The influence of flow and bed slope on gas transfer in steep streams and their implications for evasion of CO2

The evasion of greenhouse gases (including CO2, CH4 and N2O) from streams and rivers to the atmosphere is an important process in global biogeochemical cycles, but our understanding of gas transfer in steep (> 10%) streams, and under varying flows is limited. We investigated gas transfer using combined tracer injections of SF6 and salt. We used a novel experimental design in which we compared four very steep (18.4-29.4%) and four moderately steep (3.7-7.6%) streams, and conducted tests in each stream under low flow conditions and during a high discharge event. Most dissolved gas evaded over short distances (~100 and ~200-400 m respectively), so accurate estimates of evasion fluxes will require sampling of dissolved gases at these scales to account for local sources. We calculated CO2 gas transfer coefficients (KCO2) and found statistically significant differences between larger KCO2 values for steeper (mean 0.465 min-1) streams compared to those with shallower slopes (mean 0.109 min-1). Variations in flow had an even greater influence. KCO2 was substantially larger under high (mean 0.497 min-1) compared to low flow conditions (mean 0.077 min-1). We developed a statistical model to predict KCO2 using values of streambed slope x discharge which accounted for 94 % of the variation. We show that two models using slope and velocity developed by Raymond et al. [2012] for streams and rivers with shallower slopes, also provide reasonable estimates of our CO2 gas transfer velocities (kCO2; m d-1). We developed a robust field protocol which could be applied in future studies

Headwater gas exchange quantified from O-2 mass balances at the reach scale

Headwater streams are important in the carbon cycle and there is a need to better parametrize and quantify exchange of carbon-relevant gases. Thus, we characterized variability in the re-aeration coefficient (k2) and dissolved oxygen (O2) gas transfer velocity (k) in two lowland headwaters of the River Avon (UK). The traditional one-station open-water method was complemented by in situ quantification of riverine sources and sinks of O2 (i.e., groundwater inflow, photosynthesis and respiration in both the water column and benthic compartments - sediments) enabling direct hourly estimates of k2 at the reach–scale (~150 m) without relying on the nighttime regression method. Obtained k2 values ranged from 0.001 – 0.600 h-1. Average daytime k2 were a factor two higher than values at night, likely due to diel changes in water temperature and wind. Temperature contributed up to 46% of the variability in k on an hourly scale, but clustering temperature incrementally strengthened the statistical relationship. Our analysis suggested that k variability is aligned with dominant temperature trends rather than with short-term changes. Similarly, wind correlation with k increased when clustering wind speeds in increments correspondent with dominant variations (1 m s-1). Time scale is thus an important consideration when resolving physical drivers of re-aeration. Mean estimates of k from recent parametrizations proposed for upscaling, when applied to the settings of this study, were found to be in agreement with our independent O2 budget assessment (within <15%), adding further support to the validity of upscaling efforts aiming at quantifying large-scale riverine gas emissions

Presentation of research findings on the reaeration capacity of streams and estuaries

Issued as Final report, Project E-20-608 (formerly B-620)Final report has title: Proceedings of a symposium on direct tracer measurement of the reaeration capacity of streams and estuaries, July 7-8, 197