A study has been carried out to determine whether a previously derived relationship for an apparent optical property (the vertical attenuation coefficient for downward irradiance, Kd) of a water body as a function of inherent optical properties (absorption and scattering coefficients, a and b) of the water, applies to a surface water body at the extreme high-scattering end of the range of naturally occurring optical character, namely an ultraturbid lake in Australia. It was found that scattering by the very small clay particles responsible for the high turbidity of the water showed a very strong wavelength dependence in accordance with b∝(1/λ)2.26. When this wavelength dependency was taken into account, both in the laboratory determination of absorption coefficients, and in the calculation of attenuation of irradiance in the water body as a function of wavelength, then good agreement between the Kd(PAR), where PAR is photosynthetically available radiation, derived by calculation from measured a and b values and that obtained by measurement in the lake was found. Optical closure was thus achieved. ¿ American Geophysical Union 1995