| I tested the hypothesis that stream invertebrate communities from streams of different size and altitude varied in their response to metals. To evaluate how metals affected biotic interactions, I manipulated invertebrate density, predation intensity, and metals in stream microcosms. Using stream invertebrate communities, I also designed an experiment and field survey to identify reliable bioindicators of metal contamination in western streams.; Results from microcosm experiments and field studies showed that benthic invertebrate populations from high-altitude streams were more sensitive to the effects of metals than invertebrate populations from low-altitude streams. For example, densities of most groups of aquatic insects (e.g., Ephemeroptera, Plecoptera, Trichoptera) were reduced more at high-altitude metal contaminated streams than those same groups at low-altitude streams.; To determine if body size contributed to the variation in sensitivity of insects to metals, size measurements were made on species collected from a high-altitude stream and from a low-altitude stream, and from control and metal-treated stream microcosms. Most species were smaller at the high-altitude stream than the same species from the low-altitude stream. In addition, insect body size was larger in metal-dosed microcosms than in controls indicating survival was less for small individuals. Thus the variation in response of macroinvertebrates between different altitude streams observed in earlier studies may be due to differences in body size.; The effects of low levels of metals (half the chronic levels of Cd, Cu, and Zn) on some species varied in relation to invertebrate density (low and high density) and invertebrate predation (no predators added and predators added). The effects of an invertebrate predator (Hesperoperla pacifica) on Hydropsyche sp. was significantly greater in metal-dosed microcosms than in controls.; Experiments in stream microcosms showed that heptageniid mayflies were the most sensitive group to a metal mixture. Heptageniid mayflies were also consistently less abundant downstream of sources of metal contamination in the Arkansas and Eagle rivers. Therefore, results from this experiment and field survey suggest that changes in abundance of heptageniid mayflies may provide a reliable indicator of metal contamination in western streams. |
