The rate of bitumen extraction in northeastern Alberta, Canada, is outpacing the state of ecological understanding of the region, so that the extent of potential disturbances caused by atmospheric deposition remains largely unknown. Atmospheric SO2 emissions from the Fort McMurray region of Alberta (∼300 t·day–1) constitute ∼5% of the Canadian total. Combined with an estimated NOx production of ∼300 t·day–1, these emissions have the potential to acidify surface waters. Diatom assemblages in dated sediment cores from eight acid-sensitive lakes were analyzed to assess the effects of acidifying emissions on boreal lake ecosystems. There is no evidence that these lakes have become acidified. Instead, many of the lakes show characteristic changes towards greater productivity and occasionally greater alkalinity. The absence of evidence for acidification does not imply that emissions from the Oil Sands are environmentally benign, but rather suggests that the biogeochemistry of these lakes differs fundamentally from well-studied acidified counterparts in northern Europe and eastern North America. Complex interactions involving in-lake alkalinity production, internal nutrient loading, and climate change appear to be driving these lakes towards the new ecological states reported.