Ecological controls on the source and character of dissolved organic matter in an alpine/subalpine watershed, Green Lakes Valley, Colorado Front Range

Studies were conducted to identify the controls on DOM cycling at a series of sites across an alpine/subalpine ecotone during the 1998–2000 snowmelt seasons in the North Boulder Creek catchment in the Colorado Front Range. Dissolved organic nitrogen (DON) was a substantial portion of the total N load in streamwater accounting for approximately 10% of total N in the alpine and 45% of total N in the subalpine. Soil C:N ratios along the sample transect explained 78% of the variation in DOC concentrations and 70% of the variation in DON concentrations. Both soil C:N and streamwater DOC:DON ratios were useful predictors of NO₃-N export, explaining more than 50% of the variations in strearnwater NO₃-N concentrations. Additionally, export of NO₃-N from the alpine reach of the catchment appeared to subsidize the N balance of the downstream subalpine system. A comparison of DOM at an alpine and a subalpine site showed that the ratio of DOC:DON in strearnwater shifted seasonally with high values (40–55) during peak runoff in early summer and lower values (15–25) during low flows late in the runoff season. In terms of DOM export, hydrophobic fulvic acids were the primary carrier of DOC, while hydrophilic acids were the primary carrier of DON.; The fluorescence proper-ties of DOC from a selection sites indicated that during peak runoff DOC was derived primarily from terrestrial precursor material and that aquatic microbial material is a more important source of DOC above treeline during late summer and fall. Estimates based on fulvic acid fluorescence properties indicate that, in the alpine, lake ecosystems produced 40% of catchment DOC. On an areal basis, lakes produced 14 times more DOC than the surrounding terrestrial ecosystem. These results suggest that alpine lakes are hotspots of DOC production in high-elevation catchments and that high-elevation aquatic ecosystems may respond much faster than terrestrial ecosystems to increases in atmospheric deposition.; Preparative scale isolation of the two primary fractions of aquatic DOM, fulvic and hydrophilic acids, demonstrated that there were consistent chemical differences between DOM in the alpine and subalpine reaches of the catchment. In particular, organic acids collected in the alpine had a higher N content, were depleted in 13C, were enriched in 15N, and had a lower aromatic carbon content compared to organic acids collected in the forested subalpine.