Nitrate Accumulation in Aerobic Hypolimnia: Relative Importance of Benthic and Planktonic Nitrifiers in an Oligotrophic Lake

Both nitrate and nitrous oxide accumulate in the hypolimnion of the oligotrophic Lake Taupo, New Zealand, throughout stratification. The two forms of oxidized nitrogen increase in concentration with increasing depth toward the sediments, where the dissolved concentrations of reduced nitrogen are two orders of magnitude higher than concentrations in the overlying water. Nitrification rates were measured by dark [14 C]CO2 assays with and without the inhibitor nitrapyrin. The fastest rates were recorded for planktonic nitrifiers in the epilimnion and benthic species in the surficial 2.5 mm of the sediments. Nitrifying bacteria were least active in the deep hypolimnion. Deepwater accumulation of NO3 − in Lake Taupo must therefore be a product of benthic rather than planktonic nitrification.