Dark respiration rates, dissolved inorganic nitrogen (DIN) fluxes and nitrification rates were measured at two sites in the microtidal Sacca di Goro lagoon in September 2000. DIN fluxes correlated with the biomass of the dominant macrofauna species (the amphipod Corophium spp. at station Giralda and the polychaete Neanthes spp. at station Faro). Respiration (> 6 mmol O2 m-2h-1) and ammonium fluxes (> 80孯l N m-2h-1) were higher at station Giralda despite the lower organic matter content (4.5 %) and lower macrofauna biomass (4 g AFDW m-2). At both sites ammonium fluxes were significantly correlated with the biomass of the benthic infauna, but Corophium stimulated ammonium NH4+ fluxes 3-fold compared to Neanthes. The amphipod also enhanced nitrification rates (> 300 孯l N m-2h-1) due to the high density of its burrows, the higher NH4+ regeneration rates and the enhanced oxygen supply to the bacteria.