Export of dissolved organic carbon (DOC) from forested catchments is governed by competing processes of production, decomposition, sorption and flushing. To examine the sources of DOC, carbon isotopes (14C and13C) 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 (greater 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,14C 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 in14C from old groundwater values at baseflow under dry basin conditions to relatively modern 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.

Dissolved inorganic carbon (DIC) is the main acid buffer in forested lake watersheds in Canada. We usedcarbon isotopes (13C, 14C) to evaluate the production and cycling of DIC in an acid-sensitive lake watershed of thePrecambrian Shield. Soil C02, groundwater and stream DIC were characterized chemically and isotopically. Soil CO2 concentration profiles reflect both changes in production and in losses due to diffusion. S13C soil CO2 profiles (b13C values of -23%o in summer, slightly enriched during the fall and -25%o during the winter) are a reflection of the isotopic composition of the sources and changes in isotopic fractionation due to diffusion. Carbon isotopic composition (13C, 14C) of the groundwater and stream DIC clearly indicate that weathering of silicates by soil CO2 is the main source of DIC in these watersheds. 14C data show that, in addition to recent groundwater, an older groundwater component with depleted 14C activity is also present in the bedrock. The carbon isotope pattern in the groundwater also implies that, besides the main springtime recharge events, contributions to the groundwater may also occur during late winter/early spring.