Environmental Context. Iron oxides, as suspended minerals or as a colloidal phase, are common in Australian freshwater systems. Freshwater systems are also loaded with carbon-based substances, ‘dissolved organic matter’, but not all is biologically available as food to freshwater organisms. However, photochemical interactions between these iron oxides and dissolved organic matter provide a mechanism for biologically resistant carbon to re-enter the food web. Suspended iron oxides thus need to be considered in carbon cycles in aquatic ecosystems.

Abstract. The photochemical degradation of dissolved organic matter (DOM) derived from the leaves of River Red Gum (Eucalyptus camaldulensis) was examined, with a particular focus on the photochemical generation of CO₂, consumption of O₂, and the effect of particulate iron minerals on these photochemical reactions. Solutions of leaf leachate were irradiated with ultraviolet and visible light in the presence and absence of amorphous iron oxides. Addition of fresh iron oxide was found to increase the rate of photodegradation of the organic matter by up to an order of magnitude compared to the reactions without added iron oxide. The ratio of CO₂ produced to O₂ consumed was ~1:1 in both the presence and absence of iron oxyhydroxide. The reactivity of the iron oxides was dependent on the preparation method and decreased with increased storage time. These results suggest that photochemical reactions on particle surfaces should be considered when examining carbon transformation in aquatic ecosystems, especially at sites with potential for the production of iron oxyhydroxides.