Capping with layers of inert or adsorptive materials is used to control the release of polycyclic aromatic hydrocarbons (PAH) in sediment but little is known about microbial degradation processes in these materials.A rich native microbial community inhabits the sediment bed,and capping media can influence enrichment and biodegradation activity of benthic microorganisms.The aim of this study was to evaluate the effects of capping media (sand,organoclay,and powdered activated carbon[PAC]) on microbial communities under oxic conditions typical of the cap-water interface,where naphthalene degradation (model PAH) is likely to be maximized.Bench scale experiments compared naphthalene concentrations,nahAc biomarker abundance,microbial community composition,and cellular attachment in systems amended with adsorptive and non-adsorptive capping materials.Results indicate that activated carbon promoted and enhanced bioactivity;PAC treatments showed high biodegradation rates,nahAc biomarker levels,and attached biological growth consistent with enrichment of the PAHdegrading genus Pseudomonae.In contrast,sand did not enhance biological activity compared to media-free systems.Naphthalene strongly influenced microbial community composition at the species level in all treatments except organoclay,which promoted biological signatures commonly associated with impeding degradation activity.Data overall suggest that adsorptive capping materials can both promote (PAC) and inhibit (organoclay) bioactivity in the surficial layer of caps,indicating that media selection is critical to the design of bioactive capping systems.