Multiple stressor effects on stream health in the Lake Simcoe Watershed

The Lake Simcoe Watershed (LSW) is an important natural resource, supporting varied agricultural, recreational, and tourism activities for millions of people. Historically, major alterations to land uses have occurred throughout the watershed, resulting in increases of nutrients and contaminants, and alteration of hydrological and thermal regimes. The combination of these stressors has the potential to elicit greater impacts on ecosystem health than any one stressor in isolation, yet no studies have examined their effects on stream health and aquatic biota in the LSW. In this study, we quantified the impacts of multiple stressors on indices of biotic integrity, as well as assessments of benthic invertebrate and fish community composition from 2004 to 2012. Using a suite of multivariate analyses, we examined stressors across three categories of environmental variables: water quality, temperature, and hydrology. Water quality explained the largest amount of variation in stream health and biological community composition, followed by water temperature and hydrological variables, respectively. Total phosphorus, dissolved oxygen, and iron concentrations, as well as the 7-day low flow and the mean summer temperature, were identified as predictors of both fish and benthic invertebrate variance. Multiple stressor interactions were detected for fish communities, although these interactions were of lower relative importance than for individual stressors. In contrast, no significant stressor interactions were found to influence benthic invertebrate communities. Overall, our results reinforce the importance of ecosystem monitoring, and the need to consider the influence of multiple aspects of ecosystem function, when examining the health of aquatic systems.