The paleolimnoloogy (sediment chemistry, lead-210 dating, 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. Lead-210 dating suggests that the Ambrosia rise likely occurred around the 1840's, which is consistent with historical records on the European seelement in this area. Coincident with the vegetational shifts, several diatom species, Cyclotella meneghiniana, Asterionella formosa and Tabellaria fenestrata all increase while Cyclotella comta decreases somewhat after settlement. Cyclotella comta and Aulacoseira ambigua remain abundant throughout the entire core. Their relative abundances at the surface sediments read presettlement levels. It is hypothesized that Crystal Lake, a glacial lake rich in calcium carbonate, may have been able to assimilate increases of phorphorus by coprecipitating phosphorus with calcium carbonate. Such a "buffering" mechanism may be responsible for the lake's resistance to trophic changes or recovery following disburbance.