Estuarine, Coastal and Shelf Science (2002) 55, 645–654 doi:10.1006/ecss.2001.0934, available online at http://www.idealibrary.com on The Influence of Water Column Hypoxia on the Behaviour of Manganese and Iron in Sandy Coastal Marine Sediment K. D. Kristiansen a , E. Kristensen a * and M. H. Jensen b a Institute of Biology, Odense University, SDU, DK-5230 Odense M, Denmark b Nature Management and Water Quality Division, County of Fyn, Ørbækvej 100, DK-5220 Odense SØ, Denmark Received 19 September 2001 and accepted in revised form 16 November 2001 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 Mn 2+ and Fe 2+ 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 O 2 conditions in the overlying water, Mn oxides gradually disappeared followed by Fe oxides. The subsequent diffusive loss of Mn 2+ and Fe 2+ to the overlying water was inversely related to the O 2 concentration in the overlying water. The ability of the sediment to retain upward diffusion of H 2 S (sulfide retaining capacity) gradually disappeared at lowered O 2 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.  2002 Elsevier Science Ltd. All rights reserved. Keywords: sandy sediment; hypoxia; temporal changes; solid phase; dissolved; manganese; iron; flux Introduction The upper layers of marine sediments are often en- riched in oxidized manganese and iron precipitates as a result of biogeochemical processes. These processes include burial, reduction and thus dissolution of solid Mn and Fe (Friedl et al., 1997; Thamdrup, 2000), upward migration of dissolved Mn 2+ and Fe 2+ followed by oxidation and a stratified precipitation in the oxidized zone (Burdige, 1993; Cornell & Schwertmann, 1996). The oxidized forms, Mn(III- IV)—hereafter referred to as Mn(IV)—and Fe(III), are bound as (hydr)oxides with low solubility (Burns & Burns, 1975; Murray, 1979). The typical stratifi- cation with Fe(III) extending deeper than Mn(IV) is controlled by differences in reaction kinetics and thermodynamics i.e. slower oxidation rates of Mn 2+ compared to Fe 2+ and oxidation of Fe 2+ by oxidized Mn (Stumm & Morgan, 1981; Lovley & Phillips, 1988) and different solubility of the reduced and oxidized forms of Mn and Fe (Froelich et al., 1979; Aller, 1980). Mn(IV) and Fe(III) are readily reduced under anoxic conditions. They may both serve as electron acceptors in microbial oxidation of organic carbon (Lovley, 1991; Thamdrup, 2000). Mn(IV) can also be reduced by chemical reactions with reduced Fe and sulfide, and Fe(III) can react similarly with sulfide (Thamdrup et al., 1994a; Yao & Millero, 1994). After reduction, Mn 2+ may precipitate as carbonates, adsorb to clay minerals, carbonates and metal (hydr)- oxides or remain soluble and mobile as hydrated Mn 2+ (Middelburg et al., 1987; Aller, 1994). Re- duced Fe has similar properties, but the reduced solids are largely iron sulfides due to their high stability relative to e.g. carbonates (Coleman, 1993). The content of oxidized Mn and Fe in sediments is influenced by the degree to which Mn 2+ and Fe 2+ are reoxidized within the sediment. Mn 2+ and Fe 2+ may be oxidized abiologically by oxygen (Burns & Burns, 1975; Murray, 1979), but microbially mediated oxi- dation generally occurs at rates much higher than *Corresponding author. E-mail: ebk@biology.ou.dk 0272–7714/02/010645+10 $35.00/0  2002 Elsevier Science Ltd. All rights reserved.