Benthic diatoms as indicators of long-term changes in a watershed receiving passive treatment for acid mine drainage

Between 1995 and 2003, 15 reclamation projects using passive treatment systems were installed in a 70-km² watershed to reduce acid mine drainage (AMD) impacts from coal mining. Six stream sites were sampled for water chemistry and benthic diatoms on 15 dates from 1996 to 2005; 1 unimpacted reference stream, 4 downstream of treatment systems, and 1 AMD-impacted site with no treatment. Our objective was to determine if diatom communities have responded to restoration by comparing temporal trends at sites downstream of treatment to concurrent trends at untreated and reference sites. Water chemistry at the sites corresponded spatially to AMD sources within the watershed. All sites below treatment had a significant increase in pH. Diatom communities provided several lines of evidence that treatment had lessen AMD impacts over the 10 year study: (1) the percentage of circumneutral taxa significantly increased at 3 of the 4 sites below treatment; (2) the percentage of circumnuetral taxa averaged for all sites below treatment increased significantly; and (3) temporal changes in community composition were significantly directional for 3 of 4 treated sites, becoming progressively more similar to reference communities. This study emphasizes the importance of long-term data sets for assessing recovery of streams following large-scale restoration.