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 ca pacity, nutrient dynamics, trace gas exchange, and color of surface wa ters. This study focuses on factors that affect the photoreactions of the colored dissolved organic matter (CDOM) in the Satilla River, a co astal river that has high concentrations of dissolved organic carbon ( DOC) (2.0 +/- 0.2 mM C) and iron (12 +/- 2 mu M) in its freshwater rea ch. Quantum yields for the photoproduction of dissolved inorganic carb on (DIC) and carbon monoxide (CO) from Satilla CDOM decreased exponent ially 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 pho toreactions 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 t o form unreactive Fe3+ complexes, thus inhibiting iron photoreduction and slowing CDOM photooxidation. Using this method, it was demonstrate d that the photoformation of DIG, CO, and NH4+ is strongly affected by iron catalysis in the Satilla River water. Increasing the dioxygen co ntent and lowering the pH accelerated the photoreaction of the CDOM. C DOM 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 i n sunlit, well-stratified aquatic ecosystems.