Loss of optical and molecular indicators of terrigenous dissolved organic matter during long-term photobleaching

Two water samples from the Great Dismal Swamp National Wildlife Refuge with high dissolved organic matter (DOM) concentrations (51 and 121 mg C L-1) were subjected to ultraviolet (UV) light for up to 110 days. During the course of the irradiations, 74-88 % of the original dissolved organic carbon was lost along with 95-99 % of the absorption at 300 nm. Based on changes observed during light exposure, three pools of DOM were identified: photo-labile, photo-refractory, and photo-produced compounds. Solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy were used to determine structural moieties characteristic to each of these pools. These analyses showed aromatic carbons were preferentially removed while carbohydrate-like and amide/peptide-like carbons were preserved during UV exposure. An increase in carbon normalized C-13 NMR signal in the 0-50 ppm region suggests that alkyl moieties were produced, while FTIR signal at 1,745 cm(-1) and two-dimensional H-1-C-13 NMR results confirmed the photochemical production of acetate. Several properties typically used to trace terrigenous DOM in ocean margin and marine environments were significantly altered. Optical properties, including absorption spectral slopes and fluorescence indices, as well as carbon-normalized lignin yields shifted from terrestrial values towards those more typical of coastal or open ocean samples. The loss of terrestrial signatures during irradiation highlights the difficulty faced when quantifying the contribution of terrigenous DOM to aquatic carbon pools.