Abstract
Based on the data of Secchi disk depths, the beam attenuation coefficients and downward spectral irradiance, water colour and the optical characteristics of the natural light field were examined in Lake Biwa. The peaks of spectral distribution of downward irradiance in the deeper layer were found at about 560 nm in the south basin and about 540 nm in the southern part of the north basin. The colour of the lake was similar to that of the turbid coastal sea waters reported in the literature. In Lake Biwa, the value of Kv = ZSD is found to be 1.49 and the depth to which 1% of surface illumination penetrated is about 2.7 times as large as the Secchi disk depth.
