Oxygen use by nitrification in the hypolimnion and sediments of Lake Erie

Nitrification is an oxygen consumptive process, consuming 2 mol of oxygen per mol of ammonium oxidized. Hypolimnion and sediment samples were collected during the summers of 2008-2010 in Lake Erie to determine the total oxygen consumption and oxygen consumption from nitrification by blocking nitrification with selective inhibitors. Oxygen consumption by nitrification in the hypolimnion was 3.7 ± 2.9 (mean ± 1 SD) μmol O2/L/d, with nitrification accounting for 32.6 ± 22.1% of the total oxygen consumption. Nitrification in the hypolimnion contributed more to oxygen consumption in the eastern sites than western sites and was lowest in September. The nitrification rate did not correlate with environmental factors such as oxygen, nitrate or ammonium, or nitrifier numbers. Oxygen consumption by nitrification in sediment slurries was 7.1 ± 5.8 μmol O2/g/d, with nitrification accounting for 27.0 ± 19.2% of the total oxygen consumption with the lowest rates in July and the lowest percentages in June. Oxygen consumption by nitrification in intact sediment cores was 682 ± 61.1 μmol O2/m/d with nitrification accounting for 30.4 ± 10.7% of the total oxygen consumption. Nitrification rates in intact cores were generally highest in September. The proportion of oxygen consumed by nitrification corresponds closely with what would be predicted from complete oxidation of a Redfield molecule (23%). While nitrification is unlikely to be the dominant oxygen consumptive process, the rates observed in Lake Erie were sufficient to theoretically deplete a large portion of the hypolimnetic oxygen pool during the stratified period.