The chemical composition of dissolved organic carbon (DOC) in tundra streams determines the breakdown of important components of thawing permafrost carbon. We used absorbance data to examine three aspects of DOC’s fate in the Kolyma River Basin in Russia: photodegradation potential, microbial processing potential, and composition change across the landscape. Filtered and filter-sterilized water samples were incubated under light and dark conditions. Photochemical breakdown of DOC was significant, and showed evidence of light-stimulated microbial processing. DOC composition was assessed using optical measurements associated with compound structure. Microbial processing treatments increased SUVA<sub>254</sub> and decreased slope ratio, consistent with shifts towards higher percentages of aromatic, high molecular weight compounds. Photochemical treatments, however, decreased SUVA<sub>254</sub> and increased slope ratio, demonstrating lower percentages of aromatic, high molecular weight compounds, and suggesting light degrades biologically inaccessible molecules. No significant landscape trends in DOC optical characteristics appeared, despite decreasing DOC concentration down flowpaths. This indicates that, in situ, the combination of photochemical and microbial breakdown did not alter chemical composition of DOC. These results highlight the importance of each processing mechanism’s strength in altering the composition of exported tundra carbon. Furthermore, as ice cover (and therefore UV light exposure) shifts with changing climate and potentially increases photodegradation, DOC optical measurements could assist with tracking changes in breakdown pathways and carbon export.

Patterns in DOC absorbance with photodegradation and microbial processing in tundra watersheds in the Kolyma River Basin

249th National Meeting (2015)

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