Sources and variability of CO₂ in a prealpine stream gravel bar

Abstract

Gravel bars (GBs) contribute to carbon dioxide (CO2) emissions from stream corridors, with CO2 concentrations and emissions dependent on prevailing hydraulic, biochemical, and physicochemical conditions. We investigated CO2 concentrations and fluxes across a GB in a prealpine stream over three different discharge‐temperature conditions. By combining field data with a reactive transport groundwater model, we were able to differentiate the most relevant hydrological and biogeochemical processes contributing to CO2 dynamics. GB CO2 concentrations showed significant spatial and temporal variability and were highest under the lowest flow and highest temperature conditions. Further, observed GB surface CO2 evasion fluxes, measured CO2 concentrations, and modelled aerobic respiration were highest at the tail of the GB over all conditions. Modelled CO2 transport via streamwater downwelling contributed the largest fraction of the measured GB CO2 concentrations (31% to 48%). This contribution increased its relative share at higher discharges as a result of a decrease in other sources. Also, it decreased from the GB head to tail across all discharge‐temperature conditions. Aerobic respiration accounted for 17% to 36% of measured surface CO2 concentrations. Zoobenthic respiration was estimated to contribute between 4% and 8%, and direct groundwater CO2 inputs 1% to 23%. Unexplained residuals accounted for 6% to 37% of the observed CO2 concentrations at the GB surface. Overall, we highlight the dynamic role of subsurface aerobic respiration as a driver of spatial and temporal variability of CO2 concentrations and evasion fluxes from a GB. As hydrological regimes in prealpine streams are predicted to change following climatic change, we propose that warming temperatures combined with extended periods of low flow will lead to increased CO2 release via enhanced aerobic respiration in newly exposed GBs in prealpine stream corridors.

Sources, Transformation, and Fate of Dissolved Organic Matter in the Gravel Bar of a Prealpine Stream

“…In winter, when SW downwelling was the highest, this vertical gradient in DOM concentration and composition as well as EC collapsed, with DOM concentrations and composition across the GB and SC at both sample depths more similar to SW (Table S1). Furthermore, higher SW downwelling flow velocities (Boodoo et al, 2019) within GB low could explain the significantly lower DOC concentrations—more similar to the stream, at this location when compared to GB up during summer and autumn. Considered together, these interseasonal and intraseasonal patterns highlight the occurrence of substantial rates of microbial processing and DOM removal at the bedform scale, as a result of small‐scale, site‐specific GB hydrodynamics.…”

“…Hyporheic hydrodynamics, influencing SW‐GW mixing and residence time within the GB and SC subsurface, acted as a strong control on hyporheic DOM dynamics. Higher GW upwelling fluxes in summer and autumn corresponded with lower SW downwelling velocities and higher hyporheic residence time compared to winter (Boodoo et al, 2019). Thus, hyporheic hydrodynamics, controlling GW fluxes, may have acted as a major source of variability in DOM composition within the GB and SC, particularly in summer.…”

Evaluating stream CO<sub>2</sub> outgassing via drifting and anchored flux chambers in a controlled flume experiment

“…The result further enforces the robustness of the data from our steady chambers, since both the curves fall inside the literature range (represented as a light grey shaded area). According to Esters et al (2017), the coefficient α follows the strong depth dependency in ε. While Esters et al (2017) refer to standing waters, this can also be expected to be true in running waters where ε varies with depth as a result of bottom friction (Sukhodolov et al, 1998).…”

“…These chambers with internal CO 2 sensors have since been frequently used and adapted to various applications (e.g. Lorke et al, 2015;Sawakuchi et al, 2017;Rosentreter et al, 2017;Jeffrey et al, 2018;Boodoo et al, 2019). The main advantages of floating chambers are their low cost, the easyto-manage deployment and the flexibility of the chamber.…”

Evaluating stream CO<sub>2</sub> outgassing via Drifting and Anchored flux chambers in a controlled flume experiment

“…These chambers with internal CO 2 sensors have since been frequently used and adapted to various applications (e.g. Lorke et al, 2015;Sawakuchi et al, 2017;Boodoo et al, 2019). The main advantages of floating chambers are their low cost, the easy-to-manage deployment, and the flexibility of the chamber.…”