ABSTRACT: Removal of nitrogen through enhanced denitrification has been identified as an ecosystem service provided by oysters. In this study, we assessed the effects of an individual oyster (Crassostrea virginica) on nitrogen dynamics. Fluxes of N₂, O₂, nitrate/nitrite (NO_x) and ammonium (NH₄⁺) were measured from continuous-flow microcosms that contained a live oyster, sediment, or a live oyster + sediment. Net N₂ fluxes were indicative of nitrogen fixation in the sediment treatment and denitrification in the oyster and oyster + sediment treatments. Organic matter deposition and ammonium production associated with oyster biodeposits and excretion likely decreased N limitation, and thus the demand for fixation of new nitrogen, while increasing nitrification and subsequent denitrification. Oyster-mediated denitrification accounted for 48% of the total inorganic nitrogen efflux in the oyster microcosms and 35% in the oyster + sediment microcosms. Despite high rates of ammonium production, inclusion of the eastern oyster did not increase the pool of bioavailable nitrogen but shifted the microcosms from a nitrogen source to a nitrogen sink.