Variation in riparian consumer diet composition and differential bioaccumulation by prey influence the risk of exposure to elements from a recently remediated fly ash spill.

Emerging aquatic insects play a key role in transporting aquatic nutrients and contaminants to riparian consumers. However, little is known about how within- and between-year variation in the diet or patterns of element bioaccumulation in emerging insect taxa may influence the risk of exposure to wildlife. During 2 breeding seasons, the composition of the diet of tree swallows (Tachycineta bicolor) was examined at several colonies that were exposed to element contamination from a remediated coal fly ash spill to determine how variation in the diet influenced the risk of exposure to elements. The proportion of the diet that consisted of insects with an aquatic larval stage was positively related to concentrations of As, Fe, Se, and Tl in the samples. The proportion of the diet that consisted of Chironomidae (midges) was positively related to exposure to these elements at most colonies within and between years. Ephemeroptera (mayflies) contained higher concentrations of Se than midges, including 17 samples with concentrations of Se above 5 µg/g dry mass, the threshold of toxicological concern for birds. This was even the case at colonies several kilometers downstream from the spill. The results indicate that greater consideration should be given to the pattern of element bioaccumulation among different prey taxa and their relative importance in the diet to better assess the risk of contaminant exposure.