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

TLDR

Fluorescence index measurements in filtered whole water can help interpret dissolved organic carbon sources and improve understanding of carbon cycling in aquatic ecosystems. The study aims to demonstrate that the 450/500 nm emission intensity ratio, excited at 370 nm, serves as a simple index to differentiate sources of isolated aquatic fulvic acids. The authors measured fluorescence of fulvic acids isolated from terrestrial‑dominated streams and rivers and microbial‑dominated lakes, using a 370 nm excitation to calculate the 450/500 nm emission intensity ratio. Microbial fulvic acids exhibit a sharper, lower‑wavelength emission peak and a higher 450/500 nm ratio (~1.9) versus terrestrial fulvic acids (~1.4); river samples average 1.4–1.5, watershed samples show source variability, and similar fluorophores were detected in Antarctic desert oasis lakes and Rocky Mountain stream snowmelt.

Abstract

We studied the fluorescence properties of fulvic acids isolated from streams and rivers receiving predominantly terrestrial sources of organic material and from lakes with microbial sources of organic material. Microbially derived fulvic acids have fluorophores with a more sharply defined emission peak occurring at lower wavelengths than fluorophores in terrestrially derived fulvic acids. We show that the ratio of the emission intensity at a wavelength 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 fulvic acids. In our study, this index has a value of ~1.9 for microbially derived fulvic acids and a value of ~1.4 for terrestrially derived fulvic acids. Fulvic acids isolated from four large rivers in the United States have fluorescence index values of 1.4–1.5, consistent with predominantly terrestrial sources. For fulvic acid samples isolated from a river, lakes, and groundwaters in a forested watershed, the fluorescence index varied in a manner suggesting different sources for the seepage and streamfed lakes. Furthermore, we identified these distinctive fluorophores in filtered whole water samples from lakes in a desert oasis in Antarctica and in filtered 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 carbon sources for understanding carbon cycling in aquatic ecosystems.