Spectrofluorometric characterization of dissolved organic matter for indication of precursor organic material and aromaticity

We studied the fluorescence properties of fulvic acids isolated from stream s and rivers receiving predominantly terrestrial sources of organic materia l and from lakes with microbial sources of organic material. Microbially de rived fulvic acids have fluorophores with a more sharply defined emission p eak occurring at lower wavelengths than fluorophores in terrestrially deriv ed fulvic acids. We show that the ratio of the emission intensity at a wave length of 450 nm to that at 500 nm, obtained with an excitation of 370 nm, can serve as a simple index to distinguish sources of isolated aquatic fulv ic acids. In our study, this index has a value of similar to1.9 for microbi ally derived fulvic acids and a value of similar to1.4 for terrestrially de rived fulvic acids. Fulvic acids isolated from four large rivers in the Uni ted States have fluorescence index values of 1.4-1.5, consistent with predo minantly terrestrial sources. For fulvic acid samples isolated from a river , lakes, and groundwaters in a forested watershed, the fluorescence index v aried in a manner suggesting different sources far the seepage and streamfe d lakes. Furthermore, we identified these distinctive fluorophores in filte red whole water samples from lakes in a desert oasis in Antarctica and in f iltered whole water samples collected during snowmelt from a Rocky Mountain stream. The fluorescence index measurement in filtered whole water samples in field studies may augment the interpretation of dissolved organic carbo n sources for understanding carbon cycling in aquatic ecosystems.