Assessing biotic integrity of streams: Effects of scale in measuring the influence of land use/cover and habitat structure on fish and macroinvertebrates

Fish and macroinvertebrate assemblage composition, instream habitat feature s and surrounding land use were assessed in an agriculturally developed wat ershed to relate overall biotic condition to patterns of land use and chann el structure. Six 100-m reaches were sampled on each of three first-order w arm-water tributaries of the River Raisin in southeastern Michigan. Compari sons among sites and tributaries showed considerable variability in fish as semblages measured with the index of biotic integrity, macroinvertebrate as semblages characterized with several diversity indexes, and both quantitati ve and qualitative measurements of instream habitat structure. Land use imm ediate to the tributaries predicted biotic condition better than regional l and use, but was less important than local habitat variables in explaining the variability observed in fish and macroinvertebrate assemblages. Fish an d macroinvertebrates appeared to respond differently to landscape configura tion and habitat variables as well. Fish showed a stronger relationship to flow variability and immediate land use, while macroinvertebrates correlate d most strongly with dominant substrate. Although significant, the relation ships between instream habitat variables and immediate land use explained o nly a modest amount of the variability observed. A prior study of this wate rshed ascribed greater predictive power to land use. In comparison to our s tudy design, this study covered a larger area, providing greater contrast a mong subcatchments. Differences in outcomes suggests that the scale of inve stigation influences the strength of predictive variables. Thus, we conclud ed that the importance of local habitat conditions is best revealed by comp arisons at the within-subcatchment scale.