sediment core incubation method to measure the flux of dissolved organic carbon between sediment and water

PURPOSE: The exchange of dissolved organic carbon (DOC) between sediment and water likely has an impact on aquatic DOC quantity and quality. Therefore, we developed a method to measure the flux of DOC between sediment and water. MATERIALS AND METHODS: We constructed Plexiglas sediment core covers, which enable the stirring of the overlying water, continuous measuring of the oxygen concentration, adding and/or withdrawal of water, bubbling and microsensor measurements at the sediment-water interface. Sediment cores from two pre-dams of drinking water reservoirs were incubated in a set-up allowing the measurement of benthic fluxes of DOC and other solutes, including oxygen, dissolved nutrients and metals. RESULTS AND DISCUSSION: Dissolved organic carbon fluxes were low and influenced by the redox conditions in the bottom water. We tested whether UV absorption (UV₂₅₄) can be used as a fast method to monitor DOC changes in the core incubations. There was a good correlation between UV₂₅₄ and DOC only at shallow, oxic sites. In anoxic waters, UV₂₅₄ was strongly influenced by iron. The fluxes of DOC and other solutes, especially oxygen, differed by more than 1 order of magnitude. Thus, a two-step incubation, measuring first the fast fluxes followed by a 14-day incubation, is recommended. Sulfate reduction appeared to be the most important process of organic matter mineralisation under anoxic conditions, while aerobic respiration and, to a lesser extent, denitrification dominated under oxic conditions. CONCLUSIONS: We present a set-up that allows measuring the flux of DOC as well as of other solutes between sediment and water. UV₂₅₄ can be used as a proxy for DOC at oxic sites after establishing a site-specific calibration. Special care has to be taken to maintain redox conditions constant and to prevent oxygen depletion or contamination by atmospheric oxygen. The flux of DOC might be coupled to the reductive dissolution of iron minerals.