Endogenous nitrate production in an experimental enclosure during summer stratification

The biological production of nitrate was studied during the summer of 1976 in the water column of an 18.500 m3 capacity experimentalenclosure in Blelham Tarn, English Lake District, to which 11 kg NaNO3 (enriched with 9.63 atom % excess 15N) and 0.789 kg KH2PO4 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