Limnol. Oceanogr., 44(3, part 2), 1999, 804–809

St. Kevin Gulch, a headwater stream of the Rocky Mountains of Colorado, receives acid mine drainage that maintains low pH, high concentrations of heavy metals, and high rates of metal hydroxide deposition. An acidtolerant alga, Ulothrix sp., is present below the source of mine drainage in St. Kevin Gulch, but its biomass is limited by the deposition rates of iron hydroxides, which are especially high near the source. An experimental diversion of the mine drainage increased the quality of water and improved the substrate condition through a reduction of deposition rates. During the first year of the experiment, Ulothrix became abundant in this reach. During the second year, pH increased to the point at which aluminum hydroxides precipitated from the stream water onto the streambed; this change inhibited the growth of all periphyton, including Ulothrix. The deposition rate of aluminum hydroxides, however, was less than that of iron hydroxides in stream reaches with high Ulothrix biomass, suggesting that metal hydroxides vary by type in their effect on periphyton. Acid mine drainage affects many streams throughout the world and causes a variety of stresses on aquatic ecosystems. Acidity in mine drainage is generated by the oxidation of sulfide minerals, such as pyrite, which produces sulfuric acid. High concentrations of heavy metals, including zinc, cadmium, and copper, may be found in waters that receive acid mine drainage. Additionally, deposition of metal oxide precipitates onto the streambed can affect biota in receiving waters. Our objective in this study was to examine how these types of stresses (acidity, dissolved metals, hydroxide deposition) and their interactions affect periphyton in a stream prior to and following diversion of acid mine drainage, as might occur through remediation of a stream segment. The effects of mine drainage have been studied in a variety of aquatic ecosystems (Kelly 1988). As pH decreases, structural changes may occur in periphyton communities as acid-tolerant taxa become dominant. These community changes are similar to those occurring in aquatic systems affected by acidic precipitation (Elwood and Mulholland 1989; Planas 1996). Metal toxicity also alters the structure of periphyton communities (Clements 1991; Genter 1996). While species composition of periphyton may change as pH declines or metal concentrations increase, community biomass may be stable or even increase with the release of