Nitrification: A Significant Cause of Oxygen Depletion Under Winter Ice

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Abstract
Changes in concentrations of ammonium, nitrite, nitrate, and oxygen suggested the occurrence of significant nitrification throughout the water column of mesotrophic Lake St. George, Ontario, during the winter months from 1976 to 1984. The existence of nitrapyrin- and acetylene-sensitive 14C-bicarbonate incorporation confirmed that bacterial nitrification occurred. During late January to early March (water temperature 2–3 °C), nitrification occurred at an average rate of about 13 μg N∙L−1∙d−1 for the years studied. Numbers of detectable nitrifying bacteria appeared too low (by 2 to 4 orders of magnitude) to account for the observed activity. The nitrification activity observed would result in average oxygen consumption amounting to 71% of the observed oxygen depletion. This shows that winter nitrification can be an important factor in promoting oxygen depletion and possibly winter-kill of fish.