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The English-Wabigoon River system in Northwestern Ontario, Canada, was one of the most heavily mercury-contaminated waterways in the world due to historical discharges in the 1960s from a chlor-alkali plant. This study examines long-term (1970-2010) monitoring data to assess temporal trends in mercury contamination in Walleye, Northern Pike and Lake Whitefish, three species important for sport and subsistence fishing in this region, using dynamic linear modeling and piecewise regression. For all lakes and species, there is a significant decline (36-94%) in mercury concentrations through time; however, there is evidence that this decline is either slowing down or levelling off. Concentrations in the English-Wabigoon fish are elevated, and may still present a potential health risk to humans consuming fish from this system. Various biotic and abiotic factors are examined as possible explanations to slowing rates of decline in mercury concentrations observed in the mid-1980s.

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The determinants of a local-scale biological community can include both abiotic and biotic factors acting at a variety of spatial and temporal scales. Previous studies have shown that the Precambrian Shield, a broad-scale geological feature, has distinct abiotic conditions and aquatic communities in lotic systems as compared with systems off-Shield. The relative importance of historical biogeography and postglacial colonization in Ontario versus the role of contemporary abiotic conditions has been debated in determining fish community composition. In this study, we sampled fish and macroinvertebrate community data from south-central Ontario streams to examine the degree of community concordance between fishes and macroinvertebrates and to determine the main abiotic drivers for this biological distinction between Shield and off-Shield areas. This study shows that low-order lotic systems on the Shield differ both abiotically and biotically from adjacent off-Shield systems and that similar abiotic drivers are responsible for structuring both groups of taxa. However, water chemistry and land-use variables are the most important in structuring macroinvertebrate communities, whereas physical habitat plays a larger role for fish communities.