Absorption and scattering coefficients intlrondequoit Bay 1

Profiles of scalar, downwelling, upwelling, and spectral irradiance were measured on 37 dates in 1982-1983. Values of the attenuation coefficient K were 0.8-1.2 m-l, and the reflectance R was 0.03-0.06. By Kirk’s method, the absorption and scattering coefficients a and b were found to be 0.44-0.83 and 1.8-5.0 m-l. Also measured were turbidity, chlorophyll, and absorption spectra of dissolved yellow substances and of particles. Independent estimates of a, as the sum of absorption components, were in reasonable agreement with values obtained by Kirk’s method. For particles, the average value of the Chl-specific absorption coefficient k, was -0.008-0.010 m* mg-l Chl. Values of the Chl-specific scattering coefficient kb were usually -0.05-0.10 m* mg-l Chl; values were higher (-0.12) when gas-vacuolate cyanobacteria were present. Optical properties most commonly measured in lakes and seas are Secchi transparency, turbidity, and the vertical attenuation coefficient K. Properties measured less frequently include horizontal beam transmission, upwelling and scalar n-radiances, spectral irradiance, and reflectance R (the ratio of upto downwelling irradiance). All of these, except turbidity and horizontal beam transmission, are “apparent optical properties,” so called because they are functions of the radiance distribution as well as of three “inherent properties.” Inherent properties (defined for collimated light and thus independent of radiance distribution) include the volume absorption and scattering coefficients a and b and the angular scattering function. It is commonly supposed that the scattering function of Petzold (1972) applies to most natural waters. In contrast, values of the absorption and scattering coefficients vary greatly among natural waters, due to differences in kinds and concentrations of dissolved and particulate substances. Differences in a and b underlie the wide variations in apparent properties of natural waters. Significant contributions to the absorption coefficient may be made by water (a,,,), dissolved yellow substances (a,), inorganic particles (ai) if colored, detritus (ad) which The work described here was a part of an investigation of Irondequoit Bay following sewage diversion. Support by Monroe County Pure Waters is acknowledged. may contain pheoor yellow pigments, and phytoplankton (a,). Significant contributions to the scattering coefficient may be made by water and each of the three particle types. If the phytoplankton contributions are expressed in terms of chlorophyll-specific coefficients k, and kb, one can write a = a, + a,, + ai + ad + k,Chl