Hydro-climatological influences on long-term dissolved organic carbon in a mountain stream of the Southeastern United States

In the past decade, significant increases in surface water dissolved organic carbon (DOC) have been reported for large aquatic ecosystems of the Northern Hemisphere and have been attributed variously to global warming, altered hydrologic conditions, and atmospheric deposition, among other factors. We analyzed a 25-yr DOC record (1988-2012) available for a forested headwater stream in the United States and documented two distinct regimes of stream DOC trends. From 1988 to 2001, annual mean volume-weighted DOC concentration (DOCvw, mg L-1) and annual DOC flux (kg ha-1 yr-1) declined by 34 and 56%, respectively. During 1997 to 2012, the decline in DOCvw and DOC flux increased by 141 and 165%, respectively. Declining DOCvw from 1988 to 2001 corresponded to a decline in growing season runoff, which has the potential to influence mobilization of DOC from uplands to streams. Increasing DOCvw from 1997 to 2012 corresponded to increased precipitation early in the growing season and to an increase in the number and intensity of shortduration fall storms capable of mobilizing long-accrued DOC from forest litter and soils. In contrast, total annual runoffdeclined throughout the period. Rising air temperature, atmospheric acid deposition, and nitrogen depositions did not offer any plausible explanation for the observed bidirectional annual trends of stream DOCvw. Our study highlights the critical role of long-term datasets and analyses for understanding the impacts of climate change on carbon and water cycles and associated functions of aquatic and terrestrial ecosystems.