The majority of the limnological research in Sudbury, Ontario, has focused on the anthropogenic impacts of industrial emissions (SO2 and metals), with the potential effects of cultural eutrophication largely being overlooked. However, the population of the City of Sudbury has grown with the prosperity of the mining sector, which poses a risk to the quality of freshwater resources. As with many environmental issues, there is often a lack of predisturbance data that can assist in gauging the full extent of environmental change. Therefore, paleolimnological approaches were used to track long-term biological changes in sedimentary diatom assemblages related to cultural eutrophication in 4 lakes from Sudbury. Diatom assemblages were primarily dominated by oligotrophic taxa prior to watershed development; however, with the onset of urban environmental stressors (e.g., septic systems, the application of lawn fertilizers and watershed development), there was a shift toward taxa that thrive in more productive systems. Diatom assemblages also seem to track an increase in lakewater pH through time, which is likely related to increased acid neutralizing capacity as a result of watershed disturbances, algal assimilation and bacterial reduction of NO3-, and increased base cation export from the watershed due to acidic deposition. Insight into predisturbance conditions of the lakes should help lake managers set realistic biological targets for restoration and may be used to help gauge the response of these systems to future mitigation efforts. © 2011 by the North American Lake Management Society.