Recovery of benthic invertebrate communities in silver bow creek, montana, following improved metal mine wastewater treatment

Abstract

Silver Bow Creek, a tributary of the Clark Fork River in southwest Montana, has been affected by metal mining and domestic wastes for over 100 yr. The invertebrate community was monitored at five stations from 1972 through 1983 to determine both the longitudinal and temporal patterns of recovery following major improvements in mine wastewater treatment. This treatment dramatically reduced metal concentrations in the mine discharge. Despite markedly improved water quality, no invertebrates were collected in Silver Bow Creek until 1975, when small numbers of invertebrates (primarily chironomids) were collected at the furthest downstream stations. A few invertebrates (<75 m⁻²) were collected for the first time at Stations 1 and 2 in 1981, although the lower stations still had much greater densities (> 1600 m⁻²). The earliest colonizers at each station were chironomids, empidids, and oligochaetes. In 1982, 10 yr after initiation of improved water quality, aquatic insect populations were over 1800 m⁻² at all five stations. Populations dropped in 1983, but upper and lower stations had comparable densities suggesting that the stream is responding to a common stress such as high snowmelt runoff. The relatively long recovery time is partially attributed to the lack of an undisturbed headwater source of colonizers.