Photochemical reactivity of dissolved organic matter in the Delaware Estuary

Photochemical oxidation of dissolved organic matter (DOM) is an important process in the marine carbon cycle. Estuaries, in particular, play a major role in the processing of organic carbon during its transfer from land to the sea. The significance of this photochemical flux was evaluated in surface waters of the Delaware Estuary in the summer, 2002, by quantifying the main DOM photomineralization products, carbon dioxide (CO2) and carbon monoxide (CO). In order to precisely determine photochemically produced carbon dioxide (CO2) in marine waters, a semi-automated dissolved inorganic carbon (DIC) analytical system was developed. This system was designed to prevent atmospheric contamination during sample preparation, irradiation, and analysis by maintaining a closed system. Within the estuary, CO2 production was described by a single apparent quantum yield (AQY) spectrum whereas CO AQY spectra decreased with increasing salinity, suggesting the involvement of different mechanisms for the production of these two species. Production rates calculated from AQY spectra agreed reasonable well with directly measured rates (based on deckboard irradiations) but were consistently lower. Measured CO2 photoproduction rates were highest at the river (9.1 mumol CO2 L-1 d-1) and decreased non-linearly with salinity to 0.7 mumol CO2 L-1 d-1 near the mouth of the bay. CO photoproduction rates were on average ∼ 6 times lower than CO2 and decreased linearly with increasing salinity from 0.74 to 0.16 mumol CO L-1 d-1. Based on a simple depth-integrated model, a photochemical loss of ∼ 1.2 x 10 6 mol C d-1 was estimated for Delaware Estuary, which is comparable to 25% of riverine DOC input. The distribution of dissolved organic carbon (DOC) with salinity provided evidence of in situ production as well as chemical and microbial processing in the Delaware Estuary. Despite the preferential utilization of autochthonous DOM, photochemical alteration of terrestrial DOM had a positive effect on community respiration rates (but no effect on bacterial production) in post-irradiation, dark incubations. Through this coupled photochemical-biological pathway, photochemistry accounted for 14% of the ∼ 7.5 x 107 mol CO2 d-1 respired in the upper estuary (salinities < 10), contributing to the observed CO2 supersaturation (1000 to 7600 muatm CO 2) in this region.;Keywords: Delaware Estuary, photochemistry, dissolved organic matter...