Optical properties and reflectance spectra of 3 shallow lakes obtained from a spectrophotometric study

Abstract

A spectrophotometric method based on the use of an integrating sphere for measurement of absorbances of turbid (light scattering) suspensions has been applied to the estimation of the intrinsic optical properties of waters from Lakes Ngaroto, Rotoroa, and Hakanoa in the Waikato Basin. Measurements of absorbances both in the normal spectrophotometric configuration and with the integrating sphere allowed determination of the wavelength dependence of scattering and estimation of the scattering coefficient b(λ) as well as the absorption coefficient, a(λ). Filtered samples from the 3 lake waters exhibited absorption spectra which were featureless except for an exponential rise in a(λ) towards shorter wavelengths, evidently caused by the presence of dissolved/colloidal organic matter (yellow substance). The absorption spectra of particulate materials, concentrated from the lake water samples by a modification of the method of Kirk (1980), exhibited structure caused by superposition of the in vivo absorption spectra of algal photosynthetic pigments and the exponential absorption spectra of yellow substance chemically adsorbed by mineral particles. In all 3 lakes, light absorption in the visible by particulates was greater than by dissolved matter. Light scattering was apparently a more important contributor to total attenuation of a light beam than was light absorption. However, the spectral nature of light absorption was primarily responsible for giving structure to the visible reflectance spectra, which was manifested as lake‐water colour. The calculated chromaticity coordinates for synthesized reflectance spectra showed the 3 lakes to be dominantly yellow‐coloured, having a rather low degree of spectral purity which results in a brownish appearance. The chromaticity analyses were in reasonable agreement with the field descriptions of lake appearance.

Key words:

• optical properties
• colour
• water colour
• turbidity
• absorption
• algae
• transparency
• light
• Lake Ngaroto
• Lake Rotoroa
• Lake Hakanoa