Calculation of spectral weighting functions for the solar photobleaching of chromophoric dissolved organic matter in temperate lakes

The effect of solar radiation on the dissolved absorption coefficient (a(CD OM)[lambda]), which reflects the concentration of chromophoric dissolved or ganic matter (CDOM), was investigated in several lakes near Bariloche, Arge ntina and in northeastern Pennsylvania, USA. Samples of 0.2 mum filtered la ke water were exposed in quartz tubes to different portions of the solar sp ectrum using optical cutoff filters to remove parts of the ultraviolet (UV) region of the solar spectrum. Changes in the spectral absorption in these samples and the absorbed incident energy were used to calculate spectral we ighting functions (SWFs) for the photobleaching (PB) of CDOM. PB was measur ed as the loss of a(CDOM)(lambda) (the a(CDOM)[lambda] was averaged from 28 0 to 500 nm) per unit absorbed energy. CDOM from humic and clear lakes, as well as from a Sphagnum bog and an algal culture, was used in the experimen ts covering a wide range of carbon sources, We used an iterative, nonlinear optimization method to fit the measured results to a simple exponential fu nction in order to generate each SWF. Comparing individual SWFs calculated for various CDOM sources, we computed a summary SWF from the experiments us ing epilimnial CDOM from our study lakes. Our summary SWF was able to expla in 80-90% of the observed variance in our exposure experiments, and we were able to predict PB results obtained for other Argentine lakes (mean error 14.5%). Finally, we calculated that the effect of UV-B radiation on PB was small (< 20% of total decrease in the absorption coefficient) compared to U -VA and blue light radiation. This suggested that increased UV-B radiation due to stratospheric ozone depletion would not greatly increase the photobl eaching of whole water column CDOM in Patagonian lakes (< 10%).