Variation in Zn, C, and N isotope ratios in three stream insects

Total Zn concentrations and Zn isotope ratios were measured, using multicollector inductively coupled plasma (ICP)-mass spectrometry (MS), in three species of aquatic insects collected from a stream in Peterborough, Ontario, Canada. Total Zn levels averaged 193 +/- 88 mu g/g dry weight (dw) in water striders (Heteroptera: Gerridae, Aquarius remigis) and were significantly higher than the concentrations measured in stonefly nymphs (Plecoptera: Perlidae, Acroneuria abnormis) and caddisfly larvae (Trichoptera: Limnephilidae, Pycnopsyche guttifer), i.e., 136 +/- 34 mu g/g dw and 125 +/- 26 mu g/g dw, respectively. Average delta values for Zn-66/Zn-64 in the water striders were approximately 0.7 parts per thousand lighter (-1.2 parts per thousand +/- 1.0 parts per thousand) and were significantly different than those measured for stoneflies (-0.45 parts per thousand +/- 0.62 parts per thousand) and caddisflies (-0.51 parts per thousand +/- 0.54 parts per thousand). Nitrogen isotope ratios were significantly different (P < 0.05) among the three species suggesting differences in trophic positioning. Similar to the Zn isotope ratios, delta C-13 values for the water striders (-28.61 parts per thousand +/- 0.98 parts per thousand) were significantly different than those of the stoneflies and caddisflies, i.e., -30.75 parts per thousand +/- 1.33 parts per thousand and -30.68 parts per thousand +/- 1.01 parts per thousand, respectively. The data suggest that the differences observed in Zn ratios relate to food source for these insects. Similar to their carbon sources, Zn in water striders appears to be primarily of terrestrial origin, and of aquatic origin for the other two species.