Selenium uptake, trophic transfer, and toxicity in boreal lake ecosystems

Selenium (Se) is emerging as a contaminant of concern, particularly in mine and agriculture influenced areas of Canada and the United States. Due to the high site- and species-specificity of Se bioaccumulation, site-specific biodynamic modelling is the accepted approach to predict Se accumulation in aquatic systems, and fish tissue-based guidelines for the protection of aquatic life are preferred over water quality guidelines. To date, few studies have assessed Se bioaccumulation in cold-water systems, such as Canadian boreal forest lakes. These lakes, which comprise a large proportion of Canada’s freshwater, are also associated with several anthropogenic activities that can contribute to the excess release of Se to aquatic systems. Further, concern about excess Se has generally focused on the teratogenic effects on egg-laying vertebrates, with relatively little attention paid to aquatic invertebrates. The goal of this research was to improve the current understanding of Se biodynamics and toxicity in Canadian boreal lakes, with the ultimate goal of informing future ecological risk assessments of Se in Canada. Limnocorrals in two lakes located at the International Institute for Sustainable Development – Experimental Lakes Area (IISD-ELA) were used to conduct small scale whole-ecosystem experiments to study bioaccumulation and toxicity of Se. In 2017, Se was added as selenite to six limnocorrals (two treatment groups, each in triplicate) to achieve mean measured water Se concentrations of 1.0 and 8.9 µg/L and three limnocorrals were untreated controls (mean measured Se = 0.12 µg/L). Distribution coefficients (kds) ranged from 7,772 L/kg dry mass (dm) in the 8.9 µg/L treatment to 23,495 L/kg dm in the 0.12 µg/L treatment, and trophic transfer factors (TTFs) for benthic macroinvertebrates ranged from 0.49 for Gammaridae to 2.3 for Chironomidae. Selenium accumulated in fathead minnow ovaries to concentrations near or above the current British Columbia Ministry of the Environment and US Environmental Protection Agency criteria (11 and 15.1 µg/g dm for fish ovary/egg, respectively) in the 1.0 and 8.9 µg/L treatments. Chironomidae and Gammaridae densities and biomass were significantly lower in the 8.9 µg/L Se treatment relative to the 1.0 µg/L Se treatment and the control, and invertebrate diversity significantly declined in the 1.0 µg/L and 8.9 µg/L Se treatments relative to the control (0.12 µg/L Se group). In 2018, a gradient approach was used in which three limnocorrals were controls (0.08-0.09 µg Se/L), and mean measured concentrations in Se-treated limnocorrals were 0.4, 0.8, 1.6, 3.4, 5.6, and 7.9 µg/L. Total Se (TSe) bioaccumulation by organisms was generally non-linear over the gradient of water Se concentrations used, and taxonomic differences in TSe accumulation by algae (phytoplankton < periphyton) and invertebrates (Heptageniidae = Chironomidae > zooplankton) were observed. Zooplankton and benthic macroinvertebrate communities shifted according to Se exposure. Cladocera and Heptageniidae biomass and density decreased with increasing Se treatment. Overall, this work contributes to the understanding of Se trophic dynamics and toxicity in cold-water Canadian boreal lakes. These data showed how Se bioaccumulation changes with increasing aqueous exposure and with different taxa, and provided field-derived kinetic models for the saturable uptake of Se by algae and invertebrates. The levels of Se observed in algae, invertebrates and fish tissues suggest that, depending upon aqueous Se speciation, such exposures have the potential to cause Se accumulation in fish to levels of concern in cold-water, boreal lake systems. Further, these studies demonstrated that Se can have impacts on aquatic invertebrates at environmentally relevant exposure levels, and that future ecological risk assessments should consider the impacts of Se on both vertebrates and invertebrates.

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