Good, Fair, or Poor? The Reality of Stream Macroinvertebrate Biomonitoring in an Impaired Agricultural Watershed

ABSTRACT: Various biotic indices have been used to assess the condition of aquatic macroinvertebrate communities in agricultural watersheds. We collected 171 samples of benthic macroinvertebrates from 57 stream sites on three forks of an agricultural watershed in southeastern Minnesota, USA, and compared the abilities of various diversity, taxa richness, and multi-metric indices to evaluate invertebrate community health and/or level of impairment. Total invertebrate abundance (sample count), total taxa richness, Ephemeroptera-Plecoptera-Trichoptera (EPT) taxa richness, Simpson diversity, Shannon diversity, a regional benthic index of biotic integrity (BIBI), and the national Izaak Walton League’s Save Our Streams (SOS) rating score were all significantly correlated with one another. Total taxa richness, BIBI score, and SOS score displayed nearly identical influences within a principal components analysis (PCA) ordination. Only total abundance and the two diversities differed significantly among stream forks. When the distributions of sites among rating categories (poor or very poor to excellent) were compared for the various indices, nearly all indices rated community health differently from one another. In order from highest ratings to lowest ratings, the indices were: Simpson diversity, Shannon diversity, SOS score, total taxa richness, EPT taxa richness, and BIBI score. Taxa richness measures and the multi-metric BIBI appeared to rate stream health more realistically (i.e., mostly fair or poor site ratings with relatively few good to excellent) than either of the diversity indices or the SOS score (many good, very good, and excellent site ratings), suggesting that the choice of biotic index can significantly influence evaluations of stream communities in an agricultural watershed.