DETERMINING CARBON FLUX IN A SUB-ALPINE FOREST ECOSYSTEM, ANIMAS RIVER WATERSHED, SAN JUAN MOUNTAINS, COLORADO

The carbon flux from forest ecosystems may be an important climate change indicator. This study was initiated to better understand the role of dissolved organic carbon (DOC) in forest systems, to evaluate the stability of DOC in soils derived from weathering of varying bedrock compositions, and to establish baseline data for future comparisons. Samples from 22 surface waters, 10 seeps and springs, and 2 drill holes were analyzed for DOC. Samples were collected using a stainless steel filter apparatus and with pre-baked, ~ 0.7 - µm glass fiber filters, acidified with organic free HCl, filtered to pre-baked amber glass bottles, and analyzed with a Shimadzu DOC analyzer. Four samples were collected at continuously monitored hydrologic gages, with discharge data that will permit DOC fluxes to be calculated. Initial sampling was completed in June and September, 2008 to coincide with peak snowmelt runoff (1500 ft³/s) and low flow (200 ft³/s) recorded at the “Animas River gage Below Silverton”. DOC abundances were relatively low for the upper Animas River watershed near Silverton, Colo. (1.5 mg/L, average). Median DOC values (mg/L) in June were: streams (0.9), n = 17; seeps (1.2), n = 10; wells (0.45), n = 4. DOC values (mg/L) in September were: streams (0.7), n = 17; seeps (0.75), n = 6; wells (0.5), n = 5. Median DOC values decreased between June and September for seeps (0.45 mg/L), and for streams (0.2 mg/L). The data also suggest a correlation between DOC flux and bedrock alteration type. Propylitic catchments exhibited higher stream DOC concentrations in spring (median 1.5 mg/L), and lower concentrations in fall (median 0.7 mg/L). Catchments that contain soils derived from weathering of acid sulfate and quartz-sericite-pyrite altered bedrocks have consistent median DOC concentrations of 1 mg/L between high- and low-flow. The higher DOC concentrations in Spring runoff for propylitic catchments may be indicative of a higher soil microbial and vegetation productivity compared to areas affected by acidic weathering found in the more highly altered and mineralized terrain. Additional studies are investigating if soils derived from propylitic rocks may be more efficient at sequestering carbon once spring runoff is stabilized. Ongoing DOC hydrograph sampling and analysis will allow quantitative, watershed DOC flux determinations.