Landscape Influences on Habitat, Water Chemistry, and Macroinvertebrate Assemblages in Midwestern Stream Ecosystems

Abstract

Landscape characteristics of 65 subwatersheds within the Saginaw Bay Watershed of central Michigan were examined to identify relationships to stream habitat, water quality, and macroinvertebrate communities. A Geographic Information System was used to compile and analyze a series of landscape data including land use, elevation, slope, and hydrography of each watershed. Land use and landcover were quantified in 65 watersheds for both entire watersheds and 200 m stream buffers. Both watershed and buffer data were then empirically related to instream habitat and surface water chemistry using multivariate analysis. Macroinvertebrates community data from each watershed were related to stream physical and chemical data to identify which reach-scale environmental factors yhat most strongly influenced observed patterns. From these analyses, the relative influences of landscape features on macroinvertebrate communities could be inferred. Results showed that stream habitat, particularly channel morphology and substrate, were most strongly influenced by the presence of non forested wetlands. All permanent vegetative landcovers were associated with decreased values for most chemical parameters. Land use heterogeneity and average watershed slope were important predictors of total suspended solids. Landscape data accounted for over 75% of the variance in total nitrogen. In general, relationships between landscape data and stream chemistry were stronger in summer than fall. Surprisingly, the use of stream buffer data did not improve the predictions of habitat and chemistry characteristics compared with use of whole watershed data. Finer scale information may be required to depict the influence of riparian zones on midwestern streams. Macroinvertebrates were most strongly related to channel morphology, substrate characteristics, and nutrient concentrations. At the largest scale, geomorphic differences among watersheds and the extremes of land use (extensive row crop agriculture) had the strongest influence on macroinvertebrate communities, through their influence on stream habitat. At smaller scales, land use patterns (type, heterogeneity) exhibited more influence through their association with water chemistry and habitat alterations.