Defining the Reference Condition and Stressor Gradients: Making Black Boxes Transparent

Bioassessment of freshwater ecosystems using either the Reference Condition Approach (common in Canada, Australia, and EU) or the Multimetric Approach (common in USA) has developed rapidly, but is still limited by vague or subjective characterization of stressor gradients such as landscape degradation, added suspended sediments, and nutrient and pesticide additions. The struggle to adequately define what is really the Reference Condition in a given study, or what a credible gradient of stress is, permeates recent bioassessments. Using examples from hundreds of small watersheds in British Columbia, I will show how to catalogue all potential stressors of an aquatic ecosystem in a study area, and then characterize the multidimensional nature of the stressor gradient(s). This enables us to define the Reference Condition and stressor gradients empirically rather than arbitrarily. Perfectly analogous to characterizing multidimensional, natural gradients of environmental variability, we can relate the stressor gradients to gradients of biota and use such a relationship to predict the effects of increasing or reducing levels of stressors on the system.