IMPROVING STREAM BIOASSESSMENT METHODS FOR THE MONTANA VALLEYS AND FOOTHILL PRAIRIES ECOREGION

To assist in the characterization of the ecological integrity of streams of the Montana Valleys and Foothill Prairies, I investigated the distribution of aquatic macro invertebrate assemblages over that geographically variable ecoregion. DECORANA (DCA) ordination and TWINSPAN classification techniques indicated that elevation was the variable with the greatest influence on distributions of benthic taxa in the region, and that three assemblage groups are distinguishable. The Montana Department of Environmental Quality (Montana DEQ) uses an adaptation of U.S. EPA’s Rapid Bioassessment Protocol (Plafkin et al. 1989) to evaluate and monitor streams here, and the elements of the protocol were examined for sensitivity to anthropogenic impact, consistency and correlation with habitat variables. Individual attributes of benthic assemblages (metrics) were tested for ability to distinguish sites with severe anthropogenic habitat degradation from sites with minimal impact. Correlations with habitat variables were examined to determine the relevance of individual metrics. Variability in metric scores was partitioned into that attributable to elevation, the most influential natural environmental determinant of assemblage distributions, and that attributable to human-caused impact, the variable of greatest interest to resource managers. A revision of the Montana DEQ metric battery is proposed which includes seven metrics. An integrated score calculated from the currently used battery is compared with that of the revised battery. The latter was more sensitive to impact and more strongly correlated to visually assessed habitat degradation. Scores calculated by either method vary more with impact than with elevation, and residual error is about the same for both methods. The revised bioassessment method, however, is shown to be more consistent than the current Montana DEQ protocol. The habitat assessment protocol used by Montana DEQ is also evaluated and revised. Several visually assessed parameters are replaced by variables calculated from results of Wolman pebble counts. The proposed revisions of bioassessment and habitat assessment protocols provide tools which are more sensitive to impact, more consistent and more closely related to habitat variables than Montana DEQ’s current method. These tools can be applied to monitoring and evaluating stream conditions throughout the Montana Valleys and Foothill Prairies ecoregion