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 µg 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).

昆虫变态发育往往会引发显著的化学变化，这类变化可导致同位素分馏并改变污染物浓度。本研究将幼虫期的蜉蝣（Baetis tricaudatus）暴露于梯度浓度的锌水溶液（3~340 μg Zn/L）中，并测定其变态发育不同阶段体内的锌组织浓度变化。此外，本研究还测定了未暴露于锌环境的B. tricaudatus体内的稳定同位素（δ15N与δ13C）变化情况。幼虫体内的锌浓度与水溶液中的锌浓度呈正相关关系，在暴露梯度范围内其浓度最高可提升至初始的9倍。成虫体内的锌浓度同样与水溶液锌浓度呈正相关，但较幼虫低7倍。不过，这一相关关系会因成虫的亚发育阶段（亚成虫与成虫）以及性别而异。雌性成虫的体内锌浓度与暴露浓度并无关联，而其余所有性别-发育阶段组合均呈现相反的结果。变态发育还会改变同位素比值：δ15N会升高，而δ13C则无显著变化。因此，变态发育对昆虫化学组成的主要影响为锌浓度大幅下降，同时伴随δ15N信号的富集。就锌而言，这一变化在整个水溶液暴露梯度范围内基本保持一致。不过，不同性别与发育阶段间的差异提示：若利用某一昆虫发育阶段（如幼虫）测得的同位素或金属浓度，来评估以后续生命阶段（如成虫）为食的野生动物的暴露风险时，需谨慎行事。