Spectral light attenuation and the absorption of UV and blue light in natural waters

The spectral pattern of light attenuation in the ultraviolet (UV) and blue region (360-500 nm) was analyzed for seven high Arctic lakes. The best description of K-d versus wavelength was obtained with an exponential model similar to the model used for absorption by chromophoric dissolved organic compounds (CDOM), but with an additional parameter (K-back) that accounts for background scattering: K-d(lambda) = K-d(lambda(0))e(S(lambda 0-lambda)) + K-back, where K-d(lambda) is the diffuse vertical attenuation coefficient at wavelength lambda and S is the exponential slope parameter that characterizes the decrease in attenuation with increasing wavelength. The inclusion of the background parameter gave a significantly better fit and eliminated the systematic deviations over the spectrum that occur in the absence of K-back. The resultant S values (mean = 17.4 mu m(-1)) were on average 47% higher than values calculated without a background parameter, and were more sensitive to between-lake differences. The same pattern was found when S was estimated from spectral K-d values in three literature data sets, so the new equation will generally increase the estimated values of S based on K-d and provide a more accurate guide to intersystem variability. A compilation of literature data for S (based on K-d and the new equation or on absorbance) showed that its mean value (+/-SE) is significantly higher in freshwaters (17.1 +/- 0.7 mu m(-1)) than in the sea (14.0 +/- 0.4 mu m(-1)). The variability in S was highest for low values of K-d or a (340 nm values below 3 m(-1)). This pattern may be due to instrument-related problems or more likely represents the real variability in the optical properties of CDOM in low colored systems. The analysis showed that S can vary significantly between and within systems (overall range = 9.2-36.2 mu m(-1)) and that the value obtained also depends on the method of calculation, the wavelength range, and the type of optical measurement.