Spectral Distribution of Photosynthetically Active Radiation in some South-eastern Australian Waters

Abstract

The results are presented of a study of the spectral distribution of photosynthetically active radiation (PAR) in some inland, and one coastal, waters in south-eastern Australia, carried out with a submersible spectroradiometer. There is particularly rapid attenuation with depth of blue light in the 400-500-nm waveband, due to the yellow substances ('gilvin', 'gelbstoff') in the waters. Attenuation in the red region, due to absorption by water itself, is clearly evident but is generally much less steep than that in the blue. Within the rather shallow euphotic zone typical of these waters the available PAR is impoverished in blue light but still contains plenty of red (630-700-nm) light. At greater depths, in waters of moderate turbidity, a spectral distribution strongly peaked at about 580 nm, with a shoulder at about 630 nm, is obtained.

Although the contribution of phytoplankton can be significant, in general in these turbid inland waters suspended soil particles contribute more to vertical light attenuation. This is partly due to the increased pathlength of the photons caused by scattering, but direct absorption of light, especially in the blue region, by the particulate inanimate 'tripton' is suggested by the data. Turbidity and dis- solved colour of the water tend to increase together: in particularly turbid, yellow waters the spectral distribution of PAR is shifted to longer wavelengths and in an extreme case consisted of quite a sharp peak at 700 nm.

In the clear, comparatively colourIess coastal-estuarine waters of Batemans Bay (N.S.W.), blue light was attenuated less steeply than red light, so that the underwater spectral distribution, although peaked at about 570 nm, was (at 4 m) still quite rich in blue as well as red light.