The spectral distribution and attenuation of underwater irradiance in Tasmanian inland waters

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SUMMARY. 1. Measurements were made of the attenuation and spectral distribution of downwelling and upwelling photosynthetically‐available radiation (PAR) in all the principal types of natural waters found in Tasmania.2. Most lakes in the State are clear and non‐turbid, with water itself and the low concentrations of gilvin being the principal determinants of the green underwater light climate. Many others are deeply coloured by dissolved and colloidal organic material (gilvin, gelbstoff) which rapidly attenuates short wavelengths, specifying a shallow, predominantly red euphotic zone.3. A spectrophotometric measure of colour, the absorption coefficient at 440 nm, is statistically related to measurements on the platinum scale with good precision.4. Few Tasmanian lakes are turbid but in those that are the underwater light climate is almost identical to that of non‐turbid, humic lakes.5. Reflectance, R, varied with depth but not in the asymptotic way previously encountered. A linear relationship existed between the scattering coefficient, b, and nephelometric turbidity, but not at the approximate 1:1 ratio reported elsewhere.6. Most Tasmanian lakes are oligotrophic or dystrophic and phytoplankton rarely influenced the underwater light field.7. Seasonal variation in optical character is not great in natural lakes and their optical properties and light fields can be used typologically.8. Simple and multiple regression analysis showed that Secchi depth was a poor predictor of euphotic depth but the optical properties and the underwater light field of inaccessible lakes could be reasonably predicted from laboratory measurements made on small water samples, using regressions developed for a wide range of lake types and by reference to the quantaradiometric scans of lakes with comparable optical properties.9. An optical classification of Tasmanian lakes made by cluster analysis agreed reasonably well with one based on edaphic, vegetational and chemical criteria.