Impacts of urban land cover on trout streams in Wisconsin and Minnesota

We examined relations between the amounts and spatial patterns of urban land cover and fish assemblages, physical habitat, base flow, and water temperature at 39 sites in small. coldwater trout streams in Wisconsin and Minnesota. The amount of connected impervious surface area in the watershed was negatively correlated with a fish-based coldwater index of biotic integrity (IBI), catches of all coldwater and coolwater fishes and trout, and the percentage of intolerant fish; it was positively correlated with fish species richness and the percentage of tolerant fish. Nonlinear models were developed by means of 90% quantile regression to predict the maximum possible IBI score, abundance of trout, and percentage of intolerant fish at a given level of imperviousness. At a connected imperviousness of less than about 6% for a watershed, all three variables could have high values, whereas at a connected imperviousness above 11% the values were inevitably low. Between 6% and 11%, minor changes in urbanization could result in major changes in stream fishes. Canonical correspondence analysis identified water temperature and base flow its the key instream habitat factors explaining stream fish assemblages patterns. The percentage of connected impervious area in the watershed had a positive, linear relation with water temperature and a negative, nonlinear relation with base flow, suggesting pathways by which urbanization affected fish assemblages. Partial redundancy analysis indicated that land cover within 30 m of the stream channel explained considerably more variance in fish assemblages and instream habitat and physical conditions than land cover beyond 30 in. Our study Suggests that low levels of urban development can damage coldwater stream systems. Development strategies that protect the riparian area and minimize imperviousness may reduce this damage.