Zinc concentrations and isotopic signatures of an aquatic insect (mayfly, Baetis tricaudatus)

Insect metamorphosis often results in substantial chemical changes that can fractionate isotopes and alter contaminant concentrations. We exposed larval mayflies (Baetis tricaudatus) to an aqueous zinc gradient (3-340 g Zn/l) and measured the change in zinc tissue concentrations at different stages of metamorphosis. We also measured changes in stable isotopes (δ15N and δ13C) in unexposed B. tricaudatus. Zinc concentrations in larvae were positively related to aqueous zinc, increasing 9-fold across the exposure gradient. Zinc concentrations in adults were also positively related to aqueous concentrations, but were 7-fold lower than larvae. However, this relationship varied according to adult substage (subimago vs imago) and sex. Tissue concentrations in female imagoes were not related to exposure concentrations, but the converse was true for all other stage by sex combinations. Metamorphosis also altered isotopic ratios, increasing δ15N, but not δ13C. Thus, the main effects of metamorphosis on insect chemistry were large declines in zinc concentrations coupled with enriched δ15N signatures. For zinc, this change is largely consistent across the aqueous exposure gradient. However, the differences among sexes and stages suggest that caution is warranted when using isotopes or metal concentrations measured in one insect stage (e.g., larvae) to assess risk to wildlife that feed on subsequent life stages (e.g., adults).