Mercury (Hg) levels in fish respond to many ecological and physicochemical variables, and reflect cumulative effects of stressors such as watershed disturbance (e.g., industrial development, forest fire), exploitation, climate change, and transport and delivery of Hg from the watershed and atmosphere. We know that Hg levels in many traditional food fish species in the Dehcho region are high, and that these levels continue to increase in many lakes. What we don't know is why some lakes have high and/or increasing fish Hg levels whereas other lakes have relatively low and/or stable fish mercury levels. This makes it difficult to: i) predict how fish Hg levels will respond to continuing cumulative effects; ii) develop effective monitoring programs for cumulative effects; and, iii) mitigate existing "problem" lakes. Fish, benthic invertebrate, and zooplankton samples will be collected from each lake, and analyzed for mercury concentrations, stable isotope ratios (del15N, del13C), and fish age and size. Water samples will also be collected and analyzed for general chemistry and Hg levels. We will quantify Hg biomagnification and bioaccumulation in lakes with both low and high fish Hg levels (9 lakes total), and determine how to best predict fish Hg levels and biomagnification rates with ecological and/or ecosystem-level variables (e.g., fish size, age, growth rate, trophic position, lake food web structure, watershed characteristics, water chemistry, lake temperature). We will compare predictor variables between lakes with high vs. low Hg levels, integrate our results with those from other studies in the region, and evaluate how significant predictors of fish Hg levels will be affected by continuing stressors. By enhancing our ability to predict Hg levels and biomagnification rates, this research will allow a better understanding of why fish Hg levels differ among lakes, and how fish Hg levels will be affected by future cumulative impacts. Results will contribute to the development of targeted, efficient monitoring programs for cumulative effects, and identify future mitigation strategies.