Previous paleolimnological assessments have shown that many poorly buffered Canadian Shield lakes in the Sudbury (Ontario) region, including George and Lumsden lakes in Killarney Provincial Park (similar to 60 km southwest of Sudbury), acidified as a result of metal mining and smelting activities since the 1880s. In this paper, we revisit George and Lumsden lakes to conduct a paleolimnological re-assessment of these sites 25 years after the original sediment cores were collected. This updated study represents a period of chemical recovery from acidification, as well as the acceleration of regional climate warming. In agreement with the earlier paleolimnological study, our results also show that diatoms responded to acidification beginning 1920/30s, characterized by an increase in the relative abundance of acidobiontic Asterionella ralfsii. Although the most recent diatom assemblages (past 30 years) suggest that pH has likely returned to pre-acidification levels, the arrival and increases in diatom taxa absent in earlier sedimentary intervals (such as Asterionella formosa, Eunotia exigua, Frustulia magaliesmontana, and Kobayasiella subtilissima) signify the onset of new environmental conditions. The marked difference in composition between recent and pre-acidification diatom assemblages suggests a response to multiple environmental stressors, particularly regional warming, that has likely affected the trajectory of biological recovery. In these oli-gotrophic lakes, the recent increase in the relative abundance of Asterionella formosa, a rise in whole lake primary production, and a higher chrysophyte scale : diatom valve index suggest that regional warming has influenced the observed algal re-organization. Our paleolimnological investigation shows that, much like what had been reported for the Cladocera, the algal communities of these lakes have crossed climate-related limnological thresholds and are not returning to pre-acidification assemblages.
