The role of metamorphosis in regulating metal flux from aquatic to terrestrial food webs

Stream metal pollution is a global phenomenon that negatively affects aquatic insect communities. Because most aquatic insects undergo metamorphosis to become winged terrestrial adults, stream metal pollution may also have effects that extend beyond the aquatic habitat boundary. This could occur if, for example, stream metals reduce emerging insect biomass, or if insects transfer metals from aquatic to terrestrial systems upon emergence. We present the results of field surveys and lab experiments that reveal the following key insights: 1) stream metals reduce insect emergence by a combination of enhanced larval mortality and prevention of larva-to-adult metamorphosis and 2) metal concentrations in adult insects are much lower than concentrations in larval insects. These results suggest the apparent paradox that emergence is a more sensitive indicator of stream metal pollution than larval densities, even though emerged adults carry lower metal concentrations than larvae.