Spatial Heterogeneity of Surface CO₂ Fluxes in a Newly Created Eastmain-1 Reservoir in Northern Quebec, Canada

Abstract

We assessed the spatial variability in both, surface carbon dioxide (CO₂) fluxes and in pelagic respiration rates in a newly created 600-km² boreal reservoir, located in Northern Quebec. We show that total CO₂ emission to the atmosphere was highest in the first year after flooding, and that surface fluxes were spatially heterogeneous. This spatial heterogeneity was not random, but was linked to the pre-flood landscapes: reservoir areas overlying former peatbogs and mature forests had the highest average emissions, whereas areas overlying former non-forest and burned soils had the lowest emissions. Total reservoir emissions appeared to decline exponentially in the next 2 years, and so did the degree of spatial heterogeneity in surface fluxes, suggesting a progressive weakening of the link to the pre-flood landscapes, and a homogenization of reservoir processes. We show that pelagic respiration rates were also initially high and spatially heterogeneous, the latter linked to the pre-flood landscapes. A simple, first-order mass balance for the first 3 years after flooding was used to derive potential benthic CO₂ production rates, and thus to apportion the total reservoir emissions between its pelagic and benthic components. Extrapolation of the observed declines (normal exponential) in total emission, as well as of the underlying pelagic and benthic fluxes, results in a large underestimation of the fluxes for the fourth year, relative to the measured emissions. We postulate that the initial exponential decline in total emissions is driven primarily by the patterns of decomposition of surface plant biomass, whereas at later stages emission is increasingly dominated by sediment and pelagic respiration, which decline in time at a slower rate.