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SUMMARY 1. We used 94 sites within the Northern Lakes and Forests ecoregion spanning Minnesota, Wisconsin and Michigan to identify environmental variables at the catchment, reach and riparian scales that influence stream macroinvertebrates. Redundancy analyses (RDA) found significantly influential variables within each scale and compared their relative importance in structuring macroinvertebrate assemblages. 2. Environmental variables included landcover, geology and groundwater delivery estimates at the catchment scale, water chemistry, channel morphology and stream habitat at the reach scale, and landcover influences at three distances perpendicular to the stream at the riparian scale. Macroinvertebrate responses were characterised with 22 assemblage attributes, and the relative abundance and presence/absence of 66 taxa. 3. Each scale defined macroinvertebrates along an erosional to depositional gradient. Wisconsin’s macroinvertebrate index of biotic integrity, Ephemeroptera–Plecoptera–Trichoptera taxa and erosional taxa corresponded with forest streams, whereas organic pollution tolerant, Chironomidae and depositional taxa corresponded with wetland streams. Reach scale analyses defined the gradient similarly as dissolved oxygen and wide, shallow channels (erosional) opposed instream macrophytes and pool habitats (depositional). Riparian forests within 30 m of the stream coincided with an erosional assemblage and biotic integrity. 4. Next, we combined all significant environmental variables across scales to compare the relative influence of each spatial scale on macroinvertebrates. Partial RDA procedures described how much of the explained variance was attributable to each spatial scale and each interrelated scale combination. 5. Our results appeared consistent with the concept of hierarchical functioning of scale in which large-scale variables restrict the potential for macroinvertebrate traits or taxa at smaller spatial scales. Catchment and reach variables were equally influential in defining assemblage attributes, whereas the reach scale was more influential in determining relative abundance and presence/absence. |
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