Seasonal variability in the composition of dissolved organic matter in the Yukon River, Alaska

Modern analytical approaches allow for detailed characterization of the composition of dissolved organic matter (DOM) in aquatic systems. The utility of advanced FTICR-MS and 13C-NMR approaches is presented for assessing the seasonal variability in DOM composition for samples collected across the hydrograph (2008 and 2009) from the Yukon River at Pilot Station, Alaska. FTICR-MS analyses were obtained on whole water samples while one- and two-dimensional solid-state NMR analyses were performed on the hydrophobic organic acid (HPOA) and transphilic organic acid (TPIA) fractions obtained using XAD resins (accounting for 64-74% of the DOM). Both approaches indicated that lignin-derived and carboxyl-rich alicyclic molecules (CRAM) were major fractions of all samples collected throughout the year, and that contributions from black carbon were minor. Each approach also provided unique information. FTICR-MS indicated the presence in spring of compounds containing sulfur that previously were identified to be atmospherically deposited combustion products in glacial meltwaters. These likely were deposited with snowfall. NMR analyses indicated that HPOA isolates from the spring period were characterized by greater contributions from lignin residues and carbohydrate-like materials than those from summer-autumn and winter. In addition, the spring TPIA samples had a predominance of carbohydrate, which was much less evident in the structures of summer-autumn and winter TPIA isolates. Spring DOM, therefore, was representative of inputs from freshly leached plant materials. These relatively fresh organic materials were depleted in summer-fall and winter samples, indicating that summer-fall and especially winter DOM was associated with more extensively degraded DOM and older DOM pools. These results provide chemical evidence supporting observations that spring freshet DOM in Arctic rivers is more biolabile than DOM exported at other times of the year.