The 15N tracer method and the benthic chamber technique were combined to evaluate NH4+ exchanges at the sediment-water interface. This novel approach consists in measuring NH4+ fluxes during a single in situ incubation in a sample of water enclosed in a benthic chamber placed over the sediment and in a subsample thereof concomitantly incubated in a bottle. Using this combined approach, the influx and efflux of NH4+ across the sediment-water interface can be simultaneously measured along with uptake and regeneration rates of NH4+ in the water column. Details of the experimental protocol and principles behind the calculations of N transport rates are given. We applied this approach to a tropical reef on Reunion Island (Indian Ocean). Experiments were carried out in triplicate at three stations with organic-poor, sandy sediments. At the three stations, the mean flux of NH4+ from the water column to the sediment (29.6-59.2 μmol m− 2 h− 1) was much higher than the mean NH4+ uptake rate by phytoplankton (3.0-4.0 μmol m− 2 h− 1) indicating that the removal of NH4+ from the water column must be due, for the most part, to uptake by benthic microalgae in the study area. The mean flux of NH4+ from the sediment to the water column (6.7-13.7 μmol m− 2 h− 1) was comparable to the mean regeneration rate in the water (7.4-9.9 μmol m− 2 h− 1) suggesting that the sediment may constitute a significant N source for phytoplankton in the back-reef zone on Reunion Island.