Controls on Dissolved Organic Carbon Transport in Forested Headwater Streams.

Across spatial scales from headwater forests to receiving waters in coastal zones, stream loadings of dissolved organic carbon (DOC) have important influences on aquatic metabolism, transport of toxic metals, and drinking water quality.  We will discuss DOC in the context of the ecosystem processes that affect temporal and spatial patterns of DOC fluxes from catchments in headwater forests of the northern USA utilizing long-term data sets from hydrologically dissimilar systems in the northeastern and north central States. In addition, we will discuss how the sampling approach may affect DOC fluxes. For example, by comparing event-based to weekly sampling, we show that flow event and baseflow sampling is needed to accurately quantify stream DOC loadings because more than 50% of annual water and nutrient fluxes may occur during large magnitude storm flow and snowmelt events that occur less than 10% of the time.  When stream DOC fluxes were estimated from only weekly sample data, estimates were underestimated by up to 25 percent at a site in northeastern Vermont.  This result is an example of how hydrological processes regulate the flow of organic carbon from surficial sources in upland forest soils to streams.  In contrast, the hydrological connectivity of uplands is a less important control on DOC fluxes in the north central USA where contributions from peatlands dominate fluxes.  Although peatlands may dominate the overall flux of DOC, questions exist about the reactivity of peatland DOC and how important upland contributions of reactive DOC may be in chemical cycling, transformations and transport.  With stream DOC fluxes already responding to drivers of environmental change, we conclude by assessing how factors such as anthropogenic climate forcing, pollution, and forest cover change may affect the flow of DOC from source areas in catchments to streams.