We integrated basic concepts from fisheries science and toxicological risk assessment to form a potential method for setting screening-level, risk-based, site-specific water quality objectives for temperature. In summary, the proposed approach: (a) uses temperature impacts upon specific growth as a measure of chronic (cumulative) temperature effects; (b) explicitly incorporates the consequences of both magnitude and cumulative duration of exposures; (c) adjusts the result for local watershed conditions, reducing the likelihood that naturally warm systems are identified as thermally polluted while helping to ensure that naturally cool systems are closely monitored for ecologically harmful changes in thermal regime; and (d) expresses the net result both graphically and as a risk quotient, RQ, closely analogous to that used in toxicological risk assessments. The latter yields a site-specific, risk-based water quality objective and may serve as a straightforward decision rule for environmental managers. The method was applied to historical data from a small British Columbia stream subject to increasing urban development pressures. We also illustrate how the technique might be used to explore potential climatic change impacts, using coupled general circulation model predictions in conjunction with empirical downscaling. Overall, the method and results are presented as an introduction and illustration of concept, intended as a step toward the development of a logistically feasible risk-based approach to establishing screening-level, site-specific water temperature objectives, and monitoring compliance, in the context of large-scale, many-site, environmental monitoring networks. With further work, the technique offers potential to fill the gap between the temperature threshold-based targets typically specified in regulatory guidelines, which may be hydroecologically unrealistic, and detailed biophysical modelling, which typically is logistically infeasible in a day-to-day environmental monitoring and management context.
