Trophic Transfer of Inorganic Selenium Species through Representative Freshwater Food Chains

In recent decades, there has been growing interest in the toxicodynamics of the natural element selenium (Se) and its most commonly encountered chemical species, with a notable focus on freshwater ecosystems that have shown toxicological sensitivity. Exposure to elevated Se has uncovered an unusually narrow threshold between its biological necessity and toxicity, particularly among the most sensitive taxa, including oviparous vertebrates for which exposure to excess Se can lead to teratogenicity and reproductive failure. Uncertainties persist relating to the trophic transfer of dietary Se to higher organisms, originating from the bioconcentration of aqueous-borne chemical species and their biotransformation at the base of the food web. As a result, research has been ongoing to discern the influence of naturally variable biogeochemical characteristics on the enrichment, transformation, and trophic transfer of Se into the taxa of greatest concern. The purpose of this thesis was to contribute to the current understanding of Se trophic dynamics by evaluating Se trophic transfer to higher consumer species, originating from dissolved inorganic chemical species of concern. Diets of field-collected biofilms showing a diversity of community compositions were exposed to control conditions (0.3 µg Se L-1) and to graded concentrations of selenite (SeIV) and selenate (SeVI) (5 and 25 µg Se L-1), respectively, and fed to a primary consumer species common to Canadian freshwater ecosystems, Hyallela azteca. Generally, SeIV was transferred through trophic levels following a concentration-dependent relationship, with the greatest divergence among tissue Se residues occurring between the invertebrates exposed to different field-collected biofilm communities, with trophic transfer factors ranging from 0.15 – 0.51 among all SeIV exposed treatments. Final mean H. azteca tissue Se concentrations ranged from 2.7 – 6.5 µg Se g-1 dw in the low SeIV treatment and 7.3 – 18 µg Se g-1 dw in the high SeIV treatment. Uptake of SeVI into the invertebrate tissues was not significantly different from the control treatment. Differences in bioconcentration and trophic transfer between chemical species and treatments appeared to vary as a function of differences in primary producer biomass and ionic competition for uptake. For the purpose of further evaluating Se trophic transfer to a more sensitive oviparous species at a higher trophic level, an environmentally relevant freshwater food chain was created. A probable exposure scenario was simulated through aqueous SeIV exposure of green alga Stichococcus bacillarus that was fed to H. azteca, which then served as the diet for a common Canadian freshwater secondary consumer species, the fathead minnow (Pimephales promelas). Three H. azteca dietary treatments (1.6 µg Se g-1 dw [control], 6.9 µg Se g-1 dw, and 19 µg Se g-1 dw) were collected and fed to the fish species in a partial lifecycle reproductive assay. Final muscle, gonad, and liver Se concentrations showed concentration-dependent increases with greatest concentrations of 4.5 ± 1.1, 16 ± 1.8 and 17 ± 1.9 µg Se g-1 dw, respectively, in adult female fish in the high SeIV treatment. Reproduction was negatively affected by elevated Se exposure, and resulting incidence of fish fry deformities in reared F1 offspring was greatest in the low SeIV treatment (37 ± 3.0 %). Embryo Se concentrations reached up to 7.5 and 17 µg Se g-1 dw in the low and high SeIV treatments, respectively. This research showed how bioconcentration and trophic transfer of Se depended both on chemical species and primary producer community composition. Releases of Se into aquatic environments from anthropogenic activity will therefore lead to site-specific differences in Se movement along food chains and subsequent toxicity. These factors should be considerations for the future of Se management and research