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

Inland water bodies 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 five 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 being 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 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 were of greater relative importance in the former tributary inflow and the forebay, respectively. Loading of organic matter appeared to vary across the reservoir, and evidence suggests this was a key factor regulating R. Light limitation appeared to be prevalent throughout the study area, consistent with previous studies in the region. The results presented here updated the understanding of aquatic metabolism in a region characterized by hydroelectric development.