The character and bioactivity of dissolved organic matter at thaw and in the spring runoff waters of the arctic tundra north slope, Alaska

The brief spring thaw period in the arctic is an important time when carbon is transferred from terrestrial to aquatic systems as melting snow and soil waters run off into the streams, lakes, and ocean. Measurements were made of the quantity and quality of dissolved organic carbon (DOC) in thawing soil and runoff waters in the foothills region of the Kuparuk River basin of arctic Alaska, during the thaw of May 1996. Incubations were performed using DOC and soil cores at the time of thaw from the surface layers of two major tundra types of the region, nonacidic and acidic tundra. Results indicated that there are major changes in both the quantity and quality of DOC as soil waters thaw and move to the streams and lakes. DOC concentrations were found to be reduced up to 90% as soil waters thawed and became free-flowing soil waters with leachates from thawed soil cores averaging 116 mg DOC/L at thaw and soil waters averaging 20 mg DOC/L. Stream and inlet to Toolik Lake DOC concentrations averaged 12 and 10 mg DOC/L respectively indicating further dilution of DOC. Quality differences elucidated by XAD-8/4 resin fractionation of DOC were observed for waters at various points in the ecosystem. The hydrophilic neutral fraction (HIN) accounted for 71% of the DOC in waters of thawing soil cores, but was only 23% in flowing waters of soils, and 9-20% in stream waters. A 14 day 4 degrees C incubation of DOC from thawing soil core waters reduced DOC an average of 39%, with 80% of that reduction occurring in the HIN fraction.. Soil waters and stream waters were similar in DOC fraction composition, and fulvic acid fractions were dominant. Initial 14 day DOC respiration rates with high HIN waters were 10 mg CO2-C/g DOC/d, and reduced to 1.2-1.7 mg CO2-C/g DOC/d after 34 days incubation. Respiration rates of thawed cores ranged from 0.12 to 0.06 and 0.03 mg CO2-C/g C/d for nonacidic and acidic cores, respectively.