We analyzed measurements of light penetration, the angular distribution of irradiance, the spectral quality of light attenuation, chlorophyll, gelbstoff, and turbidity to depict seasonal dynamics and long-term trends in these optical characteristics for eutrophic Onondaga Lake, NY, over the 1968–1990 period. Strong seasonality in optical characteristics has been common in the lake, particularly since the closure of an adjoining soda ash/chlor-alkali manufacturing facility, associated with the dynamics of phytoplankton biomass, inputs of terrigenous particles, and probably zooplankton grazing. The New York State requirement for Secchi disc depth (SD) (1.2 m) was only met for about 20% of the observations before the closure of the soda ash/chlor-alkali facility in 1986. Since closure, there has been a shift to increased light penetration as a result of reductions in the concentrations of attenuating particles. About 65% of SD observations since the closure have met the SD standard. Blue color is not a realistic management goal for Onondaga Lake because of the high gelbstoff concentrations, that are apparently regulated by natural terrigenous inputs.