Scattering and Absorption of Light in Onondaga Lake

The temporal distribution of the partitioning of light attenuation between the processes of absorption and scattering is presented for hypereutrophic Onondaga Lake, N.Y., for a productive seven month period of 1978. The determinations were based on in-lake measurements of up- and downwelling irradiance, performed on 62 occasions during the study, and the light partitioning approach of Kirk. Further, a simple mechanistic model framework is presented to quantify the relative contributions of attenuating substances to the two attenuating processes, and therefore overall attenuation. The model is applicable to a number of ecosystems, and it requires only a rather rudimentary data set for its application. The model was applied to the time-averaged conditions of Onondaga Lake during the study, based on synoptic and subsequently collected data. Nearly 40% of the attenuation during the study was due to scattering. The time-averaged analysis indicates there were at least two significant attenuating components other than phytoplankton in Onondaga Lake, calcite precipitate and dissolved yellow substances. Transparency goals for the lake may be met by management efforts other than reduction of the loading of phytoplankton nutrients.