This document summarizes a study that assessed the ecological impacts of urban runoff from two catchments on receiving aquatic ecosystems using local periphyton communities. Periphyton were colonized for 2 weeks in rivers receiving runoff and in a non-receiving upstream reference site. The receiving communities were evaluated for photosynthetic efficiency and tolerance to copper, identified as a significant runoff stressor. The hypothesis tested was that runoff degrades communities by making them more sensitive to runoff stressors. Results indicated higher copper tolerance than water quality guidelines, showing the value of using local communities for site-specific ecological risk assessment of runoff impacts.