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Underwater ultraviolet radiation: Development of spectral models for northern high latitude lakes

, : Underwater ultraviolet radiation: Development of spectral models for northern high latitude lakes. Photochemistry and Photobiology 65(1): 107-114

The penetration of solar ultraviolet radiation (UVR) and photosynthetically available radiation (PAR) was measured in a range of subarctic takes in the forest-tundra zone of northern Quebec. The diffuse attenuation coefficients for PAR (K-dPAR) were highly correlated (r-2 = 0.78) with dissolved organic carbon (DOC) concentration and only weakly correlated with suspended particulate material as measured by chlorophyll a (r-2 = 0.48) or beam transmittance (r-2 = 0.29). Colored dissolved organic matter (CDOM) was also largely responsible for the between-lake differences in spectral attenuation of UVR. The diffuse attenuation coefficient for UVR (K-d) was a nonlinear function of wavelength (lambda) and was accurately described by the model K-d (lambda) = K-d440 exp(-S (lambda-4401). The slope coefficient S was relatively constant among lakes (mean = 0.0151 nm-1, CV = 7%), whereas K-d440 was a linear function of several CDOM-related variables and best estimated by CDOM fluorescence (r-2 = 0.98). Numerical analysis of spectra for high (subarctic) and low (Arctic) DOC lakes showed that the evaluation of the model parameters K-d440 and S was insensitive to the bandpass characteristics (2-8 nm) of different underwater radiometers. The K-d (lambda) 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-1 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 high-latitude lakes and that the UVR/PAR balance in many of these waters is sensitive to minor changes in CDOM content.


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