EFFECTS OF METALS ON STREAM MACROINVERTEBRATE ASSEMBLAGES FROM DIFFERENT ALTITUDES

We tested the hypothesis that macroinvertebrate assemblages from strea ms of different size (third- and fourth-order) and altitude (2545 and 2320 m above sea level) varied in their sensitivity to metals. We expo sed macroinvertebrates collected from these sites to metals in stream microcosms and compared their responses to those measured at reference and metal-polluted sites at different altitudes. Densities of macroin vertebrate assemblages from the small, high-altitude stream were reduc ed more in metal-dosed microcosms than those from the large, low-altit ude 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 larg er, lower-altitude streams. There are a number of factors that may con tribute to the variation in metal sensitivity we observed, such as dif ferences in abiotic factors or abundance of sensitive species between high- and low-altitude streams. In addition, differences in insect bod y size or phenology may have contributed to some of the observed varia tion in response. Since invertebrate assemblages from high-altitude si tes were 12-85% more sensitive to metals, we suggest that metal criter ia incorporate factors to allow for such differences to protect aquati c life.