The Athabasca oil sands region contains one of the world's largest deposits of oil in the form of bitumen. The extraction of oil from bitumen produces acutely toxic wastewaters, which is due primarily to the naturally occurring organic acids, naphthenic acids (NA). The ecological effects of NA and conductivity were inferred by measuring the responses of a sensitive and representative community of aquatic organisms, the phytoplankton. The current research established the findings using bioassay and microcosm experiments, combined with a comparative analysis of natural systems in the area. The bioassay experiment incorporated natural phytoplankton community assemblages from aquatic systems of varying NA exposure histories and was compared to Pseudokirchneriella subcapitata (formerly, Selenastrum capricornutum) monospecific assays to infer thresholds for ecological impact using the Pollution Induced Community Tolerance (PICT) approach. All of the assay cultures responded to even the lowest NA addition (24.5 mg/L) with increased duration of lag phases. However, a hormetic like response was witnessed in the exposure history bioassay with the highest biomass attained at the intermediate and high concentration NA additions. This response was not observed in P. subcapitata, which experienced a progressive lengthening of lag phase and decreased biomass and growth rate with increased NA additions. The PICT method defined a threshold for the ecological effect of NA on phytoplankton between 24 and 50 mg/L, a range consistent with some previous estimates. The comparative analyses of Alberta water bodies of widely varying NA and conductivity concentrations were conducted to better resolve the joint and separate effects of NA and salts on phytoplankton community composition. Canonical Correspondence Analysis (CCA) and Variance Partitioning Analysis (VPA) was performed on both 13 sites across three sampling periods and 30 sites within one sampling period to determine the relationship between the phytoplankton communities and the environmental variables. CCA determined the strongest factors in determining community structure to be NA and conductivity which correlates to previous findings, and that accounting for the temporal effects across the three seasons yielded a more accurate depiction of community structure. (