Optical properties, of waters in the Murray-Darling Basin, South-eastern Australia

Murray-Darling Freshwater Research Centre

MDFRC item.

Apparent and inherent optical properties were determined for a range of water types in the Murray- Darling basin by using a combination of field and laboratory techniques. The absorption coefficient was calculated directly from in situ irradiance measurements of photosynthetically active radiation, whereas the scattering coefficient was determined from the irradiance measurements in conjunction with published nomograms relating the apparent and inherent optical properties. The validity of the nomograms for use in these waters was confirmed by comparing values of the average cosine calculated directly from in situ measurements with those estimated from the nomograms. These were closely correlated except for sites with chlorophyll concentrations greater than 200 mg m-3. The scattering coefficients estimated from the nomograms were approximately numerically equal to the turbidity in nephelometric turbidity units, but the variability of the relationship made it unsuitable for checking the validity of the nomograms. The relative role of dissolved and particulate components in the absorption and scattering of irradiance was examined by using spectrophotometry and linear regression of inherent optical properties on concentrations of the components. Estimates of specific absorption and scattering coefficients for algae, non-chlorophyllous suspended particles, and dissolved yellow colour were comparable to similar coefficients reported in the literature. The relative importance of these components to absorption and scattering varied considerably between sites and demonstrated the need for information on inherent optical properties in understanding the factors causing changes in optical water quality.