Elevated mercury concentrations in riparian predators unaccounted for by trophic magnification

Riparian predators integrate both nutrients and contaminants from neighboring aquatic systems through their predation of emerging aquatic insects. The reliance of some riparian taxa on aquatic subsidies has led to them to being utilized as biosentinels of aquatic contamination, often justified through stable isotope studies (δ¹³C, δ¹⁵N) and trophic magnification models. However, evidence suggests that both isotopic signatures and contaminant burdens can be altered during insect metamorphosis, which could propagate to riparian predators. Here, we measured total mercury and δ¹⁵N in riparian and aquatic organisms in the headwater streams of southern Appalachia to compare biomagnification in both systems. We found that biomagnification rates were similar, but that mercury concentrations in riparian organism were approximately six times higher than neighboring aquatic organisms. We propose possible ecological mechanisms, occurring separately or simultaneously, that might explain these surprising observations: (1) metamorphosis bioamplifies mercury, (2) metamorphosis enriches δ¹⁵N values, and/or (3) the fractionation of δ¹⁵N that occurs during predation is less for the observed riparian predators than for the aquatic organisms. Further study is needed to elucidate the exact mechanism driving elevated mercury concentrations in riparian predators.