Differential Selenium Uptake by Periphyton in Boreal Lake Ecosystems

Selenium (Se) is a naturally occurring trace element with a narrow margin between essentiality and toxicity in many organisms. Selenium is a contaminant of concern in the boreal forest region of North America because certain anthropogenic activities increase the loading of Se into cold-water aquatic ecosystems, which can have adverse effects on higher trophic levels such as fish, amphibians, and birds. Selenium is rapidly and efficiently assimilated from the water column into organisms at the base of the food web and transferred to higher trophic levels through dietary pathways. This initial step of aqueous Se uptake by organisms at the base of the food web is the greatest step in Se assimilation into aquatic food webs and has much uncertainty surrounding it. Complex assemblages of algae, bacteria, fungi, and detritus that exist at the sediment-water interface, also known as periphyton, play a key role in Se incorporation and biotransformation to more harmful organic forms and in energy cycling in aquatic systems. There are significant site-specific differences that exist in Se enrichment into aquatic food webs by organisms at the base of the food web, which makes predicting the ecotoxicological effects of elevated Se loading uncertain, varying 102 to 106-fold among different systems. Most field studies focused on the ecological risk assessment of Se have been conducted in warm-water systems and more research is needed regarding the effects of increased Se loading in cold freshwater ecosystems, including how certain water quality variables influence the incorporation of Se into food webs by organisms like periphyton. Additionally, boreal lakes specifically can be at a greater risk to Se toxicity at elevated levels due to the generally low presence of buffering ions like sulfate and phosphate which are known to interfere with Se uptake by various organisms. The goals of my research were to further address these research gaps to better understand the biodynamics of Se assimilation by organisms at the base of cold freshwater food webs. Specifically, an experiment was performed examining the bioaccumulation of low environmentally relevant concentrations of Se as selenite reflecting the current Se guidelines in naturally grown periphyton from multiple boreal lakes. The Se exposure concentrations used were 0.5, 1, 2, 4 μg/L, corresponding to the current freshwater lentic Se guidelines of 1 μg/L in Canada, 1.5 μg/L in the United States, and 2 μg/L in British Columbia. The results of the research revealed that periphyton rapidly and variably accumulated Se at low aqueous Se concentrations in a concentration-dependent manner. A range of periphyton tissue Se concentrations of 8.0 – 24.9 μg/g dm was seen in the 1 – 2 μg/L treatments surrounding the current freshwater Se guidelines, reaching 30.9 – 50.2 μg/g dm in the highest treatments in certain boreal lake systems. Previous studies have reported adverse effects in invertebrates fed periphyton at similar Se concentrations, suggesting that systems exposed to low levels of Se could experience adverse effects in certain higher trophic level populations. Differential uptake of Se into periphyton among the five studied lakes was also observed, where periphyton from mesotrophic lakes generally accumulated more Se than periphyton from oligotrophic lakes. The differences in Se uptake were likely explained by periphyton community composition and water chemistry differences, however significant correlations between these variables were observed. Higher proportions of the specific algal phylum known as the charophytes in periphyton grown in more oligotrophic lakes corresponded to decreased periphyton Se uptake, as well as in the presence of water with higher dissolved inorganic carbon content. Increased proportions of another algal phyla known as the bacillariophytes or diatoms in periphyton from more mesotrophic lakes corresponded to increased periphyton Se uptake, as well as in the presence of higher total dissolved phosphorus content. The trends demonstrated by different water chemistry and periphyton community variables in this experiment among multiple boreal lakes could serve as representative factors to consider when assessing potential risks of Se toxicity in different lentic systems. The results of this research provide further insight on the biodynamics of Se assimilation at the base of boreal lake food webs at environmentally relevant concentrations, which can potentially inform Se ecological risk assessments in cold, freshwater ecosystems in North America