Controls on Dissolved Organic Nitrogen (DON) in the Surface Waters of the Green Lakes Valley, Colorado Front Range

Hood, E W
INSTAAR and Dept. of Geography University of Colorado, Boulder CB 450 Boulder, CO 80309 United States

Mark W Williams
Department of Geography and Institute of Arctic and Alpine Research University of Colorado, Boulder


The role of dissolved organic matter in the biogeochemical evolution of surface waters in seasonally snow-covered catchments is still poorly understood in mountain areas of the western US. Here we report on the controls on dissolved organic nitrogen (DON) export in surface waters draining forested and alpine areas of the Green Lakes Valley in the Colorado Front Range. These include: landscape type, inorganic N, dissolved organic carbon (DOC), and C:N rations in terrestrial and aquatic environments. In alpine streams, DON concentrations were inversely related to inorganic N concentrations (r2 = 0.40, p$<$ 0.001, n = 39). A synoptic survey showed that tundra areas (n = 21) had significantly higher DON (5.3 然oles/L) and lower inorganic N (10.2 然oles/L) compared to DON (0.6 然oles/L) and inorganic N (21.3 然oles/L) in talus areas (n = 15). DOC concentrations were also higher in tundra streams (3.7 mg/L) compared to talus streams (1.0 mg/L). Weekly stream samples along a longitudinal transect 3,650 m to a mixed spruce and fir forest at 2,930 m showed that inorganic N concentrations decreased sharply to near detection limits from the alpine to subalpine zones while DON concentrations increased from near detection limits to about 5 然oles/L at the alpine/subalpine transition. A potential explanation for these patterns is that tundra and forested areas of the catchment are N-limited while talus areas are not. Consequently, mineralization rates are higher in tundra and forested areas, resulting in higher rates of export of recalcitrant dissolved organic matter from terrestrial to aquatic environments.