Mercury dynamics in the anadromous Arctic char (Salvelinus alpinus) and food webs of inner Frobisher Bay, Nunavut

Mercury (Hg) is a ubiquitous toxic metal that bioaccumulates in organisms and biomagnifies in food webs. Evaluating Hg bioaccumulation and biomagnification in Arctic marine ecosystems is critical for understanding Hg dynamics and estimating exposure to fish and wildlife consumed by humans. In this thesis, I investigated inter-individual variability in biological factors affecting Hg accumulation in anadromous Arctic char (Salvelinus alpinus), as well as food web structure and Hg biomagnification in the benthic, pelagic, and benthopelagic marine food webs of inner Frobisher Bay, in Nunavut, Canada. Stable isotope ratios of carbon (δ13C) and nitrogen (δ15N), as well as concentrations of Hg were measured in 119 anadromous Arctic char and 62 taxa of fish, invertebrates, and zooplankton that had been sampled in inner Frobisher Bay in 2018 and 2019. Mean concentrations of total mercury (THg) and relationships between THg and biological variables known to influence Hg concentrations in fish (e.g., fork length, weight, age, growth rate, trophic position, carbon source, Fulton’s condition factor, gonadosomatic index, and hepatosomatic index) were compared between immature and mature Arctic char. The immature Arctic char exhibited greater inter-individual variability in factors affecting THg accumulation compared to the mature Arctic char, and δ15N (i.e., marine prey reliance) was a strong predictor of THg concentrations for all individuals. Biomagnification of methyl mercury (MeHg) in each food web was quantified with Trophic Magnification Slopes (TMS; calculated as the slope of the linear regression of log10 MeHg concentrations and δ15N values) and Trophic Magnification Factors (TMF; calculated as the antilog of the regression slope). Rates of MeHg biomagnification were highest in the benthopelagic food web (TMS = 0.201; TMF = 1.59), followed by the pelagic food web (TMS = 0.183; TMF = 1.52), and lastly the benthic food web (TMS = 0.079; TMF = 1.20), and δ15N explained 88%, 79%, and 9% of variation in MeHg concentrations in each food web, respectively. Results from food web structure analyses indicated that the benthic food web had the greatest trophic diversity, trophic redundancy, and largest isotopic niche area of all food webs studied. The results indicated that greater food web complexity reduces rates of MeHg biomagnification. The research presented in this thesis demonstrated that there are a variety of biological and ecological factors that influence Hg bioaccumulation and biomagnification in Arctic marine organisms and food webs. The acquisition of comprehensive Hg and food web structure data, in association with Canada’s Coastal Environmental Baseline Program, broadened the scope of understanding of Hg dynamics in an Arctic marine ecosystem, and provides important baseline information pertinent for future spatial and temporal comparisons and evaluations of effects of resource development and climate change