Toxicity of Selenium and Total Dissolved Solids at the Base of Aquatic Food Webs: Assessing the Causes of Biological Impairment Resulting from Mountaintop Removal-Valley Fill Coal Mining

Streams impacted by mountaintop removal -- valley fill (MTR-VF) coal mining in the southern Appalachians display severely degraded biological communities. These communities are typically dominated by aquatic insects (particularly the orders Ephemeroptera, Plecoptera, and Trichoptera (EPT)) in terms of species abundance and diversity. The loss of aquatic insect taxa from impacted streams has been correlated with changes in water chemistry, specifically elevated selenium (Se) and total dissolved solids (TDS) concentrations. Selenium is a dietary toxicant with well documented effects in egg-laying vertebrates (i.e., fish and birds), however, very little research has been conducted on the effects of dietary Se exposure to aquatic insects. Further, we know very little about the effects of elevated TDS (and the myriad assortment of ionic signatures) to EPT taxa. Here, we investigated the toxicity of dietary Se and elevated TDS exposure using a laboratory microcosm in order to determine the proximal chemical stressor to EPT taxa in MTR-VF impacted streams. Our microcosm included naturally grown periphyton biofilms and the parthenogenetic mayfly Centroptilum triangulifer as a representative, laboratory aquatic insect in full life-cycle experiments. We found that dissolved selenite (SeO3) was readily absorbed by periphyton with bioconcentration factors ranging from 800 -- 3000-fold after 7-9 days of exposure to a range of environmentally relevant dissolved Se concentrations (∼2 -- 20 microg L-1). Mayflies grown to adulthood on these diets displayed trophic transfer factors ranging from 2.5 -- 3.2-fold when available food rations were limited, however when food rations were doubled mayfly trophic transfer decreased (0.9 -- 1.4-fold) resulting in lower tissue Se concentrations. Mayflies raised on the limited ration diet suffered significant, dose responsive, adverse effects of Se exposure including reduced survival (≥11 microg g-1; dietary Se concentration, dry weight basis), reduced secondary production (≥11 microg g-1), and reduced fecundity (≥4 microg g-1), whereas those fed a greater ration performed similar to controls across all exposure levels (≥40 microg g-1). Importantly, Se toxicity and food web dynamics are dominated by the incorporation of dissolved inorganic Se by primary producers. We found that dissolved selenite (SeO3) was much more readily bioconcentrated by periphyton than selenate (SeO4, the dominant Se species in MTR-VF impacted streams), however the extent of bioaccumulation over an eight day exposure period was not significantly different due to microbially-mediated reduction of dissolved SeO4 into SeO3. Further, both SeO3 and SeO4 exposed periphyton contained nearly identical Se speciation (dominated by organo-selenide) and were equally toxic to C. triangulifer in life cycle exposures. Next, we examined the toxicity of elevated TDS to C. triangulifer in life cycle exposures to laboratory reconstituted waters generated to mimic the ionic chemistry of streams impacted by valley filling. Waters with MTR-VF ionic signatures (elevated Ca2+, Mg2+, SO4 2-, and HCO3- concentrations) significantly reduced C. triangulifer survival and secondary production at ≥33% strength dilution (∼950 microS cm-1) and were completely lethal at 100% strength dilution (∼1800 microS cm -1). In contrast, waters with similar conductivity but different ionic compositions (elevated Na+, HCO3-, and Cl- concentrations) were not toxic. Overall, it required dietary Se concentrations higher than what is normally observed in valley fill impacted streams (∼2-3 microg g-1) in conjunction with food limitation conditions to produce adverse effects on C. triangulifer performance, whereas elevated TDS was overtly toxic even when diluted and dietary rations were not limited. As such, it appears elevated TDS (with specific MTR-VF ionic signatures) is likely the proximal chemical stressor driving the degradation of aquatic insect communities in MTR-VF impacted streams, however dietary Se may play a role in adversely affecting aquatic insect abundance and diversity.