Scale, assessment components, and reference conditions: issues for cumulative effects assessment in Canadian watersheds.

Recent years have witnessed an increase in the use of watershed-based cumulative effects assessment (WCEA) in Canada; however, several challenges remain regarding its effective implementation and execution. Fundamental to WCEA is the establishment of linkages between environmental stressors and particular and measurable components of the aquatic environment. Dynamic and often synergistic relationships between the multiple physicochemical stressors in the landscape can affect water quantity, quality, and the health of aquatic species. Essential decisions must be made about what to measure to characterize both stressors and aquatic effects, what scale is appropriate for measurement, and to what the measurements should be referenced. This review presents lessons learned from case studies conducted in 6 different watersheds across Canada, each focused on advancing the science behind WCEA, but with varied objectives and approaches. Issues of scale, selection of aquatic environmental components or indicators for assessment, and reference conditions were compared and contrasted to highlight common challenges that can affect the implementation and outcome of a WCEA. The lack of long-term monitoring data and data inconsistencies were identified as frequently limiting factors for the advancement of WCEA science and the application of WCEA. Recommendations were made for developing a comprehensive and integrated methodology for WCEA in Canada.