Spectral slope is a term used to parameterize featureless absorbance spectra according to their shape. Spectral slope is obtained by calculating the slope of the log-linearized absorption spectrum over a given range of wavelengths. Past studies have shown that spectral slope is related to molecular size in fulvic acids. A simple method of spectral analysis that compares spectral slopes obtained from two distinct regions of the UV spectra of aquatic dissolved organic matter is demonstrated. The ratio of these slopes (RS) shows considerable change during photo-oxidation, variation within estuaries, and substantial shifts with depth in the upper 1000 m of open ocean waters. Evidence is presented that these variations in RS are strongly related to molecular size shifts within dissolved organic matter (DOM) in a water sample. UV-visible spectrophotometric analysis of 0.2 mum filtered, estuarine waters coupled with stirred cell ultrafiltration and subsequent analysis of the size fractions show that the RS parameter exhibits significant correlation with shifts in molecular weight distribution that occur during photo-oxidation of DOM and during the mixing of high-molecular weight (HMW) terrigenous DOM and low-molecular weight (LMW) marine DOM. The RS parameter is applicable to natural waters as diverse as the Great Dismal Swamp and the Saragasso Sea, acting as a qualitative or semi-quantitative indicator of molecular weight and DOM source. In addition, RS is a faster and simpler tool than fluorescence excitation emission matrices (EEMs), which have been proposed as a means to determine the source of ballast water in ships from foreign ports. RS can serve as a quick screening step for determining which samples need to be examined using EEMs or other methods.
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