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

Abstract

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)(λ) = K(d)(λ0)e(S(λ0-λ) + K(back), where K(d)(λ) is the diffuse vertical attenuation coefficient at wavelength λ 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 μm-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 μm-1) than in the sea (14.0 ± 0.4 μm-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 μm-1) and that the value obtained also depends on the method of calculation, the wavelength range, and the type of optical measurement.