Pre-restoration macroinvertebrate assemblages related to environmental conditions in a stream impaired by cattle grazing

The purpose of this study was to determine the environmental factors responsible for the benthic macroinvertebrate assemblage structure of a stream reach impaired by cattle grazing that was scheduled for restoration. The impaired reach to be restored was a 3.14-km section of Smith Creek known as Bruce Farm (BR), near Harrisonburg, VA. BR was heavily grazed by cattle for decades and suffered from obvious erosion, organic loading, and lack of a forested riparian zone. BR was compared to a section of a nearby similar stream that was considered to be mixed use (MU) because it had a mature forested riparian zone and no access by cattle, but it still received nutrient and sediment inputs from upstream cattle grazing. Total density and taxa richness were greater at BR (52,438 organisms/m2; 84 taxa) than MU (3,982 organisms/m2; 63 taxa). Univariate and multivariate statistical techniques indicated that biological variables related to the growth of plants on rocks (ash-free dry mass, chlorophyll a, epilithic biomass) had the strongest relationships with benthic macroinvertebrates, while physical variables related to the substrate composition (% fines, % gravel, Trask's sorting coefficient) had weaker relationships. Predicted changes in the post-restoration benthic macroinvertebrate assemblage are discussed.