Impacts of Urban Land Use on Macroinvertebrate Communities with Comparison of Samples from Snags and Riffles for Southeastern Wisconsin Streams

Abstract

Macroinvertebrates were used to assess the impacts of urbanization on stream quality across a gradient of watershed imperviousness in 43 southeastern Wisconsin streams. Decreased Shannon diversity, percent of pollution intolerant Ephemeroptera, Plecoptera, and Trichoptera individuals, and generic richness resulted as watershed imperviousness increased. Values of the Hilsenhoff Biotic Index increased with watershed imperviousness, indicating that stream quality had declined with increased urbanization. Functional feeding metrics indicated a shift in macroinvertebrate composition from little to highly urbanized sites. Percent composition of collectors and gatherers increased, while percent filterers, scrapers, and shredders decreased with increased watershed imperviousness. This shift in macroinvertebrate taxa also suggested decreased stream quality. At low levels of watershed imperviousness, stream quality assessments were variable. However, for most stream quality metrics, a threshold existed between 10 and 20 percent watershed imperviousness, past which stream quality declines were minimized. Future studies should focus on physical and chemical characteristics of watersheds with less than 10-20 percent watershed imperviousness. If aspects elucidating high stream quality levels of such watersheds can be pinpointed, future development may be planned to ensure high stream quality levels in developing areas. Benthic macroinvertebrate community structure was assessed at both snag ·and riffle habitats in the same 43 southeastern Wisconsin streams across a range of watershed urbanization to determine differences in taxa assemblage and a variety of stream quality metrics. Discriminant analyses indicated that dominant species at riffle and snag habitats differed; Hydropsychid caddisflies (Hydropsyche betteni and Cheumatopsyche spp.) and elmid beetles (Optioservus spp. and Stenernlis spp.) discriminated riffle habitats from snag habitats where isopods (Asellus intermedius) and amphipods (Hyalella azteca and Gammarus pseudolimnaeus) predominated. Results of analysis of covariance indicated snag and riffle habitats differed across a gradient of urbanization for the Hilsenhoff Biotic Index, Shannon diversity index, and percent of pollution intolerant insects, filterers, and shredders at each stream site (p </= 0.10). Specific and generic richness were not significantly different between snags and riffles (p ~ 0.10). Macroinvertebrate communities found in snags suggested decreased stream quality when compared to communities found in riffles at the same sites. Thus, metrics derived from each habitat should not be interchanged when assessing stream quality during biomonitoring. However, metric values derived from snag data may be calibrated according to those obtained from riffle habitat under the same level of environmental degradation to predict stream quality. There were 30.2 fewer percent Ephemeroptera, Plecoptera, and Trichoptera individuals, 0.58 fewer Shannon index points and 0.29 greater biotic index points at snags than at riffles. Snag communities had 1.5 more percent shredders and 44.2 fewer percent filterers than riffles. With additional study, these calibrated metric values might be used for stream quality analysis in streams without riffles or in which snags were sampled.