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.

Abstract Northern peatlands contain up to 20% of the ∼3000 Pg of global soil organic carbon. Carbon-rich peatlands cover upwards of 65% of the landscape in northern Canada where resource extraction activities disturb both the carbon pools and the future carbon sequestration capacity of the landscape. Previous estimates of the carbon losses from this disturbance predict a complete loss of the region’s peatland carbon pool. Mining industries operating in these sensitive environments have recently begun constructing closure landscapes which are intended to develop carbon cycle processes similar to undisturbed northern peatlands. This study investigates eddy covariance fluxes of carbon dioxide (CO2) at one of Canada’s first fully constructed boreal plains watersheds, the Sandhill Fen Watershed. During the first three years since inception, only the lowland region had an annual net ecosystem exchange of CO2 (NEE) indicative of increasing CO2 sink potential. The lowland region was characterized by saturated salvaged peat soils, standing water, thriving communities of Typha and Carex spp. and was a net CO2 sink of 77 g C m−2 in the third year. At the same time the upland and the midland regions characterized by moist salvaged peat soils and a mix of herbaceous, shrub and planted Picea glauca and Pinus banksiana remained net sources of CO2. Despite similar rates of gross primary production, ecosystem and plot-level respiration rates in the lowland were significantly lower than in the midland region, likely due to very low reduction potentials within the lowland’s saturated soils. With no other significant outflows of carbon, the lowland of the Sandhill Fen Watershed may be in the early stages of organic matter accumulation. Due to limited oxidation of the salvaged peat substrate in the lowland region, wetland reclamation employing these techniques may reduce the disturbance loss of the carbon pool in the boreal plains.