Export of DOC from forested catchments on the Precambrian Shield of Central Ontario: Clues from C-13 and C-14

Export of dissolved organic carbon (DOG) from forested catchments is governed by competing processes of production, decomposition, sorption and flushing. To examine the sources of DOG, carbon isotopes (C-14 and C-13) were analyzed in DOC from surface waters, groundwaters and soils in a small forested catchment on the Canadian Shield in central Ontario. A significant fraction (geater than 50%) of DOC in major inflows to the:lake is composed of carbon incorporated into organic matter, solubilized and flushed into the stream within the last 40 years. In contrast, C-14 in groundwater DOC was old indicating extensive recycling of forest floor derived organic carbon in the soil column before elution to groundwater in the lower B and C soil horizons. A small upland basin had a wide range in C-14 from Old groundwater values at baseflow under dry basin conditions to relatively modem values during high flow or wetter antecedent conditions. Wetlands export mainly recently fixed carbon with little seasonal range. DOC in streams entering the small lake may be composed of two pools; an older recalcitrant pool delivered by groundwater and a young labile pool derived from recent organic matter. The relative proportion of these two pools changes seasonally due the changes in the water flowpaths and organic carbon dynamics. Although changes in local climate (temperature and/or precipitation) may alter the relative proportions of the old and young pools, the older pool is likely to be more refractory to sedimentation and decomposition in the lake setting. Delivery of older pool DOC from the catchment and susceptibility of this older pool to photochemical decomposition may consequently be important in governing the minimum DOC concentration limit in lakes.