STRUCTURAL RESPONSES OF BENTHIC MACROINVERTEBRATE COMMUNITIES FROM DIFFERENT STREAM ORDERS TO ZINC

It is well established that benthic invertebrate community structure and function shift in a predictable fashion along longitudinal stream gradients as a result of variation in environmental conditions. Our research is concerned with experimentally testing whether this shift in community structure influences the response of benthic macroinvertebrates to heavy metals. Using artificial streams, we compared effects of Zn on natural assemblages of benthic macroinvertebrate communities collected from Little Beaver Creek (LBC; a third-order stream) and the Big South Fork of the Cache la Poudre River (SFP; a fourth-order Stream) of the Cache la Poudre, Colorado, catchment. Organisms collected from LBC and SFP were exposed to 0 or 130 mug/L Zn in indoor experimental streams for 7 d. In general, similar taxa were found at both sites, but densities were generally higher at SFP than at LBC, We observed significant effects at the community and population level as a result of Zn, stream order, and the interaction between Zn and stream order. Specifically, mayflies from both sites were sensitive to Zn, but the magnitude of the response varied between sites. Our results indicate that benthic macroinvertebrate communities from different stream orders may vary in sensitivity to Zn.