Optical absorption and fluorescence properties of chromophoric dissolved organic matter in natural waters

Abstract

Complete optical absorption and fluorescence spectra were collected for a diverse suite of 0.2-pmfiltered marine, riverine, and estuarine waters, as well as for colored dissolved organic matter (CDOM) isolated from several of thcsc waters by solid-phase C,, extraction. Absorption and fluorescence parameters for these samples arc reported. For surface waters, variations in the fluorescence quantum yields obtained with 355-and 337-nm excitation fell within a narrow window (<2.5-fold variation about the mean values), demonstrating that fluorescence measurements can be used to determine absorption coefficients of CDOM in the ultraviolet region with reasonably good accuracy. Methods for predicting absorption coefficients and line shapes from the fluorescence data arc introduced and tested. The absorption and fluorescence spectra of CDOM extracted from some seawaters differed significantly from those of the original waters, demonstrating that material isolated by hydrophobic adsorption is not necessarily representative of the suite of colored organic matter present in aquatic systems. These results clearly illustrate that great care must be taken when extracted material is used to infer the optical properties of natural waters.

Photogeneration of singlet oxygen and free radicals in dissolved organic matter isolated from the Mississippi and Atchafalaya River plumes

“…Small changes were also seen in the zero salinity samples, with the February samples giving a slightly lower S. Our values fell within the range of 0.0159-0.0191 nm y1 , with a mean of 0.0173 " 0.0007 nm y1 . These values are consistent with those reported for coastal waters Ž by other studies Blough et al, 1993;Green and . Blough, 1994;Vodacek et al, 1997 which used similar wavelength regions in determination of S.…”

“…Because the values along the transects changed only a small amount, this could indicate that singlet oxygen production is independent of terrigenous or marine origin of the CDOM. However, it is interesting to note that the optical absorption S values of coastal waters, both from our data and others, Ž tend to increase at higher salinities Blough et al, 1993;Green and Blough, 1994;Vodacek et al, . 1997 , indicating a higher percentage of small molecular weight material in the bulk CDOM at the Ž high salinities Strome and Miller, 1978;Green and .…”

“…Mopper et al 1996 observed similar changes for productive, mid-latitude coastal water, concentrated with Amicon filters as used in the present study. The change in S arising in the concentration step, during which lower molecular mass material is lost, can be explained if it is assumed that the CDOM fraction of smaller size absorbs more in the shorter wavelengths ŽStrome and Miller, 1978;Green and Blough, 1994;. Mopper et al, 1996 .…”

Untitled

“…The detection limits of most laboratory spectrophotometers equipped with 10-cm cells are insufficient to measure low concentrations of CDOM present in many shelf and open-ocean waters (Miller et al 2002;Nelson and Siegel 2002). Extraction and concentration of CDOM from natural waters had been used to overcome Analytical intercomparison between type I and type II longpathlength liquid core waveguides for the measurement of chromophoric dissolved organic matter problems associated with low concentrations of CDOM (Carder et al 1989;Green and Blough 1994). However, these methods suffered from problems associated with sample handling (concentration and dilution) and preferential concentration of components of the CDOM pool rather than the entire suite of CDOM present in aquatic systems.…”

“…The standard method for determining CDOM absorption coefficient spectra used conventional spectrophotometers equipped with a 10-cm quartz cell to measure the optical density of filtered seawater samples relative to purified fresh water (Green and Blough 1994;Vodacek et al 1997) or a 25-cm-pathlength in situ spectrophotometer (WETLabs, ac-9; e.g., Twardowski et al 1999). The detection limits of most laboratory spectrophotometers equipped with 10-cm cells are insufficient to measure low concentrations of CDOM present in many shelf and open-ocean waters (Miller et al 2002;Nelson and Siegel 2002).…”