UNDERWATER ULTRAVIOLET RADIATION : DEVELOPMENT OF SPECTRAL MODELS FOR NORTHERN HIGH LATITUDE LAKES

Ë :. ABSTRACT The penetration of solar ultraviolet radiation ([IVR) and photosynthetically available radiation (PAR) was measured in a range of subarctic lakes in the forest-tundra zone of northern Québec. The diffuse attenuation coefficients for PAR (Kd*AJ were highly correlated (l = 0.78) with dissolved organic carbon (DOC) concentration and only weakly correlated with suspended particulate material âs measured by cNorophyll a (fl = 0.48) or beam transmittance (f = 0.29). Colored dissolved organic matter (CDOM) was also largely responsible for the between-lake differences in spectral attenuation of LIVR. The diffuse attenuation coefficient for trvR (fJ was a norùinear function of wavelength (À) and was accurately described by the model Ko ()t') = K6a* exp(-S tÀ-,1401). The slope coefficient S was relatively constant âmong lakes (mean = 0.0151" -tt-r, CV = 77o\, whereas K6aa6 was a linear function of several CDOM-related variables and best estimated by CDOM fluorescence (l = 0.98). Nurnerical analysis of spectra for high (subarctic) and low (Arctic) DOC lakes showed that the evaluation of the model parameters K6aa6 and S was insensitive to the bandpass characteristiçs (2-A nm) of different underwater radiometers. The Ka (À) model was then used to develop a nondimensional index of relative spectral composition (RI) to characterize different water masses as a function of dissolved organic matter (DOC and CDOM fluorescence). Below about 4 mg DOC L-r there is a sharp nonlinear rise in this index with decreasing DOC. These results show that CDOM controls the spectral composition of underwater UVR in northern highJatitude lakes and that the UVW PAR balance in many of these waters is sensitive to minor changes in CDOM content.

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