Sources and chemical character of dissolved organic carbon across 2 an alpine / subalpine ecotone , Green Lakes Valley

12 [1] We investigated how the source and chemical character of aquatic dissolved organic 13 carbon (DOC) change over the course of the runoff season (May–November, 1999) in 14 Green Lakes Valley, a high-elevation ecosystem in the Front Range of the Colorado Rocky 15 Mountains. Samples were collected on North Boulder Creek from four sites across an 16 alpine/subalpine ecotone in order to understand how the transition from the lightly 17 vegetated alpine to the forested reaches of the catchment influences aquatic DOC. 18 Concentrations of DOC at the four sites peaked between 2.6 and 8.9 mg C L 1 on the 19 ascending limb of the snowmelt hydrograph, with the higher concentrations at the two 20 subalpine sites. Seasonally, the chemical fractionation of DOC showed that there was a 21 large range (29–72%) in the fulvic acid content of DOC at the four sites, with the highest 22 fulvic acid percentages on the ascending limb of the hydrograph. Yields of DOC at the 23 two gauged sites ranged from 10 to 11 kg ha 1 over the season with between 45 and 50% 24 of the yield occurring as fulvic acid DOC and the remainder as nonhumic material. The 25 fluorescence properties of DOC from all four sites indicated that during peak runoff, 26 DOC was derived primarily from terrestrial precursor material. However, seasonal 27 changes in the fluorescence properties of fulvic acids at the highest elevation sites suggest 28 that DOC derived from algal and microbial biomass in the lakes is a more important 29 source of DOC above tree line during late summer and fall. We hypothesize that much of 30 the autochthonous DOC production is a result of algal growth in alpine lakes. Further, 31 comparison to a forested control catchment suggests that processes in the alpine reach of 32 the catchment may alter both the amount and chemistry of DOC incident to the 33 downstream subalpine aquatic system.