Industrialization over the past 150 years has drastically modified the greater Sudbury area. Agents of this disturbance have included logging, agriculture, acidification, and metal deposition. Revitalization of these environments, particularly the lakes, requires an understanding of natural variability prior to industrialization, and knowledge of how anthropogenic terrestrial modifications manifest themselves in aquatic ecosystems. Paleolimnological analyses of sediment records from Clearwater Lake were used to compare species composition and diversity in an important group of algal primary producers (the diatoms) before and after industrialization. Diatom analysis in the 30-cm sediment core consisted of identifying taxa and calculating relative species abundance and changes in species richness. The radioisotope 210-Pb was used to date the sediments. The results showed a diversification of pennate diatoms and an increase in species diversity from approximately 15 to 25 taxa at a depth of 18 cm, likely before significant human impact. These changes could have resulted from natural watershed disturbances such as fire or drought. Samples deposited within the past 100 years also displayed increases in species richness, ; especially among acidophilic taxa such as Eunotia spp., Pinnularia spp., and Tabellaria flocculosa. Further studies using modern species-environment relations for diatoms could be used to provide quantitative models of species responses to environmental change. This research demonstrates the magnitude of natural variability prior to anthropogenic influence.