The Influence of Water Column Hypoxia on the Behaviour of Manganese and Iron in Sandy Coastal Marine Sediment

The influence of bottom water hypoxia on manganese, iron and sulfur biogeochemistry was examined in sandy sediment from the shallow coastal lagoon, Fællesstrand, Denmark. The organic-poor sediment at Fællesstrand experiences occasional coverage of floating macroalgae and variable degrees of hypoxia at the sediment-water interface, resulting in dramatic changes in metal behaviour. The narrow peaks and steep gradients in Mn and Fe oxides as well as porewater Mn2+ and Fe2+ observed in the upper 2–3 cm of the sediment under fully oxic conditions indicate intense metal reduction-oxidation cycles. The Fe zones were generally displaced about 1 cm downwards compared with the Mn zones due to differences in reactivity. At lowered O2 conditions in the overlying water, Mn oxides gradually disappeared followed by Fe oxides. The subsequent diffusive loss of Mn2+ and Fe2+ to the overlying water was inversely related to the O2 concentration in the overlying water. The ability of the sediment to retain upward diffusion of H2S (sulfide retaining capacity) gradually disappeared at lowered O2 concentrations in a temporal pattern closely related to the changes in reactive Mn and Fe present. The sulfide retaining capacity is sustained for about 14 days under anoxia in Fællesstrand sediment. After 28 days of anoxia, 30–35% of the total Mn and Fe pools initially present in the sediment was lost. Despite the relatively low metal content, this organic-poor sediment may withstand hypoxic conditions in the bottom water (e.g. caused by coverage with floating macroalgae) and is thus capable of maintaining an intact benthic community for extended periods of time.