Changes in the character of stream water dissolved organic carbon during flushing in three small watersheds, Oregon

Abstract

The hydrologic and biogeochemical responses of forested watersheds to inputs of rainfall and snowmelt can be an indicator of internal watershed function. In this study, we assess how the quantity and quality, both chemical and spectroscopic, of stream water DOC changes in response to a 6‐day storm event during the wet season of 2003 in three small (<1 km2) basins in the H. J. Andrews Experimental Forest, Oregon. The watersheds included one old‐growth watershed (WS02) and two previously logged watersheds (WS01 and WS10). Prestorm concentrations of DOC ranged from 1.5 to 2.2 mg C L−1 in the three watersheds and increased approximately threefold on the ascending limb of the storm hydrograph. Concentrations of DOC were both highest in the unharvested, old‐growth watershed. The specific UV absorbance (SUVA, 254 nm) of DOC in the three watersheds increased by 9 to 36% during the storm, suggesting that DOC mobilized from catchment soils during storms is more aromatic than DOC entering the stream during baseflow. The increase in SUVA was most pronounced in the previously harvested catchments. Chromatographic fractionation of DOC showed that the percentage of DOC composed of non‐humic material decreasing by 9 to 22% during the storm. Shifts in the fluorescence properties of DOC suggest that there was not a pronounced change in the relative proportion of stream water DOC derived from allochthonous versus autochthonous precursor material. Taken together, these results suggest that spectroscopic and chemical characterization of DOC can be used as tools to investigate changing sources of DOC and water within forested watersheds.