Influences of water and substrate quality for periphyton in a montane stream affected by acid mine drainage

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 acid-tolerant 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.