Urban stream rehabilitation through a decision-making framework to identify degrading processes and prioritize management actions

SUMMARY: Rehabilitation of physical habitat in urban streams is unlikely to incre ase instream biological diversity if factors other than habitat simplification limit community development. We used a study of regional variation in macroinvertebrate communities to develop a model predicting which degrading processes limit community comp osition in urban streams. Organic pollution (as indicated by biochemical oxygen demand, BOD) and increased run-off (as indicated by proportion of catchment that is impervious) —both products of catchment -scale perturbations—were the best environmental variables to explain differences between communities in metropolitan sites and those in surrounding rural sites. We hypothesized that catchment-scale processes limited community development in metropolitan sites. This was supported by a second study that showed little change in community composition in metropolitan sites after the placement of artificial riffles, which were designed to increase habitat complexity. We thus propose a decision-making framework for urban stream rehabilitation, wherein community structure of a physically degraded site is assessed. If the community is similar to those of surrounding rural sites, site-specific physical habitat restoration is considered likely to be successful. If the community is similar to those of metropolitan sites, rehabilitation measures to improve water quality or hydrology are recommended as first priority. Levels of BOD and catchment impervious area are used to prioritize appropriate actions.