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

• Published in: Limnology and Oceanography
• Main Authors: Markager, Stiig, Vincent, Warwick F.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2000
• Subjects: Arctic
• Online Access: http://dx.doi.org/10.4319/lo.2000.45.3.0642; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.4319%2Flo.2000.45.3.0642; https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2000.45.3.0642

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: urn:x-wiley:19395590:media:lno20004530642:lno20004530642-math-0001 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.