Nitrification, mediated by ammonia-oxidizing bacteria (AOB) and archaea (AOA) is important in global nitrogen cycling. In estuaries where gradients of salinity and ammonia concentrations occur, there may be a differential selection for ammonia-oxidizer populations. The aim of this study was to examine the activity, abundance and diversity of AOAs and AOBs in surface oxic sediments of a highly nutrified estuary that exhibits gradients of salinity and ammonium. AOB and AOA communities were investigated by measuring ammonia monooxygenase (amoA) gene abundance and nitrification potentials both spatially and temporally. Nitrification potentials differed along the estuary and over time; with greatest nitrification potentials occurring mid-estuary, (8.2 μmol N gdw^-1 d^-1 in June; increasing to 37.4 μmol N gdw^-1 d^-1 in January). At the estuary head, nitrification potentials were 4.3 μmol N gdw^-1 d^-1 in June; increasing to 11.7 μmol N gdw^-1 d^-1 in January. At the estuary head and mouth, nitrification potentials fluctuated throughout the year. AOB amoA gene abundances were significantly greater (by 100-fold) than AOA both spatially and temporally. Nitrosomonas spp. were detected along the estuary by DGGE band sequence analysis. In conclusion, AOB dominated over AOA in the estuarine sediments with the ratio of AOB:AOA amoA gene abundance increasing from the upper (freshwater) to lower (marine) regions of the Colne estuary. These findings suggest that in this nutrified estuary, AOB (possibly Nitrosomonas spp.), were of major significance in nitrification.