Energy and carbon fluxes from an oil sands pit lake

Currently, post-mining landscape plans in the Athabasca Oil Sand Region include large watersheds terminating in pit lakes. In 2012, Base Mine Lake (BML), was constructed with the aim of demonstrating technologies associated with lake reclamation in the region. This paper examines the first 6.5 years of lake-atmosphere energy and carbon exchange. Energetically, BML behaved similar to other northern lakes, storing large quantities of heat in the spring and releasing it in the fall as sensible and latent heat fluxes. At various times a hydrocarbon sheen formed on the lake, which may have suppressed evaporation. However, simple linear relationships failed to statistically quantify the impacts and more comprehensive modelling of the variability may be required. At daily scales, variability in evaporation was well explained by the product of vapour pressure deficit and wind speed as well as the available energy (R² = 0.74), while sensible heat was explained by the product of wind speed and the difference in air and surface temperature as well as available energy (R² = 0.85). Spring CH₄ fluxes were high, particularly around ice melt, with a maximum flux of 3.3 g m^-2 day^-1. Otherwise fluxes were low, except during irregular periods. The peak flux of these periods occurred following ~58 h of continuously falling pressure, relating cyclone activity to these large periods of methane emissions. Annually, CO₂ and CH₄ fluxes were initially high, with median fluxes of 231 mg CO₂ m^-2 h^-1 and 23 mg CH₄ m^-2 h^-1 in 2014. However, the median fluxes reduced quickly and over the least three years of the study (2017 through 2019) the median fluxes declined to 36 mg CO₂ m^-2 h^-1 and 10 mg CH₄ m^-2 h^-1. Overall, BML behaves similar to other boreal lake ecosystems with above average carbon fluxes compared to other constructed reservoirs.