The Aquatic Insects, with Emphasis on Trichoptera, of a Colorado Stream Affected by Coal Strip-Mine Drainage

Abstract

Benthic sampling was conducted year-round over a 2 year period (July 1975-June 1977) on Trout Creek, a mountain stream in northwestern Colorado, to assess the effects of coal mine drainage on the stream insect community. Samples were taken monthly from rubble riffles above and below drainage from unreclaimed mine spoils. Unexpectedly, aquatic insects exhibited similar mean density and biomass values at each site and accounted for at least 95% of the total benthic standing crop above and below the mine. The number of insect taxa was similar and diversity index values did not vary significantly between sites. However, the community structure of the two sites did differ. Certain taxa were most abundant at the upstream site (Prostoia besametsa andLepidostoma moneka), while others were more abundant at the downstream location (Agapetus boulderensis, Glossosoma ventrale, and Hydropsyche spp.). The greatest difference between the two sites was the differential importance of Trichoptera. At the upper site, caddisflies accounted for 39% of the benthic density due to the abundance of four families -Lepidostomatidae, Glossosomatidae, Brachycentridae, and Hydroptilidae). Trichopterans accounted for 60% of the benthic density below the mine, due primarily to the abundance of Hydropsychidae, and especially Glossosomatidae. This study points to the importance of community structure analysis in assessing effects of changes in environmental conditions in stream systems. The detrimental effects'of drainage from mine spoils on aquatic life have been well documented in the East, where acid mine drainage from high-sulfur coal strip-mine spoils has been a major pollutant of streams (see e.g. Parsons 1968, Roback and Richardson 1969, Koryak et al. 1972, Tomkiewicz and Dunson 1977). However, little is known of the effects of drainage from the low-sulfur coal strip-mines in the West (Canton and Ward 1978). Coal has been extracted from the Edna Mine, adjacent to Trout Creek in northwestern Colorado, for approximately 30 years. With the increased importance of western coal, the effects of mine drainage on aquatic life should be elucidated. Research was undertaken to determine the effects of drainage from the Edna Mine on the aquatic insects of Trout Creek. STUDY AREA. Trout Creek flows roughly south-north beginning in the Dunkley Flat-Tops area and entering the Yampa River west of Milner, Colorado. The two study sites were located on rubble riffles in a third order section of the stream. Site C2 (Fig. 1) was immediately above the Edna coal mine at an elevation of 2208 m. At this point the stream was relatively undisturbed as it left a narrow valley and entered a flood plain. The substratum was coarse rubble overlying sand and gravel. The creek was well shaded at this site with willows and alders. The lower site, C4, was 2 km downstream at an elevation of 2150 m. Site C4 was below drainage from unreclaimed mine spoils, where mining ceased approximately 20 years ago, and also received drainage from more recent mining activity. The substratum was similar to that of C2. The riparian vegetation was not as dense due to cattle grazing; the grasses and sparse willows provided little shade for the stream. In addition, some irrigation return water entered Trout Creek above this site.