Variation in metal concentrations across a large contamination gradient is reflected in stream but not linked riparian food webs.

Aquatic insects link food web dynamics across freshwater-terrestrial boundaries and subsidize terrestrial consumer populations. Contaminants that accumulate in larval aquatic insects and are retained across metamorphosis can increase dietary exposure for riparian insectivores. To better understand potential exposure of terrestrial insectivores to aquatically-derived trace metals, metal concentrations in water and tissues were analyzed from different components of streams and riparian food webs across a large (2-3 orders of magnitude) metal gradient (e.g., Zn, Cu, Cd, Pb) in the Rocky Mountains (USA). Our research indicates that the trace metal concentration gradient present among streams was lost during metamorphosis of aquatic larval insects into terrestrially flying adults, decoupling terrestrial exposures from aquatic concentrations. This pattern was caused by declines in 1) among-stream variation in trace metal concentrations, 2) relationships between metal concentrations in paired water and food web components, and 3) mean metal concentrations within aquatic food webs and across the aquatic-terrestrial boundary. Specifically, among-stream variation in trace metal concentrations was highest for water and aquatic vegetation, intermediate for aquatic insect larvae (30% lower than water) and lowest for adult aquatic insects and riparian spiders (65% lower). Metal concentrations in paired water and food web components ranged from highly related across the stream-metal gradient (slopes 1) for water and aquatic vegetation, to less related (slopes closer to 0) for aquatic vegetation and aquatic insect larvae, to unrelated (slopes ~0) for aquatic larval and adult insects. Finally, mean metal concentrations were highest in aquatic vegetation and lowest in adult aquatic insects emerging from streams (50% lower than aquatic vegetation). Our results indicate less efficient trophic transfer and higher metamorphic loss of trace metals from high metal streams (i.e., exposure-dependent transfer). For many trace metals, aquatic-terrestrial dietary transfer is unlikely to be an important source of exposure for terrestrial insectivores of adult aquatic insects.