EFFECTS OF HEAVY-METALS ON A MACROINVERTEBRATE ASSEMBLAGE FROM A ROCKY-MOUNTAIN STREAM IN EXPERIMENTAL MICROCOSMS

Natural assemblages of stream benthic macroinvertebrates were collecte d using artificial substrates from a Rocky Mountain stream and exposed for 10 d to a mixture of heavy metals (Cd, Cu, and Zn) in stream micr ocosms. Metal levels were 0, 1x, 5x, and 10x where x = 1.1, 12, and 11 0 mug/L Cd, Cu, and Zn, respectively. The 1x treatment was similar to chronic criteria values recommended by the US Environmental Protection Agency (EPA) for each metal and total metal levels measured in water at the Arkansas River, Colorado, a US EPA superfund site. Most ephemer opterans and plecopterans were sensitive to metals; however, some taxa within these groups were metal tolerant. Densities of Baetis tricauda tus (Ephemeroptera:Baetidae), Epeorus longimanus and Rhithrogena hagen i (Ephemeroptera:Hepatageniidae), and Drunella grandis and D. doddsi ( Ephemeroptera: Ephemerellidae) were reduced in the 1x treatment. The r esponse of D. grandis to metals was size-dependent with small larvae b eing more sensitive than large ones (p = 0.02). Chironomids were gener ally tolerant to metals. These data show that a metal mixture was extr emely toxic to stream macroinvertebrates from a Rocky Mountain stream. Our results were similar to field biomonitoring studies at the Arkans as River and Eagle River, Colorado, that examined the effects of metal s on stream macroinvertebrate communities. We suggest that multispecie s experiments using indigenous stream organisms be combined with field biomonitoring to rigorously define the biological effects of heavy me tals on lotic systems.