This experimental study examined the interactive effects of bioturbation and settling organic matter (OM) on the fate (burial and remobilisation) of 2 surface-deposited contaminants in Baltic Sea sediment: the metal Cd and a hydrophobic organic pollutant, the flame retardant BDE-99. Three macrofaunal species with diverse feeding and bioturbation strategies were used: the amphipod Monoporeia affinis, the clam Macoma balthica and the polychaete Marenzelleria spp. Radiolabelled contaminants were added to the sediment surface in association with 3 different OM types: (1) phytoplankton, (2) terrestrial lignin and (3) Baltic sediment. Bioturbation by all species increased the retention of both contaminants in the sediment, most effectively M. affinis and M. balthica. A decoupled transport of Cd and BDE-99 by Marenzelleria was observed. Generally, Marenzelleria buried the highest amount of Cd into the sediment but also caused the highest remobilisation to the water, indicating an effective transport of (soluble) Cd over the sediment -water interface via bioirrigation. Lack of the highly hydrophobic and mainly particle-associated BDE-99 below the sediment surface suggests that Marenzelleria caused no significant particle mixing. The addition of various OM types significantly affected the distribution of Cd, but not of BDE-99. There was an interactive effect between bioturbation (species) and OM type, generally showing an increased burial and release of Cd when associated with phytoplankton in the presence of Marenzelleria. Our results emphasise the importance of understanding the complex interactions between ecological (e.g. infaunal feeding and bioturbation activities) and physiochemical processes (contaminant speciation and sorption kinetics) when assessing the fate of contaminants in aquatic ecosystems. KEY WORDS: Cadmium · BDE-99 · HOC · POP · Flame retardant · Marenzelleria spp. · Macoma balthica · Monoporeia affinis · RemobilisationResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 356: 25-38, 2008 cycling of sediment-associated organic contaminants and metals (Ferro et al. 2003, Caradec et al. 2004. Bioirrigation, on the other hand, increases the flux of solutes, i.e. dissolved compounds in the pore water and over the sediment -water interface (Aller & Aller 1998). The partitioning of a contaminant between the solid and water phase in sediments depends on the contaminant's inherent properties and the biogeochemical state of the sediment. Hydrophobic organic contaminants (HOCs) preferentially adsorb to particulate OM in sediments, and their distribution between phases generally can be calculated based on the physiochemical properties of the compound, such as water solubility and lipophilicity, and the amount of OM in the sediment (Schwarzenbach et al. 2003). The partitioning of metals is strongly affected by the sediment chemical environment, such as pH, redox conditions and the presence of other chemical species (Chapman et al. 1998). Under oxic condition...

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