Effects of Metals on Stream Macroinvertebrate Assemblages from Different Altitudes

We tested the hypothesis that macroinvertebrate assemblages from streams of different size (third‐ and fourth‐order) and altitude (2545 and 2320 m above sea level) varied in their sensitivity to metals. We exposed macroinvertebrates collected from these sites to metals in streams microcosms and compared their responses to those measured at reference and metal‐polluted sites at different altitudes. Densities of macroinvertebrate assemblages from the small, high‐altitude stream were reduced more in metal‐dosed microcosms than those from the large, low‐altitude stream. Results from the field survey were similar to experimental results, as densities of organisms from metal‐polluted sites in small, high‐altitude streams were lower than those of the same taxa in larger, lower‐altitude streams. There are a number of factors that may contribute to the variation in metal sensitivity we observed, such as difference in abiotic factors or abundance of sensitive species between high‐ and low‐altitude streams. In addition, differences in insect body size or phenology may have contributed to some of the observed variation in response. Since invertebrate assemblages from high‐altitude sites were 12‐84% more sensitive to metals, we suggest that metal criteria incorporate factors to allow for such differences to protect aquatic life.