No abstract

No abstract

A series of controlled photolysis experiments using simulated sunlight was conducted in the presence and absence of estuarine bottom sediments to address the role of sediment resuspension on dissolved organic carbon (DOC) cycling in estuarine and coastal waters. In 0.2-mm-filtered estuarine water samples with no added sediment, DOC concentrations decreased from 0.4 to 3 mmol L 21 h 21 as a result of photooxidation. When 0.2-mm-filtered water samples with the addition of 1-2 grams of estuarine sediment per liter were irradiated, DOC was produced at rates of 3 to 150 mmol L 21 DOC per gram dry sediment. Photoproduction of DOC from resuspended sediments increased in direct proportion to the percent organic carbon content of the added sediment. Highenergy ultraviolet light was the most effective for photodegrading DOC in filtered samples and for photoproducing DOC from resuspended sediments. Photosynthetically active radiation (PAR; 400-700 nm) did not significantly degrade DOC in filtered water. However, PAR did produce half as much DOC from resuspended sediments in organic-rich regions of the estuary relative to full spectrum sunlight irradiations. The photoproduction of DOC from resuspended sediments, calculated for the top 1 m of coastal waters, resulted in fluxes that were significantly larger than benthic and riverine fluxes of DOC. Photoproduction from resuspended sediments therefore represents an episodically significant but previously unrecognized source of DOC to estuarine and coastal ecosystems receiving large sediment plumes.

No abstract

More than 80% of the iron(II) present in a dilute (pH 4.5) H2SO4 solution was oxidized by hydrogen peroxide (3 microM) in 24 h, whereas in rainwater Fe(II) remained stable for days indicating that a complexed form of Fe(II) exists in rainwater that protects it against oxidation. When a rain sample was irradiated for 2 h with simulated sunlight, there was a 57 nM increase in Fe(II) resulting from photoreduction of organic Fe(III) complexes. Once irradiation ceased, the photoproduced Fe(II) rapidly oxidized back to its initial concentration of 32 nM prior to irradiation, but not to zero. These photochemical studies demonstrate that during the daytime when sunlight is present there are dynamic interconversions between complexed and uncomplexed Fe(II) and Fe(III) species in rainwater. During the night, after the photochemically produced Fe(II) is reoxidized to Fe(III), virtually all remaining Fe(II) is complexed by ligands which resist further oxidation. Rain samples oxidized under intense UV light lost their ability to stabilize Fe(II), suggesting the ligands stabilizing Fe(II) are organic compounds destroyed by UV-irradiation. Additional UV-irradiation studies demonstrated that on average 25% of the Fe-complexing ligands in rainwater are extremely strong and cannot be detected by spectrophotometric analysis using ferrozine. The stability of organically complexed Fe(II) has important implications for the bioavailability of rainwater-derived Fe in the surface ocean.