Concentrations and quality of DOC along the terrestrial–aquatic continuum in a boreal forested catchment

Abstract

C cycling and dissolved organic C (DOC) inputs to boreal aquatic systems probably will change substantially with climate change. DOC concentrations already are increasing in surface waters. Terrestrial C is a major source of C to boreal freshwater ecosystems, but the interface between these 2 ecosystems, the riparian zone, has not been studied often. To improve our understanding of the importance of terrestrial inputs of DOC to aquatic systems from surrounding forests, we followed the changes of DOC concentration along a continuum of precipitation, throughfall, soil water, ground water, lake, and brook water in a pristine, boreal, forested headwater catchment and developed a lake C balance based on terrestrial and lacustrine C fluxes. We also examined DOC quality changes using the ratio of absorbance at 465 and 665 nm (E4/E6). DOC concentrations increased from 2.4 mg/L in precipitation to 132.3 mg/L in soil water as water passed through the terrestrial ecosystem. DOC concentrations in the riparian zone were correlated with DOC concentrations in the adjacent outflowing brook but not in the headwater lake. E4/E6 ratios indicated that the DOC in precipitation and throughfall was dominated by higher molecular weight compounds and that the DOC in soil and ground water was dominated by lower molecular weight compounds. The input of terrestrial DOC to the aquatic ecosystem was estimated to be 5 to 13 g C m−2 y−1, which is small compared with the C fluxes between atmosphere and vegetation, but can significantly decrease the net ecosystem exchange of an old-growth forest catchment. Terrestrial DOC was a major source of C in the lake, rendering it heterotrophic. The DOC export (3 g DOC m−2 y−1) made up almost 70% of total C export.