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

Abstract

Inland water bodies play a significant role in the global cycling of greenhouse gases (GHGs). Impoundment of rivers changes their GHG dynamics and leads to a pulse of emissions, primarily due to the respiration of introduced organic matter (OM). Aquatic metabolism estimates using free water oxygen curves were calculated at five sites on the lower Nelson River, Manitoba, Canada to assess the impact of impounding a new hydroelectric reservoir at the Keeyask Generating Station. After impoundment, the greatest ecosystem respiration (R) rates were seen 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 that appeared unimpacted (-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 allochthonous OM was a key factor regulating R. Further, evidence of "priming," wherein labile OM facilitates the breakdown of more recalcitrant OM, was observed. Light limitation appeared to be prevalent throughout the study area, consistent with previous studies in the region. Despite the unique water chemistry present (i.e., high total phosphorus, high turbidity), aquatic metabolism in the study area appeared similar to other boreal impoundments. The results presented here updated the understanding of aquatic metabolism in a region characterized by hydroelectric development. Further, challenges in the methodology (e.g., gas transfer velocity estimation) were identified and discussed.