Factors Influencing Photoreactions of Dissolved Organic Matter in a Coastal River of the Southeastern United States

Photoreactions of dissolved organic matter can affect the oxidizing capacity, nutrient dynamics, trace gas exchange, and color of surface waters. This study focuses on factors that affect the photoreactions of the colored dissolved organic matter (CDOM) in the Satilla River, a coastal river that has high concentrations of dissolved organic carbon (DOC) (2.0 ± 0.2 mM C) and iron (12 ± 2 μM) in its freshwater reach. Quantum yields for the photoproduction of dissolved inorganic carbon (DIC) and carbon monoxide (CO) from Satilla CDOM decreased exponentially in the 300−450 nm spectral region. Photoreaction of the CDOM in sunlight caused a decrease in UV and visible absorbance that occurred most rapidly in the UV-B (280−315 nm) region, indicating that CDOM photoreactions can enhance exposure of aquatic organisms to DNA-damaging UV radiation. The role of iron in the photooxidation was investigated by adding fluoride ions or deferoxamine mesylate (DFOM) to the water to form unreactive Fe3+ complexes, thus inhibiting iron photoreduction and slowing CDOM photooxidation. Using this method, it was demonstrated that the photoformation of DIC, CO, and NH4+ is strongly affected by iron catalysis in the Satilla River water. Increasing the dioxygen content and lowering the pH accelerated the photoreaction of the CDOM. CDOM photoreaction was accompanied by conversion of dissolved iron and carbon to particulate forms. Such “photocoagulation” may contribute to the transport of iron and carbon from surface waters to sediments in sunlit, well-stratified aquatic ecosystems.