Benthic Invertebrate Community Responses to Heavy Metals in the Upper Arkansas River Basin, Colorado

Abstract

This research examined effects of heavy metals on benthic macroinvertebrate communities in the upper Arkansas River, Colorado. Benthic macroinvertebrates were collected seasonally (fall, spring, and summer) over a 2-yr period upstream and downstream from two sources of metals, Leadville Mine Drainage Tunnel and California Gulch. Community responses to metals observed in this study were similar to those reported previously for other streams. Benthic communities at reference stations were dominated by mayflies (Ephemeroptera), which generally constituted >50% of the total individuals collected. In contrast, highly polluted and moderately polluted sites were dominated by Orthocladiinae chironomids and caddisflies (Trichoptera); respectively. The spatial distribution of specific taxa at the Arkansas River was influenced by several factors, including their tolerance for heavy metals, their recolonization ability, and natural environmental conditions. Distribution patterns of some groups (e.g., mayflies, stoneflies, beetles, and Orthocladiinae chironomids) appeared to be directly affected by heavy metals, whereas the distribution of other groups (e.g., caddisflies and black flies) was probably more influenced by factors such as food availability. Macroinvertebrate community responses to metals were complicated by natural seasonal and longitudinal changes in community composition. The greatest effects on community composition and abundance of sensitive taxa were observed during spring, when metal levels were highest. Some longitudinal changes in species richness and abundance of dominant taxa were independent of heavy metals. Determination of the effects of heavy metals on benthic communities requires careful evaluation of natural changes in community composition.