Spectral adsorption properties of natural watres: contribtion of the soluble and particulate fractions to light absorption in some inland watres of south-eastern Australia

Abstract

A method has been developed for measuring the absorption spectra, and calculating the in situ absorption coefficients, of the particulate fraction (tripton/phytoplankton) of natural waters. The procedure involves concentrating the particulate fraction by filtration followed by resuspension in a smaller volume, and measuring the absorption spectrum with the help of an integrating sphere to minimize the effects of light scattering. The method has been applied to seven inland water bodies in the southern tablelands of New South Wales and the Australian Capital Territory. For comparative purposes the absorption spectra. and in situ absorption coefficients, of the soluble colouring matter (gilvin, gelbstoff) have also been measured. The absorption coefficients have been used to calculate what part of the total absorbed photosynthetically active radiation (PAR) is captured by each of the three major absorbing components. In clear but rather coloured waters, gilvin absorbs most of the quanta, followed by water itself. In two waters with fairly high phytoplankton levels, the particulate fraction absorbs as much PAR as the water, but gilvin still absorbs rather more than either. In highly turbid waters, the particulate fraction (consisting mainly of tripton) absorbs most of the quanta. The data show that tripton has an absorption spectrum in the visible region. which rises steadily with decreasing wavelength: this is attributed to humic materials. It is clear that in turbid waters the inanimate particulate fraction (tripton) is a major light absorber, in addition to being the most important light scatterer.