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Gas exchange in streams and rivers

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Abstract 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
Model for gas exchange for a sparingly soluble gas where diffusion through water (and not air) controls gas flux
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Decision tree for applying various methods of estimating gas exchange as a function of research question, stream size, and gas exchange rates. Thresholds are somewhat arbitrary and will depend on properties of specific ecosystems
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Gas exchange (k600) increases with increasing energy dissipation rate (εD = gSv, where g is acceleration of gravity, S is stream channel slope, and v is stream flow velocity). Solid line is the model fit (dotted lines are the 95% prediction intervals) from piece‐wise linear regression analysis (R2 = 0.78) where εD < 0.02 m2 s−3 (dashed lines: 95% confidence interval [CI]: 0.016, 0.026), ln k600 = 0.35 ln εD (95% CI: 0.31, 0.41) + 3.10. Whereas in streams where εD > 0.02 m2 s−3, ln k600 = 1.18 ln εD (95% CI: 1.10, 1.30) + 6.43.(Reprinted with permission from Ulseth et al. (). Copyright 2019 Springer nature)
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Modeled oxygen curves at varying rates of K600 (per day). Gross primary production and ecosystem respiration vary in each model scenario to keep nighttime equilibrium and daytime peaks approximately equal so that the shape of curve as a function of variation on K600 is evident. Note that as K600 declines, daytime peak on dissolved O2 occurs later in the day and that O2 declines more slowly after sunset. Yellow box is daytime. Modeling approaches (Appling, Hall, et al., ; Holtgrieve, Schindler, Branch, & A'mar, ) use the gas exchange‐induced shape in the curve to evaluate K600 separately from metabolism
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Gas exchange velocities normalized to Sc = 600 vary as a function of temperature and gas type for several sparingly soluble gases
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Water and Life > Methods
Science of Water > Water Quality
Water and Life > Nature of Freshwater Ecosystems

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