Photochemical processing is an important way to transform terrestrial dissolved organic matter (DOM) but was rarely investigated by ultra-high resolution mass spectrometry. We performed an irradiation experiment with water from a shaded forest stream flowing into a lit reservoir. Bacterial activity explained only 1% of dissolved organic carbon (DOC) decline in a combined bacterial and photodegradation approach. Photodegradation decreased the DOC concentration by 30%, the specific ultraviolet (UV) absorption by 40%–50%, and fluorescence intensity by 80% during six days. The humification index (HIX) decreased whereas the fluorescence index (FI) did not change. Two humic-like components identified by parallel factor analysis (PARAFAC) of excitation–emission matrices followed the decrease of fluorescent DOM. Changes of relative peak intensities of Fourier transform-ion cyclotron resonance mass spectroscopy (FT-ICR MS) elemental formula components as a function of cumulated radiation were evaluated both by Spearman’s rank correlation and linear regression. The FT-ICR MS intensity changes indicate that high aromatic material was photochemically converted into smaller non-fluorescent molecules or degraded by the release of CO2. This study shows the molecular change of terrestrial DOM before the preparation of drinking water from reservoirs.
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