Gedig, D. P. | Search bioRxiv

bioRxiv2024

Aquatic metabolism throughout 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 (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.

ecology↗

bioRxiv2022

Mercury contamination of an introduced generalist fish of intermediate trophic level

Mercury contamination is a global issue because mercury concentrations in aquatic systems are influenced by both natural and anthropogenic pathways, including the burning of fossil fuels and flooding during hydroelectric development. Mercury biomagnifies in aquatic ecosystems, leading to higher concentrations in piscivore fishes than those at lower trophic levels. Here, liver and muscle total mercury (THg) concentrations in black crappie Pomoxis nigromaculatus from three lakes in southeastern Manitoba, Canada were related to age, morphology and physiological traits to better understand the dynamics of mercury accumulation in an introduced generalist fish species. Black crappie liver and muscle samples from Big Whiteshell Lake (relatively large lake, 17.5 km2; n=30), Caddy Lake (small lake surrounded by wetlands, 3.1 km2; n=42) and Lac du Bonnet (river widening influenced by hydroelectric dams, 84.0 km2; n=29) were analyzed for THg content. These THg concentrations were then compared to black crappie mercury concentrations in other Canadian water bodies to assess within species relative contamination levels, as well as to mercury concentrations in other freshwater fishes to examine biomagnification. Age and size had strong positive correlations (r>0.60) with muscle mercury concentrations. No evidence of acute point source contamination was found in the study area when compared to black crappie muscle mercury concentrations in other water bodies, and tissue THg concentration was not correlated with a reduction in gonadosomatic index (GSI) or hepatosomatic index (HSI). Analysis of liver THg in addition to muscle THg revealed the possible impacts of seasonal and ontogenetic differences in diet on exposure. Furthermore, THg analysis of liver and muscle tissue showed how generalist foraging techniques of black crappie may curb the progressively greater mercury exposure and resultant physiological consequences expected from ontogenetic diet shifts from invertebrates to fishes. Although there appeared to be temporally varied levels of mercury exposure (i.e., liver THg) by sex, there was no sex effect observed in long-term accumulation in the muscle. Flood risk is believed to be a key driver of differences in black crappie THg concentrations between lakes in the region. Black crappie bioaccumulated less mercury at age than primary piscivore species in the region. These results will help foster a better understanding of mercury biomagnification in boreal shield lakes within a region impacted by legacy mercury.

pharmacology and toxicology↗