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UBC Theses and Dissertations

Determining gas transfer velocities and CO₂ evasion fluxes from streams using carbon dioxide as a tracer McDowell, Mollie Jean


Evasion of carbon dioxide (CO₂) from headwater streams is a dominant process controlling the fate of terrestrially-derived carbon (C) in inland waters. However, methodological limitations associated with determining the gas transfer velocity of carbon dioxide (kCO₂) in headwater streams inhibit efforts to accurately quantify CO₂ emissions. In this thesis, I present a proof of concept for a tracer gas method that mitigates common issues associated with conventional methods for determining kCO₂. In this method, a datalogger controls in situ stream sensors that measure the partial pressure of CO₂ (pCO₂) and other stream parameters as well as a solenoid valve connected to a compressed CO₂ cylinder. Automated injections of CO₂ were made via an aquatic diffuser located on the stream bed. Infrared gas-analyzing (IRGA) CO₂-type sensors enclosed in waterproof, gas-permeable membranes located downstream from the diffuser continuously measured aqueous pCO₂ and equilibrate to elevated values during CO₂ injections. The difference between upstream and downstream pCO₂ values during CO₂ injection relative to pre-injection concentrations permitted calculation of both the CO₂ flux from the reach and kCO₂. This method improves upon conventional methods due to its automation, in situ measurement, and use of CO₂ as a tracer rather than another gas, thereby reducing analytical error and increasing the frequency and timing with which measurements can be made relative to conventional methods. I tested this method in a headwater stream in southwestern British Columbia. I calculated kCO₂ and continuous CO₂ emissions from the reach and compared both datasets to hydrogeomorphic parameters as well as values in the literature. Values of kCO₂ were generally above the average values reported in the literature, but they corresponded well to values reported for steep, turbulent headwater streams. Values of kCO₂ varied in relation to discharge, flow velocity, and stream temperature. CO₂ emissions from the stream were highest during high flow events. Headwater streams, which have been shown to be "hotspots" for CO₂ emissions, can also be considered as exhibiting "hot moments" of CO₂ evasion.

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