Distinct air–water gas exchange regimes in low- and high-energy streams

Ulseth, Amber J.; Hall, Robert O.; Canadell, Marta Boix; Madinger, Hilary L.; Niayifar, Amin; Battin, Tom J.

doi:10.1038/s41561-019-0324-8

Cited by 136 publications

(214 citation statements)

References 47 publications

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“…Based on the contrasting emissions estimates across high latitudes, it is clear that individual regions have distinct properties resulting in different magnitudes of emissions. In our study area, the drivers of CO 2 concentrations (NPP and network position) are very similar to those in other boreal regions (Hutchins et al, ) but landscape fluvial emissions are the result of a complex interplay between stream network architecture and gas exchange which are poorly understood (Ulseth et al, ; Wallin et al, ). Future work should focus on developing cross‐regional relationships of the broad and overarching controls on fluvial emissions in the landscape.…”

Section: Discussionsupporting

confidence: 69%

Fluvial CO₂ and CH₄ patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

Hutchins

Tank

Olefeldt

et al. 2020

Global Change Biology

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Despite occupying a small fraction of the landscape, fluvial networks are disproportionately large emitters of CO2 and CH4, with the potential to offset terrestrial carbon sinks. Yet the extent of this offset remains uncertain, because current estimates of fluvial emissions often do not integrate beyond individual river reaches and over the entire fluvial network in complex landscapes. Here we studied broad patterns of concentrations and isotopic signatures of CO2 and CH4 in 50 streams in the western boreal biome of Canada, across an area of 250,000 km2. Our study watersheds differ starkly in their geology (sedimentary and shield), permafrost extent (sporadic to extensive discontinuous) and land cover (large variability in lake and wetland extents). We also investigated the effect of wildfire, as half of our study streams drained watersheds affected by megafires that occurred 3 years prior. Similar to other boreal regions, we found that stream CO2 concentrations were primarily associated with greater terrestrial productivity and warmer climates, and decreased downstream within the fluvial network. No effects of recent wildfire, bedrock geology or land cover composition were found. The isotopic signatures suggested dominance of biogenic CO2 sources, despite dominant carbonate bedrock in parts of the study region. Fluvial CH4 concentrations had a high variability which could not be explained by any landscape factors. Estimated fluvial CO2 emissions were 0.63 (0.09–6.06, 95% CI) and 0.29 (0.17–0.44, 95% CI) g C m−2 year−1 at the landscape scale using a stream network modelling and a mass balance approach, respectively, a small but potentially important component of the landscape C balance. These fluvial CO2 emissions are lower than in other northern regions, likely due to a drier climate. Overall, our study suggests that fluvial CO2 emissions are unlikely to be sensitive to altered fire regimes, but that warming and permafrost thaw will increase emissions significantly.

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Section: Discussionsupporting

confidence: 69%

Fluvial CO₂ and CH₄ patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

Hutchins

Tank

Olefeldt

et al. 2020

Global Change Biology

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“…Efforts to calibrate acoustical instruments are minimized in our approach because we utilize cross‐band differences and not absolute SPLs. These benefits make acoustic techniques unique in their potential to enhance the spatial and temporal resolution of k 600 estimates in turbulent running waters, and to provide a tool urgently needed to constrain the role of mountain streams in global estimates of gas fluxes (Ulseth et al ). We also show the potential to use sound as an indicator of the different sources of k 600 , enabling an improved mechanistic understanding of recently recognized distinct regimes in air–water gas exchange (Ulseth et al ).…”

Section: Discussionmentioning

confidence: 99%

“…Air-water gas exchange rates are controlled by dissipation of near-surface turbulence which drives interfacial exchange (Lamont and Scott 1970) and are further enhanced by bubblemediated exchange (Cirpka et al 1993;Woolf 1993). Bubblemediated gas exchange is an important but poorly quantified component of k in running waters (Ulseth et al 2019). Bubble contributions increase with channel slope (Hall et al 2012;Hall and Madinger 2018) and can dominate over interfacial exchange in cascading channels with slopes ≥ 1% (Cirpka et al 1993;Chanson 1995).…”

mentioning

confidence: 99%

Listening to air–water gas exchange in running waters

Klaus

Geibrink

Hotchkiss

et al. 2019

Limnology & Ocean Methods

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Air–water gas exchange velocities (k) are critical components of many biogeochemical and ecological process studies in aquatic systems. However, their high spatiotemporal variability is difficult to capture with traditional methods, especially in turbulent flow. Here, we investigate the potential of sound spectral analysis to infer k in running waters, based on the rationale that both turbulence and entrained bubbles drive gas exchange and cause a characteristic sound. We explored the relationship between k and sound spectral properties using laboratory experiments and field observations under a wide range of turbulence and bubble conditions. We estimated k using flux chamber measurements of CO2 exchange and recorded sound above and below the water surface by microphones and hydrophones, respectively. We found a strong influence of turbulence and bubbles on sound pressure levels (SPLs) at octave bands of 31.5 Hz and 1000 Hz, respectively. The difference in SPLs at these bands and background noise bands showed a linear correlation with k both in the laboratory (R2 = 0.93–0.99) and in the field (median R2 = 0.42–0.90). Underwater sound indices outperformed aerial sound indices in general, and indices based on hydraulic parameters in particular, in turbulent and bubbly surface flow. The results highlight the unique potential of acoustic techniques to predict k, isolate mechanisms, and improve the spatiotemporal coverage of k estimates in bubbly flow.

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“…If one wants to estimate gas exchange for an entire large watershed, it is impractical to measure k 600 at the reach scale for all reaches and then sum them up. There must be some sort of scaling exercise where one measures gas exchange for a few reaches of varying size and extrapolates, or uses published models, for example, Raymond et al (), Ulseth et al (), or Wallin et al (), that enable scaling based on hydraulics and geomorphology. Hydraulic estimates at network scales are improving greatly (Gomez Velez, Harvey, Cardenas, & Kiel, ; Wallin et al, ) enabling estimates of velocity at large spatial scales.…”

Section: What Methods To Use?mentioning

confidence: 99%

“…This rough channel morphology increases air entertainment and bubble formation. In these “white waters,” gas exchange accelerates with increasing ε D , in contrast in calmer flowing waters, where turbulent diffusion drives an attenuating increase (i.e., scaling exponent <1) of k 600 with ε D (Ulseth et al, ) (Figure ). The accelerating increase of k 600 with ε D in “white” waters is because turbulence originating from the stream bottom ( ε S , Table ) reaches the surface and enhances gas exchange (Moog & Jirka, ; Raymond et al, ).…”

Section: Controls On Variation Of Gas Exchangementioning

confidence: 99%

Gas exchange in streams and rivers

Hall

Ulseth

2019

WIREs Water

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Gas exchange across the air–water boundary of streams and rivers is a globally large biogeochemical flux. Gas exchange depends on the solubility of the gas of interest, the gas concentrations of the air and water, and the gas exchange velocity (k), usually normalized to a Schmidt number of 600, referred to as k600. Gas exchange velocity is of intense research interest because it is problematic to estimate, is highly spatially variable, and has high prediction error. Theory dictates that molecular diffusivity and turbulence drives variation in k600 in flowing waters. We measure k600 via several methods from direct measures, gas tracer experiments, to modeling of diel changes in dissolved gas concentrations. Many estimates of k600 show that surface turbulence explains variation in k600 leading to predictive models based upon geomorphic and hydraulic variables. These variables include stream channel slope and stream flow velocity, the product of which, is proportional to the energy dissipation rate in streams and rivers. These empirical models provide understanding of the controls on k600, yet high residual variation in k600 show that these simple models are insufficient for predicting individual locations. The most appropriate method to estimate gas exchange depends on the scientific question along with the characteristics of the study sites. We provide a decision tree for selecting the best method to estimate k600 for individual river reaches to scaling to river networks. This article is categorized under: Water and Life > Nature of Freshwater Ecosystems Science of Water > Water Quality Water and Life > Methods

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Distinct air–water gas exchange regimes in low- and high-energy streams

Cited by 136 publications

References 47 publications

Fluvial CO₂ and CH₄ patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

Fluvial CO₂ and CH₄ patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

Listening to air–water gas exchange in running waters

Gas exchange in streams and rivers

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Distinct air–water gas exchange regimes in low- and high-energy streams

Cited by 136 publications

References 47 publications

Fluvial CO2 and CH4 patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

Fluvial CO2 and CH4 patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

Listening to air–water gas exchange in running waters

Gas exchange in streams and rivers

Contact Info

Product

Resources

About

Fluvial CO₂ and CH₄ patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

Fluvial CO₂ and CH₄ patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change