Hydrologic Controls on In-Stream Optical Dissolved Organic Matter Characteristics in an Old-Growth Forest of the Oregon Cascades

Dissolved organic matter (DOM) is a critical component of the carbon cycle linking terrestrial and aquatic ecosystems, yet DOM composition representative of DOM sources at headwater catchments in the western U.S is poorly understood. This study examined the effect of forest management history and hydrologic patterns on DOM chemistry at nine experimental watersheds located in the H.J. Andrews Long Term Ecological Research Experimental Forest of the Oregon Cascades. Stream water samples representing a three-week composite of each watershed were collected between May 2013 and February 2015 (32 events). DOM chemistry was characterized by examining UV and fluorescent properties of stream samples. Specific UV absorbance at 254 nm (SUVA254; Weishaar et al. 2003), generally indicative of aromaticity, showed the lowest value at the high elevation clear-cut site (watershed 6, 1,030 m) and the highest value at the low elevation clear-cut site (watershed 10, 680 m) throughout the study period. DOM fluorescent components, identified by this study using a multivariate statistical model, Parallel Factor Analysis (PARAFAC), did not differ significantly among experimental watersheds with varying forest management history. However, a protein-like DOM component exhibited temporal variations. Correlation analysis between the protein-like DOM and hydrologic patterns indicate that stream water during dry seasons come from protein-rich groundwater sources. This study shows UV and fluorescent spectroscopy DOM characterization is a viable finger printing method to detect DOM sources in pristine headwater streams at the western Cascades of Oregon where characterization of the stream water source with low DOC and DON concentrations is difficult.