Ecological studies conducted in apparently pristine ecosystems throughout the world have shown that these systems are being altered as an unconsidered consequence of human activities. A paleoecological examination of eight lake ecosystems located along a north-south transect through the northern Alberta boreal ecoregion was conducted using siliceous microfossil and geochemical proxies. The structure of microfossil assemblages has undergone substantial shifts in some of the study lakes, but remains largely unaltered in others. This heterogeneous ecological response suggests that the effects of these stressors are mediated on a local scale, by processes occurring in the lake and drainage basin. This proximal mediation of environmental stressors is particularly important in relation to surface water acidification. Although the rate of acid deposition associated with bitumen extraction and processing has increased substantially over the past thirty years, there is so far no indication that this has caused acidification of any of the study lakes.