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

Abstract

Terrestrially derived dissolved organic matter (DOM) impacts the optical properties 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 the southeastern United States that is dominated by vascular plant‐derived organic 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 (FDOM) loss (50% and 56% of the initial CDOM and FDOM), and analysis of kinetics suggested a reservoir of DOC that was resistant to photodegradation. In contrast, CDOM photodegradation closely followed first‐order kinetics over two half‐lives with no indication of a nondegradable component. FDOM loss was slightly biased toward fluorophores considered representative of terrestrial humic substances. Additional changes in optical properties included increases in spectral slope and shifts in fluorescence excitation/emission maxima that were generally consistent with previous observations from field studies of photobleached DOM. Biological degradation of photobleached DOM was more rapid than that of unbleached material, and this net positive effect was evident even for extensively photodegraded material. Bacterial degradation caused shifts in the opposite direction from photochemical degradation for both spectral slope and excitation/emission maxima and thus dampened but did not eliminate changes in optical properties caused by photobleaching.