Landscape Influences on Stream Habitats and Biological Assemblages

<em>Abstract.</em>—Conducting biological assessments at a national scale requires a classification scheme to report results, define reference conditions, and interpret data. We believe that such a classification should be based largely on biota. Analyzing stream biological assemblage data across the conterminous United States is difficult due to the lack of synoptic assemblage data. In the last 10 years, however, the number and scope of bioassessments has increased dramatically. We compiled a national-scale database of lotic fish assemblages containing 5,951 sample sites from available national and state agency data. Cluster analysis (Bray-Curtis distance) and indicator species analysis were used to cluster the data, identify clusters, and describe them. We developed 12 national clusters of fish assemblage groups that were well described by indicator fish species and predicted using both discriminant function analysis and classification tree analysis. We also examined the relationship of existing landscape classification schemes to fish assemblage similarity. Existing schemes captured about half the within-group similarity expressed in biologically derived clusters. Schemes based on ecoregion, physiography, hydrologic units, and geopolitical boundaries had very similar mean within-group fish assemblage similarities. Cluster and mean similarity analyses were not strongly influenced by using data subsets that removed nonnative fish species and disturbed sites. This suggests that the underlying mechanisms responsible for controlling fish assemblage patterns at the national scale are fairly robust to the effects of nonnative species and anthropogenic disturbances.