In this thesis mercury concentrations in biota of the upper South Saskatchewan River Basin are assessed in three non-industrialized systems. Mercury levels in large sport fish (northern pike, walleye, lake trout) frequently exceeded the consumption limit of 0.5ppm. Goldeye and mooneye of the Oldman River and lake whitefish of Waterton Lakes were below 0.5ppm total mercury. Agricultural and urban effluents constituted no sources of significant mercury loadings to the Oldman River. A doubling of mercury biomagnification factors between longnose dace and their food suggests bioenergetic heterogeneity of these fish along the river gradient. Basin-specific mercury levels were detected for the upper and middle basins in Waterton Lakes, and are associated with food web characteristics, and fish bioenergetics. High mercury levels in a new reservoir were in part attributed to increased loadings from flooded soils, as is commonly observed, but also to bioenergetic constraints and growth inefficiency as a result of non-piscivory of this population.

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.