Laboratory
assessments of aqueous metal toxicity generally demonstrate
aquatic insects tolerate relatively high concentrations of metals
in aqueous exposures; however, mesocosm experiments and field biomonitoring
often indicate effects at relatively low metal concentrations. One
hypothesis proposed to reconcile this discrepancy is an increased
sensitivity of smaller size classes of organisms. We exposed field
colonized benthic communities to aqueous metals in a series of mesocosm
experiments. In addition, a novel single-species test system was used
to expose first instar, mid-instar, and late instar mayflies (Ephemeroptera, Baetis tricaudatus) to Zn. These experimental approaches
tested the hypothesis that small invertebrate size classes are more
sensitive than large, mature size classes. Mesocosm results demonstrated
strong size-dependent responses of aquatic insects to metals. Smaller
organisms generally displayed greater mortality than large, mature
individuals, and models were improved when size was included as a
predictor of mortality. Size-dependent responses of Baetis spp. occurred in mesocosm experiments and in our single-species
test system. The median lethal concentration (LC50) for
early instar B. tricaudatus was less than 6% of the
previously reported LC50 for late instars. Together, these
results suggest that aquatic insect body size is an important predictor
of susceptibility to aqueous metals. Toxicity models that account
for insect phenology by integrating the natural size progression of
organisms have the potential to improve accuracy in predicting effects
of metals in the field.
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