Long-Term Trends in DOC Concentrations and Fluxes in a Southern Appalachian Headwater Stream

Abstract:

Dramatic increases in dissolved organic carbon (DOC) of stream water have been reported for aquatic ecosystems of the Northern Hemisphere and have been attributed variously to global warming, recovery from acid rain, or to altered hydrologic connections between watersheds and receiving streams. Here, we analyzed one of the longest continuous records of stream water DOC available in the Southeastern US. The record comes from a forested headwater stream at Coweeta Hydrologic Laboratory in the Southern Appalachians (NC, USA). In contrast to the increasing DOC trends reported for northern temperate watersheds, we observed steep declines in both the volume-weighted concentrations of stream DOC (43% decline) and DOC fluxes (55% decline) between 1988 and 2005. Annual mean runoff declined by 38%, which we attributed mainly to a 47% decline in baseflow during the study period. Increased soil [SO₄^2-] and ionic strength of soil water indicate that soils within the watershed are becoming more acidic through time. Together, these results suggest that the dramatic decline in DOC concentrations can be attributed to: 1) a decline in runoff, which affected the mobilization of DOC from uplands to the stream, and 2) gradual soil acidification, which probably restricted the formation of DOC within the watershed. Declining DOC in headwater streams has implications not only for carbon and other nutrient cycles but also for the health of aquatic habitats throughout this region. More broadly, these results emphasize that long-term trends in DOC may differ for streams across the Northern Hemisphere. Long-term datasets have the potential to reveal the range of these trends and the underlying processes that drive them.