It is critical to identify and use appropriate ecological indicators to detect, monitor and assess the impacts of environmental stressors on natural systems in order to effectively legislate, protect and restore ecosystems. Based on strong and predictable community responses to acid-stress, crustacean zooplankton has the potential to be a crucial indicator of acidification in freshwater ecosystems. This thesis is based on evaluating the potential of zooplankton as indicators to detect and track the degree of impact of recent acid deposition on Canadian Shield lakes in north-west parts Saskatchewan. These systems may now be at risk as the area is downwind of growing oil-sand operations near Fort McMurray, Alberta, a significant emission source of acidifying agents. Yet, to this point chemical data from past and recent studies of these lakes have not led to any consensus in respect to the degree of impact. In contrast to chemical inferences, concurrent sampling of crustacean zooplankton communities in these lakes indicated strong symptoms of acid-stress, including: 1) distinction of lakes based on species assemblages with different acid tolerances, 2) species-environment relationships strongly related to acid sensitivity, 2) unusually low species richness, 4) low frequency of occurrence of acid-sensitive taxa/species, and 5) opposite trends in changes of relative abundances of acid-sensitive and acid-tolerant taxa/species along the acid-sensitivity gradient. I propose that these responses resulted from seasonal or short-term pH alterations (rather than chronic acidification), which can be critical during early stages of acidification, This emphasizes the importance of biological indicators to assess acid-stress, because biological responses can occur prior to long-term chemical responses are detectable. Subsequently, I developed a framework to use the presence and absence of individual zooplankton species as indicators to characterize lakes along the acid-stress gradient as well as for relatively uninfluenced reference lakes. To evaluate the performance of presence vs. absence of indicator species I adopted statistical methods originally proposed by Dufrêne and Legendre (1997). Although statistical analyses identified presence indicator species for highly stressed lakes, further evaluation revealed that these species were not appropriate indicators as they were ubiquitous generalists. In contrast, absence indicator species were more appropriate as these habitat specialists were absent from acid stressed lakes. On the other hand, presence indicator species for the low acid-stress category were largely habitat specialists and therefore appropriate indicators for this category. No presence or absence indicator species were identified for lakes at the intermediate acid-stress level. For non acid-stressed reference lakes that were categorized into three groups based on water chemistry (nutrients, acidity and colour), presence indicator species were appropriate because lakes were represented by the habitat specialists that were well adapted to the conditions within the individual groups. Thus the combined use of both presence and absence indicators is recommended to characterize lakes along stress gradients, whereas the sole use of presence indicators should be adequate to reflect different reference conditions. In conclusion, this thesis highlights the appropriateness of zooplankton as an extremely sensitive barometer of acid stress. Furthermore it demonstrates a framework of using community attributes of zooplankton to reflect the degree of acidification of lakes across an ecoregion, while using presence and absence of individual indicator species to characterize the level of acid-stress to individual lakes.