THE INFLUENCE OF ELEVATION ON BENTHIC COMMUNITY RESPONSES TO HEAVY-METALS IN ROCKY-MOUNTAIN STREAMS

We examined the influence of elevation on benthic macroinvertebrate co mmunity responses to heavy metals at 33 sites in six Colorado streams where zinc (Zn) concentration ranged from 2 to 691 mu g/L and elevatio n ranged from 2200 to 3500 m above sea level. We assigned the 33 sites to one of four categories, on the basis of the total Zn concentration and the hardness-based criterion for Zn. Number of taxa, abundance, a nd species richness of mayflies (Ephemeroptera), and abundance of most mayfly and stonefly (Plecoptera) taxa were significantly reduced at s ites where Zn concentration exceeded the hardness-based criterion. Alt hough most measures of benthic community structure were not affected a t sites where the Zn concentration was less than the criterion, the sp ecies richness of mayflies and the abundance of Heptageniidae, Drunell a, and Rhyacophila were significantly reduced. Results of stepwise mul tiple regression analysis and canonical discriminant analysis showed t hat the effects of Zn on benthic community structure were influenced b y elevation. These natural changes in benthic communities confound res ults of biomonitoring studies and must be considered when assessing he avy metal impacts. Total abundance and species richness of mayflies, a nd abundance of Heptageniidae were the most reliable indicators of hea vy metal impacts in our southern Rocky Mountain streams.