A three-year study of seasonal variation in water-column and sediment nitrogen species was conducted in the transition zone of the Potomac River 35 m from the Virginia shore at a site with an average water-column depth of approximately 1 m over sandy sediment. A diffusion-controlled sampler was used to collect water samples from the water column, at the interface between the water column and sediment, and at several tens of centimeters into the sediment. Nitrate was the predominant dissolved nitrogen species in the water column. The importance of denitrification was inferred by nitrate fluxes which were directed into the sediment from the water column during approximately 75% of the sampling periods and ranged from 0·02 to 0·69 mmol m⁻² day⁻¹. Flux of nitrate from the sediment into the water column, ⩾0·1 mmol m⁻² day⁻¹, due possibly to nitrification in surficial sediment, occurred during one spring and two summer sampling periods. Ammonium fluxes were less than 0·1 mmol m⁻² day⁻¹ during 90% of the sampling periods. Of the ammonium fluxes that were >0·05 mmol m⁻² day⁻¹, all were fluxes into the sediment during sampling periods when sediment resuspension occurred, and all were into the water column during periods of calm. The mean value of ammonium flux (0·005 ± 0·05 mmol m⁻² day⁻¹) from the sandy, shallow-water sediments was two orders of magnitude less than the ammonium fluxes from the deeper, silty channel sediments in the same reach of the river. Diffusive flux calculations suggest that approximately one order of magnitude more nitrate than ammonium is cycled between the shallow-water column and the sandy sediment in the transition zone of the Potomac River.