Aquatic nitrogen transformations at low oxygen concentrations.

Nitrite and nitrous oxide made up 40% of the hypolimnetic dissolved inorganic nitrogen in mesotrophic Lake Rotoiti, New Zealand, prior to hypolimnetic anoxia. Up to 120 mg of N m-3 as nitrite and 20 mg of N m-3 as nitrous oxide accumulated, whereas dissolved-oxygen concentrations remained between 1.0 and 0.2 g m-3 and were totally consumed when the hypolimnion became completely anoxic. Assays of water column nitrification potentials, together with measurements of the relative rates of nitrate and nitrite reduction, suggested that at low dissolved-oxygen concentrations both nitrite and nitrous oxide were produced mainly by ammonium-oxidizing bacteria, with nitrous oxide being a product of nitrifier denitrification.