Shades of Brown: Changes in dissolved organic matter (DOM) with storm-event and seasonal hot moments

Abstract:

Storm events and seasonal “hot moments” can produce sudden changes in dissolved organic matter (DOM) - both for bulk concentrations as well as individual DOM constituents. Such episodes can also yield disparate responses for individual DOM moieties which may not be apparent otherwise. Here, we highlight key shifts in stream water DOM in response to episodic events. The study site is an intensively-studied forested watershed (12 ha) located in the Piedmont region of Maryland, USA. DOM has been characterized for stream water and 10 distinct watershed DOM sources over a period of seven years (2008-2014). Stream water sampling was performed for storms using automated ISCO samplers and manual grab samples during baseflow. The composition of DOM was characterized using a variety of UV and fluorescence metrics. Data were also available from an in-stream, high-frequency (15-30 minutes) UV-vis based sensor. Hydrologic flow paths for stream runoff were identified using an end-member mixing model. While small to moderate storms yielded sharp increases in stream water dissolved organic carbon (DOC) concentrations and aromatic and humic constituents, a pronounced dilution of DOC was observed for peak flows associated with tropical storms. This dilution pattern did not, however, extend to aromatic and humic DOM constituents suggesting that while rainfall input may outpace DOC supply, the composition and/or sources of DOM may not change. Usually, stormflow also resulted in a decrease in the proportion of protein-like DOM, but a different response was observed for storms following dry, hydrologically-disconnected periods that produced a sharp increase in protein-like DOM. The increase was attributed to the “first-flush” of labile DOM that had mineralized and accumulated in streams over the preceding dry period. Similar pulse increases in protein-like DOM for stream runoff were also observed for a few low-intensity, long-duration storms that occurred early in autumn. This DOM response likely resulted from leaching of DOM from freshly-fallen leaves. Understanding these “hot moments” of DOM provides crucial insights into the leaching kinetics and sensitivity of individual DOM chemical species and can help separate out the various mechanisms that regulate the supply and transport of DOM within watersheds.