To better elucidate the ecological effects of naphthenic acids and major ions liberated in oil sands development, the summer-time composition of phytoplankton communities in ten water bodies near Fort McMurray (northeastern Alberta) was studied in 1997. The water bodies varied in degree of process water influence, and in age, size and ancillary chemical characteristics. Community biomass of phytoplankton was not systematically related to naphthenic acid or major ion concentrations, even though the higher naphthenate concentrations exceeded published EC50's for acute effects on several different aquatic species. Chlorophyta were frequently dominant, particularly where naphthenate and major ion concentrations were highest. Canonical Correspondence Analysis (CCA) revealed gradients in taxonomic composition at a finer (genus and species) taxonomic level. Despite the simultaneous and uncontrolled variation of other environmental factors, naphthenate and major ion concentrations (as indexed by conductivity) explained a highly-significant 40% of the variation in taxonomic composition. Systems with naphthenates <6.5 mg l(-1) and conductivity <800 PhiS cm(-1) were clustered together near the origin of the CCA plots, suggesting little ecological effect at such concentrations. Taxa associated with elevated naphthenate and/or major ion concentrations were derived from six different algal divisions and included many that were identified as tolerant in previous bioassay experiments. Over the range of concentrations encountered (1.5-45 and 100-3000 mg l(-1) for naphthenates and ions, respectively), CCA indicated that the ecological effect of major ions appeared to be at least as great as that of naphthenates.