Dynamic changes in size-fractionated dissolved organic matter composition in a seasonally ice-covered Arctic River

Arctic rivers are sensitive to climate and environmental change, but the biogeochemical response remains poorly understood. Monthly size-fractionated dissolved organic matter (DOM) samples from the lower Yukon River were characterized using UV-visible, fluorescence, and Fourier transform-infrared (FT-IR) spectroscopy techniques. The EEM-PARAFAC analysis revealed three major fluorescent DOM components, including two humic-like components (C-480 and C-400) and one protein-like component (C-310), with their relative importance following the order of C-480 >= C-400 > C-310 in the high-molecular-weight DOM (1 kDa-0.4 mu m) and C-400 > C-480 > C-310 in the low-molecular-weight DOM pool (< 1 kDa). Transformation in DOM and change in sources were manifested in major fluorescent components and optical properties, including biological index (BIX), humification index (HIX), spectral slope (S275-295) and specific UV absorbance at 254 nm (SUVA(254)). These changes occurred within different DOM size-fractions and among ice-covered, spring freshet, and open seasons. Joint analysis of EEM and FT-IR spectra using a data fusion technique showed that humic-like DOM is mostly associated with CH, CC, and CO bonds, while protein-like DOM is correlated more with CN and NH related structures. DOM aromaticity and the ratios of HIX to BIX and protein-like to humic-like components may be used as a compelling proxy to measure change in source waters and to infer permafrost dynamics. Our results provide insight into the seasonal variation in DOM composition for different size-fractions in the lower Yukon River, and a baseline dataset against which future changes can be understood in the context of arctic basin biogeochemical cycling.