gradients, rather than absolute temperature, are likely a more transferable parameter for assessing the influence of temperature on f _CO2 across a wide range of streams. Given the small range of streamwater and GB surface temperatures across all study sites, we are confident that the observed pattern was not driven merely by temporal variation but rather by processes that actually drive f _CO2.

The observed diurnal patterns of f _CO2 from the GB are likely due to the combined effects of in-stream respiration, GB metabolism and the delivery of respiratory CO₂ from adjacent soils via groundwater²². We suggest that streamwater downwelling close to the head of the GB and enriched in CO₂ from over-night respiration in the stream^9,36 spikes the GB subsurface water, itself CO₂-enriched by groundwater inputs. This would lead to a temporally lagged increase in GB f _CO2 around 2 pm. Conversely, lower overnight (5 am) f _CO2 from the GB may be attributable to downwelling streamwater that is low in CO₂ owing to photosynthesis and degassing during the daytime⁹ and is flowing through the GB at night. These time-lagged CO₂ outgassing patterns are in fact corroborated by estimated lag times (0.3 to 17 h) in electrical conductivity between downwelling streamwater and the GB head, crest and tail (see Supplementary Table S4), and are likely the reason for overall lack of diurnal patterns of GB f _CO2 in OSB. Our findings suggest no major contributions from groundwater discharge to the porewater in the GB in OSB. However, groundwater adjacent to OSB is typically super-saturated in CO₂ ⁹, and may therefore still be a significant source of CO₂ from the catchment to the f _CO2 from the GB.

Within the stream corridor, GBs represent a direct interface between groundwater, typically enriched in CO₂, and the atmosphere. Fluxes of CO₂ from the OSB gravel bar were higher and more variable than that of the stream. Higher f _CO2 from the GBs than from the adjacent streamwater may be attributable to enhanced biogeochemical reaction rates in the GB sediments. In fact, several studies have highlighted the streambed and particularly GBs as sites of increased metabolism and nutrient cycling, owing to increased residence time and large reactive surface areas^36,37,38,39; these processes may ultimately contribute to the observed f _CO2. Furthermore, recent findings by Rasilo et al.⁴⁰ showed that a substantial percentage of CO₂ within the streambed originates from sub-surface oxidation of soil derived DOC and CH₄ in groundwater, in addition to direct transport of CO₂ from groundwater to the streambed. Thereby, CO₂ from soil respiration⁴¹ and/or chemical weathering⁴² in the catchment can evade through the porous system of the unsaturated sediments in the GBs.

GBs also trap and burry organic matter during floods, a process that can be enhanced by vegetation growing on the GBs⁴³, and potentially fuel respiration in the sediments⁴⁴. Here we have not studied the distribution of particulate organic carbon in the sediments and can therefore not assess its effect on the production of respiratory CO₂ within GBs. However, our results do not support a significant relationship between DOC concentration in the GB porewater and f _CO2 from the GB. We argue therefore that DOC as the intermediary to the carbon cycle² is likely not a major driver or limiting factor of f _CO2 released from the GB in OSB. Furthermore, f _CO2 did not significantly vary with the small changes in stream discharge and water level within the GB (Fig. 2), indicating that variability in related drivers of f _CO2 such as sediment saturation^26,29,30 and gas diffusivity^27,29 were likely insignificant during sampling at OSB.

While our results suggest temperature as an important driver of GB and streamwater f _CO2, we cannot exclude the possibility of a range of potentially unaccounted drivers of CO₂ outgassing from GBs, some of which may also co-vary with temperature and season. For instance, respiration is typically controlled by a combination of retention time^45,46,47, temperature^48,49,50 and limiting reactant concentrations (e.g., oxygen, labile DOC, total nitrogen and phosphorous)^51,52,53. While GB sediments within OSB were never found to be oxygen limited, oxygen concentrations varied spatially and seasonally (see Supplementary Table S1). We suspect that high temperatures in summer and downwelling of bioavailable, autochthonous organic carbon^2,54,55 would stimulate heterotrophic respiration within the GB⁵⁶. This would result in summer oxygen lows and peak levels of CO₂ outgassing as observed in our study⁵⁷ (see Supplementary Table S1). During autumnal leaf litter fall, concentrations of DOC (possibly also particulate organic carbon) in the OSB stream and sub-surface peak⁵⁷ (see Supplementary Table S1). Although soil-derived DOC comprises a wide range of components, including those readily degradable by bacteria^58,59,60, lower temperatures and the prevalence of comparatively “less bioavailable” organic carbon stream- and soilwater inputs^61,62,63 during autumn would likely lead to decreased microbial activity and autumn f _CO2. Furthermore, several studies have shown that the frequency and intensity of sediment re-wetting are determinants of sub-surface DOC quality^64,65,66,67. This variation would likely control microbial activity within the sediments and additionally contribute to the spatial and temporal variability of CO₂ outgassing on diurnal to sub-seasonal timescales. Following these notions, a dependence of f _CO2 on seasonally varying availability of labile DOC, rather than total DOC within the GB can be proposed.

Our GB coverage survey at baseflow revealed that GB spacing (corresponding to 3 to 6 stream widths; see Supplementary Table S3) was bracketed by 5 to 7 channel widths for regular pool spacing in natural streams^68,69,70 and by 1 to 4 channel widths for low-order stream step-pool reaches^71,72. We acknowledge that our upscaling estimates based on the assumption of an elliptical GB area, and rough estimates of wetted stream area (see Supplementary Methods) may lead to an unknown error in our GB f _CO2 calculations. The low contribution and small variability of GB f _CO2 in the 5^th order stream is likely an underestimate owing to the relatively short reach length (1 km) and low number (n = 9) of GBs sampled (see Supplementary Table S3). The high sensitivity of our calculations to GB f _CO2: streamwater f _CO2 ratios was primarily due to its large variability (see Supplementary Table S3), an inherent characteristic of stream corridors over space and time^9,10, and its direct influence as a multiplicative factor on f _CO2 from the stream corridor. Nevertheless, our findings highlight the relevance of GBs for f _CO2 from entire stream networks at baseflow.

Our findings expand current knowledge on the relationships between hydrodynamics, thermal dynamics, and carbon fluxes within stream corridors^32,73,74. Accounting for f _CO2 from the corridor and notably from the GBs, rather than from the active channel solely¹⁴, will likely increase current f _CO2 estimates from streams⁵ on the catchment to regional scale. As highlighted by Wohl and colleagues¹¹, carbon dynamics in stream and river corridors is particularly prone to human alterations. Our findings add yet another dimension to this. Our study calls for wider surveys to further assess and predict the relevance of GBs for CO₂ evasion fluxes from stream ecosystems, including their corridor, as extended baseflow and droughts, owing to global warming, may lead to increased exposure of GBs to solar radiation.

Material and Methods

Field sites

Our core study site was a point GB (ca. 42 m long, 8 m wide at baseflow (430 L s⁻¹) in the 3^rd-order gravel stream Oberer Seebach (OSB; Ybbs River catchment, Austria) (see Supplementary Figs S1 & S2, Supplementary Methods). OSB is a cold-water stream (6.87 °C, average 2010 to 2016) with temperatures ranging from 0.4 °C in winter to 19.9 °C during summer baseflow. The OSB flow regime (average discharge: 752 L s⁻¹, 2010 to 2016) is characterized by snowmelt in spring and pronounced storm flow (up to 28,065 L s⁻¹) in summer. Topography adjacent to the OSB channel influences subsurface hydrological flow paths, with groundwater flowing from the orographic left hillslope through the OSB streambed where it mixes with streamwater or partially recharges groundwater in the opposite right floodplain⁵⁷ (see Supplementary Fig. S3). The hydraulic conductivity (mean ± standard deviation) of GB sediments in OSB varied from 3.9 ± 2.6 × 10⁻⁴ m s⁻¹ to 1.1 ± 0.5 × 10⁻³ m s⁻¹ and 8.9 ± 4.5× 10⁻⁴ m s⁻¹ at the head, crest and tail, respectively. Average water travel times (measured at 0.75 m and 1.25 m below surface) within the GB approximated 10 h, 1.5 h and 1.3 h to the head, crest and tail, respectively (see Supplementary Table S4 & Supplementary Methods).

We expanded our study sites from OSB to 12 additional GBs within 2^nd to 5^th-order streams throughout the catchments of the Ybbs River and the Grosse Erlauf River where we focused on discrete sampling in time during August and September 2016 (see Supplementary Fig. S1, Supplementary Table S2). We extended our f _CO2 measurements to the catchment scale (see Supplementary Fig. S1), estimating stream area and percentage stream coverage by GBs across mid-order streams (2^nd to 5^th) within the Ybbs catchment during a roaming survey (~0.25–0.60 km for 2^nd order streams and ~1.0 km for 3^rd–5^th order streams), complimented by median f _CO2 data for ~150 stream reaches within the Ybbs catchment¹⁰ (see Supplementary Methods, Supplementary Fig. S1).

Temperature distribution

We measured temperature every 10 minutes at 8 depth levels (0.07 m above to 1 m below surface) at the head, crest and tail of the GB in OSB using LogTrans6-GPRS sensors (Umwelt und Ingenieurtechnik GmbH, Dresden, Germany–range: −20 °C–50 °C, accuracy: ±0.1 °C). Temperature of shallow groundwater was measured in three wells adjacent to OSB at a fixed depth below the long-term minimum water level; streamwater temperature was measured upstream and downstream of the GB. Groundwater and streamwater temperatures were monitored at 10 and 30 min intervals, respectively. We also recorded air temperature (HOBO U30, Onset Computer Co., MA) and global radiation (ZAMG, Austria) as potential drivers of the thermal regime within the GB at OSB. Temperature in the ancillary GBs was measured at the surface, at 0.25 and 0.50 m depth using piezometers and a TSUB21-CL5 (PyroScience GmbH, Aachen, Germany) dipping probe. Vertical gradients of temperature were calculated as the difference between the temperature at the GB surface and at 1-m and 0.5-m depth for OSB and the ancillary sites, respectively, and normalized by depth.

Porewater chemistry

We sampled porewater from 13 piezometers (0.75 and 1.25 m below sediment surface) distributed over the GB in OSB and analyzed it for DOC (Sievers TOC Analyzer GE) and for electrical conductivity (WTW Cond 3310, Weilheim, Germany) (see Supplementary Methods).

CO₂ evasion fluxes

f _CO2 from the streamwater, and from the head, crest and tail of each GB were determined using flux chambers^7,75. On the OSB GB, we measured diurnal f _CO2 (5 am, 2 pm, 8 pm) at baseflow over 6 to 7 consecutive days in August and September 2015, November 2015 and March 2016, respectively. At the ancillary sites, discrete f _CO2 measurements at 8 am (7 to 9 am) and 6 pm (5 to 7 pm) were taken within one week in August and September 2016 during baseflow (see Supplementary Table S2). At the start of each incubation, CO₂ in the chamber was equalized with air, and CO₂ concentrations recorded every minute for a minimum of 0.5 hours. f _CO2, was calculated from f _CO2 = [CO₂]_mol × RMM_C × 1000]/(A × T), where RMM_C is the relative molar mass of carbon, A is the surface area (m²) covered by the chamber, and T is the incubation period (h). The molar CO₂ concentration [CO₂]_mol was determined as ΔCO_{2 (ppmv)} × 10⁻⁶ × V _chamber/V _m, where V _chamber refers to the volume (L) of the chamber and V _m refers to the molar gas volume defined as R × T/p, where R is the gas constant (8.314 J mol⁻¹ K⁻¹) and p the gas pressure (hPa). The gradient of the linear section of CO₂ increase over 10–30 minutes was used to derive f _CO2, with visual inspection of each CO₂ curve to confirm consistent linearity. Uncertainty in individual f _CO2 measurements was calculated via equipment error propagation (see Supplementary Methods), where atmospheric pressure, estimated from altitude, was assumed constant (930 hPa).

To assess the implications of GBs as sites of increased f _CO2 for an entire stream network, we first estimated the percentage areal cover of GBs within the wetted channel boundaries of 2^nd to 5^th order streams of the Ybbs catchment. We used streamwater f _CO2 data derived from a detailed catchment stream study (n = 148) of the Ybbs catchment¹⁰, assuming similar stream order-specific proportionality between GB and streamwater f _CO2 to that determined in our catchment-wide f _CO2 survey in order to estimate corresponding GB f _CO2. Correcting catchment-wide streamwater f _CO2 for percentage area presence of GBs, we calculated the increase in overall stream corridor f _CO2 considering GB contributions to f _CO2 under baseflow conditions. We utilized a Monte Carlo approach to constrain our estimates (see Supplementary Methods) of increase in stream corridor f _CO2. As a result of only one 2^nd order stream being sampled during the Ybbs/Grosse Erlauf catchment survey, 2^nd and 3^rd order stream data were combined for calculations.

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Acknowledgements

This study received funding from the European Union (INTERFACES, Seventh Framework Program, 607150) to TJB and from the Austrian Science Foundation (Y420-B17) to TJB. We thank G. Steniczka, H. Kraill, S. Schmid, C. Preiler, M. Mayr, R. Niederdorfer, C. Fasching and M. Vanek for assistance in the field and laboratory and the Zentralanstalt für Meteorologie und Geodynamik, Austria (ZAMG) for climatic data and R. Poeppl and B. Groiss for assistance in conducting field topographical surveys and providing equipment.

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Department of Limnology and Bio-Oceanography, University of Vienna, Althanstrasse 14, A-1090, Vienna, Austria
Kyle S. Boodoo & Jakob Schelker
WasserCluster Lunz GmbH, Dr. Carl Kupelwieser Promenade 5, 3293, Lunz am See, Austria
Kyle S. Boodoo & Jakob Schelker
Helmholtz Centre for Environmental Research – UFZ, Department of Hydrogeology, Permoserstraße 15, 04318, Leipzig, Germany
Nico Trauth & Christian Schmidt
Stream Biofilm and Ecosystem Research Laboratory, ENAC, Ecole Polytechnique Fédérale de Lausanne, CH - 1050, Lausanne, Switzerland
Tom J. Battin

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Kyle S. Boodoo
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T.J.B., K.S.B. and J.S. developed the study design; K.S.B. conducted fieldwork and data analyses; N.T. and C.S. contributed to data analyses; T.J.B. and K.S.B. wrote the manuscript.

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Boodoo, K.S., Trauth, N., Schmidt, C. et al. Gravel bars are sites of increased CO₂ outgassing in stream corridors. Sci Rep 7, 14401 (2017). https://doi.org/10.1038/s41598-017-14439-0

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Received: 13 July 2017
Accepted: 10 October 2017
Published: 31 October 2017
DOI: https://doi.org/10.1038/s41598-017-14439-0

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