SOURCES AND SINKS OF DISSOLVED ORGANIC-CARBON IN A FORESTED SWAMP CATCHMENT

Concentrations of dissolved organic carbon (DOC) were measured in precipitation, throughfall, stemflow, and soil, peat and stream water in a 50 ha catchment with a central 5 ha swamp at Mont St. Hilaire, Quebec. DOC concentrations in precipitation were low (2.0 mg L-1), but increased in passage through the tree canopies as throughfall (9.1-14.6 mg L-1) and stemflow (23.1-30.1 mg L-1). For the period July 1-November 15, 1987, 0.5 g DOC m-2 was imported as precipitation, and forest canopies contributed a further 1.4-1.7 g m-2 to the soil surface. DOC concentrations were higher (46.0 and 67.6 mg L-1) in upland soil organic horizons, but decreased with depth because subsoil mineral horizons acted as a major sink of DOC. A laboratory experiment using leaf leachate revealed that subsoil horizons were able to adsorb DOC, with equilibrium DOC concentrations ranging from 3 to 19 mg L-1. Soil organic carbon appeared to be an important determinant of equilibrium DOC concentrations. The swamp was a major source of DOC, with an overall average DOC concentration of 58.6 mg L-1 and showed strong spatial and temporal variations related to hydrologic and thermal regimes. During base flow periods, stream DOC concentrations were small (< 3 mg L-1), dominated by water fed from springs draining upland soils. During high flows, stream DOC concentrations increased through the contribution of DOC-rich water originating in the swamp. Sources, sinks and transport of DOC are thus a function of a complex set of inter-related biotic and abiotic process.