Fluxes of CO2 and CH4 from a small, eutrophic, managed reservoir as determined by eddy covariance

Abstract:

Lakes and reservoirs emit a disproportionately large portion of the global carbon (C) greenhouse gas (GHG) budget relative to their spatial extent. Eutrophication of freshwater bodies could further promote GHG emissions from these systems. Therefore, understanding the temporal patterns and main drivers of GHG fluxes from inland waters is important to develop strategies to mitigate the impacts of eutrophication and avoid climate change feedbacks. In this study, we used an eddy covariance (EC) tower equipped with a closed-path CO₂/H₂O analyzer and an open-path CH₄ sensor to study the temporal patterns and drivers of CO₂ and CH₄ fluxes from eutrophic Falling Creek Reservoir (FCR). FCR is a small (0.12 km², maximum depth of 9.3 m) drinking water reservoir located in Vinton, VA, USA (37°18’12” N, 79°50’14”W). In 2012, the Western Virginia Water Authority, who owns and operates the reservoir, installed a hypolimnetic oxygenation (HOx) system that injects dissolved oxygen into the bottom waters without altering the reservoir’s thermal stratification or temperature. We studied the influence of the HOx system during the summer stratified period on CO₂ and CH₄ fluxes from the reservoir. Our results show that FCR was a source of CO₂ throughout the study period, with greater fluxes observed during the nighttime and during May and June. CH₄ fluxes did not have a clear diel pattern, but greater fluxes were also observed at night. FCR was a weak CH₄ source in April and a weak sink in May, but became a consistent source in June and July as emissions increased more than three fold due to warmer water temperatures. Although higher temperatures generally led to higher flux emissions, CO₂ emissions in July were less than 50% of June emissions as primary production increased. Here, we present the fluxes over the summer 2020 period, including discussion of the role of storm events and hypolimnetic oxygenation on CO₂ and CH₄ fluxes.