Heavy metals structure benthic communities in Colorado mountain streams

The development of held sampling designs that employ multiple reference and polluted sites has been proposed as an alternative to the traditional upst ream vs, downstream approach used in most biomonitoring studies. Spatially extensive monitoring programs can characterize ecological conditions within an ecoregion and provide the necessary background information to evaluate future changes in water quality. We measured physicochemical characteristic s, heavy-metal concentrations, and benthic macroinvertebrate community stru cture at 95 sites in the Southern Rocky Mountain ecoregion in Colorado, USA . Most sites (82%) were selected using a systematic, randomized sampling de sign. Each site was placed into one of four metal categories (background, l ow, medium, and high metals), based on the cumulative criterion unit (CCU), which we defined as the ratio of the instream metal concentration to the U .S. Environmental Protection Agency criterion concentration, summed for all metals measured. A CCU of 1.0 represents a conservative estimate of: the t otal metal concentration that, when exceeded, is likely td cause harm to aq uatic organisms. Although the CCU was less than 2.0 at most (66.3%) of the sites, values exceeded 10.0 at 13 highly polluted stations. Differences amo ng metal categories were highly significant for most measures of macroinver tebrate abundance and all measures of species richness. We observed the gre atest effects on several species of heptageniid mayflies (Ephemeroptera: He ptageniidae), which were highly sensitive to heavy metals and were reduced by >75% at moderately polluted stations. The influence of taxonomic aggrega tion on responses to metals was also greatest for mayflies. In general, tot al abundance of mayflies and abundance of heptageniids were better indicato rs of metal pollution than abundance of dominant mayfly taxa. We used stepw ise multiple-regression analyses to investigate the relationship between be nthic community measures and physicochemical characteristics at the 78 rand omly selected sites. Heavy-metal concentration was the most important predi ctor of benthic community structure at these sites. Because of the ubiquito us distribution of heavy-metal pollution in the Southern Rocky Mountain eco region, we conclude that potential effects of heavy metals should be consid ered when investigating large-scale spatial patterns of benthic macroinvert ebrate communities in Colorado's mountain streams.