Abstract The biological production of nitrate was studied during the summer of 1976 in the water column of an 18.500 m 3 capacity experimental enclosure in Blelham Tarn, English Lake District, to which 11 kg NaNO 3 (enriched with 9.63 atom % excess 15N) and 0.789 kg KH 2PO 4 had been added. Nitrification was detected in the water column during stratification and was maximal within a 2 m deep zone centered on a depth of 8.0 m in the metalimnion and upper hypolimnion, where numbers of autotrophic nitrifying bacteria were highest. Calculated rates of nitrification in this zone based on BOD measurements. N-Serve-sensitive bicarbonate incorporation, regression analysis of the daily changes in nitrate-nitrogen at 8.0 m and integrated changes in nitrate-nitrogen throughout the metalimnion and hypolimnion during late August 1976 were respectively: 55.4, 45.0, 22.4 and 39 μg N 1 −1 day −1. The importance of sediment-derived ammonium for nitrifying bacteria and the fate of nitrate produced by nitrification are discussed.