Size-dependent response of macroinvertebrates to metals in experimental streams

Our previous research has shown that the effects of metals on stream benthic invertebrate populations and communities can vary within and between locations. With this in mind, we examined whether invertebrate body size could explain some of the variation in metal sensitivity within a species. Benthic macroinvertebrates from a pristine Rocky Mountain foothills' stream were collected using artificial substrates and exposed to a mixture of Cd, Cu, and Zn in stream microcosms for 10 d at their respective Colorado chronic criterion levels (4.0, 5.0, and 50 μg/L). The effects of metals on the ephemeropterans Baetis tricaudatus (Baetidae), Ephemerella infrequens (Ephemerellidae), and Rhithrogena hageni (Heptageniidae) and the plecopteran Pteronarcella badia (Pteronarcyidae) were size dependent, as there was an inverse relationship between body size and survivorship. These results may have important implications for setting water-quality criteria for metals and for using benthic invertebrates in biological assessments.