Effect of an Alternating Oxic/Anoxic Regime on a (Freshwater) Yarra River Sediment

Abstract

The uptake and release of phosphorus, nitrogen, iron, and manganese from Yarra River sediments have been examined using laboratory reactors. Both slurried and static sediments were exposed to an alternating regime of oxic and anoxic conditions. Experiments examined the effect of changing the oxygen status on daily and weekly time frames. In all experiments, after anoxia was re-established, oxidant consumption followed the expected thermo-dynamic order: O₂ > Mn^IV ≈ NO_x > Fe^III. Contrary to predictions based on the standard iron–phosphorus model, significant phosphorus release was observed under oxic conditions. This was attributed to the mineralization of organic matter. Nitrate was shown to minimize phosphorus release from anoxic sediments by ‘redox buffering’ which prevented iron(III) reduction. The high ambient water column nitrate concentration (40–45 μM) in the Yarra River should effectively limit phosphorus release from the sediments unless long term (multiweek) anoxia occurs. Reduction of nitrate concentration occurred predominantly via denitrification. This work clearly demonstrated that on a time scale of hours to weeks, the iron, manganese, nitrogen, and phosphorus cycles are interacting closely.