Reconciling stream dissolved organic matter with snowmelt-driven subsurface flowpaths in a montane, headwater catchment.

Abstract:

Dissolved organic matter (DOM) transport is a key biogeochemical link across the terrestrial-aquatic interface in headwater catchments, and therefore is intimately linked with the hydrologic connectivity of the catchment to the stream. This study evaluated the mobility of specific chemical constituents of DOM during snowmelt in a montane, semi-arid catchment in the Boulder Creek CZO. Dissolved organic matter quality was monitored using fluorescence spectroscopy on a daily basis from water sampled from the soil (10 – 25 cm depth) via tension lysimeters and from the stream during snowmelt and was compared to approximately bi-monthly groundwater samples (~18 m depth). In the stream, a transition occurred from fluorescent DOM (FDOM) being dominated by protein-like material to FDOM being dominated by more humic-like material. The FDOM in the interstitial water of the soils and the groundwater did not change in character. Thus, the stream transition is indicative of an engagement of DOM originating from hillslope soils during snowmelt. Dissolved organic carbon (DOC) normalization of these fluorescent loadings suggest that the peak in DOC concentration seen in the stream is mainly controlled by the non-fluorescent fraction of DOM. These results indicate that shifts in hydrologic connectivity of different watershed units to the stream are a major control on DOM export from the watershed.