We examined nitrification rate and inorganic nitrogen distribution and their seasonal variation in a large perturbed estuary,the Pearl River Estuary based on three cruises conducted in winter (Jan.2005),summer (Aug.2005) and spring (Apr.2006).Concentrations of dissolved inorganic nitrogen,and nitrifying bacteria abundance were measured in the water column.On site incubations were carried out to estimate the nitrification rates (ammonium and nitrite oxidation).A dramatically decrease of NH4 +,increase of NO3 and no significant variation of NO2 within salinity 0-5.This pattern appears to occur throughout the year.The river end members of inorganic nitrogen species change substantially with extremely high concentrations in the dry season.