Effects of Whole-Lake Base Addition on the Optical Properties of Three Clearwater Acidic Lakes

Data describing changes in the attenuation, absorption, and scattering of photosynthetically active radiation (PAR) were collected over a 5-yr period from three limed lakes and an untreated reference site. Light attenuation rose significantly after liming. Short-term changes in light attenuation were attributed to increased scattering by suspended calcite particles; such changes persisted for fewer than 15 d due to the rapid dissolution of the calcite. Long-term increases in light attenuation corresponded to increases in light absorption, which were attributed to higher concentrations of light-absorbing substances (gelbstoff) as indicated by higher lake dissolved organic carbon (DOC) concentrations. Changes in chlorophyll did not play an important role in regulating light attenuation except during the first summer following liming. Reacidification of one of the three limed lakes was accompanied by a decrease in lake DOC concentrations, followed by a gradual decrease in light attenuation and absorption. Within 3 yr after liming, the chemical and optical characteristics of this site were comparable with the pretreatment period. Results from lake liming experiments are consistent with the hypothesis that changes in the photochemical properties and/or the concentration of light-absorbing organic compounds are important mechanisms by which acidification alters the optical properties of lakes.