Aquatic metabolism after impoundment of a low productivity boreal reservoir using free water oxygen

Inland waterbodies play a significant role in the global cycling of greenhouse gases. Impoundment of rivers changes their greenhouse gas dynamics and leads to a pulse of emissions, primarily due to the respiration of introduced organic matter. Aquatic metabolism estimates using free water oxygen curves were calculated at 5 sites on the lower Nelson River, Manitoba, Canada, in 2021 to assess immediate conditions after impounding a hydroelectric reservoir at the Keeyask Generating Station. Net ecosystem production varied significantly across the study area (<i>P</i> &lt; 0.001), with the most heterotrophic sites found in the reservoir. More specifically, ecosystem respiration (R) and gross primary production rates varied significantly (<i>P</i> &lt; 0.001) within the reservoir itself. The greatest R (mean [standard deviation]) was observed in a former tributary inflow (−1.94 [1.03] g C m⁻² d⁻¹) and then in the forebay (−1.06 [0.74] g C m⁻² d⁻¹), an order of magnitude greater than the upstream extent of the reservoir area (−0.13 [0.29] g C m⁻² d⁻¹). Benthic and pelagic R had greater relative importance in the former tributary inflow and the forebay, respectively. Loading of organic matter seemed to vary across the reservoir, and evidence suggests it was a key factor regulating R. Light limitation seemed prevalent throughout the study area, consistent with previous studies in the region. The results presented here update understanding of aquatic metabolism in a region characterized by hydroelectric development.