A sediment core incubation method to measure the flux of dissolved organic carbon between sediment and water
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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.
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.
KeywordsBenthic flux Dissolved organic carbon DOC Sediment core incubation UV254
This work was financially supported by the TALKO project (BMBF 02WT1290A). Thanks to Lionel Denis and Jo Banks for providing information about their set-ups. Thanks to the UFZ workshop for the excellent construction of the stirring heads. We thank Juliane Schmidt for her assistance in the laboratory and the UFZ GEWANA for sample analysis. Thanks to Dr. Wolf Von Tümpling for his advice on statistical analysis and to an anonymous reviewer whose suggestions significantly improved the manuscript.
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