Landscape Influences on Stream Habitats and Biological Assemblages

<em>Abstract.</em>—The influence of land use and instream physical habitat on biotic condition of fish assemblages was investigated for 48 stream reaches in the Huron and Raisin rivers. The amount of agriculture and wetland in the catchment and 100-m stream buffers had the strongest relationships with instream physical habitat, and these two categories of land use/cover were negatively correlated with each other (<em>r </em>= –0.70, <em>p </em>= <0.01). Agriculture was associated with high levels of sedimentation and reduced flow stability, while wetland was associated with low sedimentation and stable flows. The index of biotic integrity (IBI) was positively related to low sedimentation, stable flows, and the presence of fine gravel (2–8 mm). It was not significantly correlated with agricultural land use, but was positively related to natural land cover (forest + wetland combined) in the buffer. The best linear regression model using physical habitat and land-use variables from all sites adequately predicted IBI scores (adjusted <EM>R</EM><SUP>2</SUP> = 0.52). However, when the Huron and Raisin basins were treated separately, some of the included variables differed, and model fit increased (Huron adjusted <EM>R</EM><SUP>2</SUP> = 0.76, Raisin adj. <EM>R</EM><SUP>2</SUP> = 0.79), indicating that relations of fish assemblages to physical habitat and land use differed between basins. The Raisin model included land cover variables, while the Huron model included only variables related to physical habitat. Thus instream habitat and land cover may play different roles in these basins, suggesting the benefit of forming separate models for individual basins when sufficient data are available.