Sources of dissolved organic matter (DOM) in a Rocky Mountain stream using chemical fractionation and stable isotopes

Dissolved organic matter (DOM) is an important vehicle for the movement of nutrients from terrestrial to aquatic systems. To investigate how the source and composition of aquatic DOM change in both space and time, we used chemical, spectroscopic, and isotopic analyses to characterize DOM in a headwater catchment in the Colorado Front Range. Streamwater samples for DOM analyses were collected from 2 sites, a lightly vegetated alpine site and a forested, subalpine site, in the North Boulder Creek catchment during the snowmelt runoff season (May–September). Concentrations of dissolved organic carbon (DOC) peaked on the ascending limb of the snowmelt hydrograph at both the alpine (2.6 mg C l⁻¹) and the subalpine sites (7.0 mg C l⁻¹) and decreased sharply on the descending limb of the hydrograph. Fractionation of DOM into operationally defined humic and non-humic components showed that the fulvic acid content of DOC decreased through the season at both sites and that spectroscopic (fluorescence and ultraviolet) properties of the humic DOM fraction shifted in a manner consistent with an increase in the proportion of humic DOM derived from instream sources as compared to terrestrial catchment sources. Humic and non-humic fractions of DOM isolated near peak flow in June and during low flows in September showed a seasonal enrichment in ¹⁵N and ¹³C as well as a seasonal decrease in the ratio of aromatic to aliphatic carbon, both of which were correlated with a decrease in the C:N ratio of the DOM fractions. These results suggest that seasonal shifts in the isotopic and chemical characteristics of DOM are a result of changes in catchment sources of DOM. In particular, it appears that DOM production in alpine lakes is an important contributor to the streamwater DOM load during late season low flows, especially in the alpine reach of the catchment. Our results further suggest that stable isotopes of C and N are useful tools, particularly when combined with ancillary data such as elemental analyses and catchment discharge, for evaluating sources and transformations of DOM at the catchment scale.