The paleolimnology (sediment chemistry, 210 Pb dating, pollen, and diatoms) of Crystal Lake, McHenry County, Illinois, USA was studied to investigate the impact of European settlement on lake trophic status. Pollen clearly indicates a vegetational shift from Quercus dominance to Ambrosia. 210 Pb dating suggests that the Ambrosia rise likely occurs around the 1840’s, which is consistent with historical records on the European settlement in this area. Coincident with the vegetational shifts, several diatom species, Cyclotella meneghiniana, Fragilaria crotenensis, Asterionella formosa and Tabellaria fenestrata, all increase while C. comta decreases after the settlement. However, C. comta and Aulacoseira ambigua, two dominant species, remain abundant throughout the entire core. Their relative abundances at the surface sediments reach presettlement levels. It is hypothesized that Crystal Lake, a glacial lake rich in CaCO3, may have been able to assimilate increases of phosphorus by coprecipitating phosphorus with CaCO3. Such a ’buffering mechanism’ may be responsible for the lake’s resistance to trophic changes or recovery following disturbance.