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

Abstract

The effect of solar radiation on the dissolved absorption coefficient (a CDOM []), which reflects the concentration of chromophoric dissolved organic matter (CDOM), was investigated in several lakes near Bariloche, Argentina and in northeastern Pennsylvania, USA. Samples of 0.2 m filtered lake water were exposed in quartz tubes to different portions of the solar spectrum 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 weighting functions (SWFs) for the photobleaching (PB) of CDOM. PB was measured as the loss of a CDOM () (the a CDOM [] was averaged from 280 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 experiments covering a wide range of carbon sources. We used an iterative, nonlinear optimization method to fit the measured results to a simple exponential function in order to generate each SWF. Comparing individual SWFs calculated for various CDOM sources, we computed a summary SWF from the experiments using epilimnial CDOM from our study lakes. Our summary SWF was able to explain 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 (Ͻ20% of total decrease in the absorption coefficient) compared to UV-A and blue light radiation. This suggested that increased UV-B radiation due to stratospheric ozone depletion would not greatly increase the photobleaching of whole water column CDOM in Patagonian lakes (Ͻ10%).

Photochemical production of hydrogen peroxide and methylhydroperoxide in coastal waters

“…This approach, calculation of a spectral weighting function (SWF) using the form of Eq. (10), is similar to prior studies that examined the wavelength dependence of absorption coefficients (Kirk, 1994;Vodacek et al, 1997;Osburn et al, 2001) and photochemical quantum yields for chemical species (Johannessen and Miller, 2001).…”

“…The parameter C was an offset value, and was subtracted from the entire absorption spectrum. Photobleaching of CDOM for the photolysis experiments was characterized as the average loss of absorbance (L avg , m À 1 ) over the range of 280 to 500 nm (Osburn et al, 2001).…”

“…Apparent quantum yields for photobleaching (AQY) were calculated for each surface water sample using procedures described by Osburn et al (2001). Calculating photobleaching apparent quantum yields using the average loss of absorbance (L avg ), as in this work, sacrifices wavelength specific information.…”

Determination of apparent quantum yield spectra for the formation of biologically labile photoproducts

“…This long-pass cutoff filter approach has been used successfully on seawater samples in the determination of AQY spectra for dissolved inorganic carbon (DIC) (Johannessen and Miller 2001) and CO (Ziolkowski 2000). Similarly, Vähälato and Salonen (2000) used the radiation filtering effects of natural lake waters during in situ incubations in place of defined cutoff filters to produce different spectral irradiation for use in calculating AQY spectra for DIC photoproduction, and Osburn et al (2001) used cutoff filters to calculate spectral weighting functions for DOM bleaching.…”

Increases in the longwave photobleaching of chromophoric dissolved organic matter in coastal waters

“…Moreover, CDOM photobleaching is a function of iron availability and photochemistry (Gao and Zepp 1998;White et al 2003). Osburn and Morris (2003) suggested that CDOM photoreactivity can increase with salinity across an estuarine gradient, but Minor et al (2006) found a decrease in CDOM photobleaching at 280 nm when humic CDOM was added to an artificial salinity gradient used to mimic coastal mixing. Although many studies have investigated factors controlling CDOM photoreactivity, discrepancies in results can arise from different metrics used to define CDOM photoreactivity (Hu et al 2002).…”