Carbon loss and optical property changes during long-term photochemical and biological degradation of estuarine dissolved organic matter

Terrestrially derived dissolved organic matter (DOM) impacts the optical pr operties of coastal seawater and affects carbon cycling on a global scale. We studied sequential long-term photochemical and biological degradation of estuarine dissolved organic matter from the Satilla River, an estuary in t he southeastern United States that is dominated by vascular plant-derived o rganic matter. During photodegradation, dissolved organic carbon (DOC) loss (amounting to 31% of the initial DOC) was much less extensive than colored dissolved organic matter (CDOM) or fluorescent dissolved organic matter (F DOM) loss (50% and 56% of the initial CDOM and FDOM), and analysis of kinet ics suggested a reservoir of DOC that was resistant to photodegradation. In contrast, CDOM photodegradation closely followed first-order kinetics over two hall-lives with no indication of a nondegradable component. FDOM loss was slightly biased toward fluorophores considered representative of terres trial humic substances. Additional changes in optical properties included i ncreases in spectral slope and shifts in fluorescence excitation/emission m axima that were generally consistent with previous observations from field studies of photobleached DOM. Biological degradation of photobleached DOM w as more rapid than that of unbleached material, and this net positive effec t was evident even for extensively photodegraded material. Bacterial degrad ation caused shifts in the opposite direction from photochemical degradatio n for both spectral slope and excitation/emission maxima and thus dampened but did not eliminate changes in optical properties caused by photobleachin g.