Hypolimnetic O₂Consumption, Denitrification, and Methanogenesis in a Thermally Stratified Lake

Hypolimnetic O 2 consumption, methanogenesis, and denitrification were estimated from concentration changes in O 2 , CH 4 , and nitrogen oxides (NO 3 − , NO 2 − , N 2 O), respectively, in the eastern basin of Lake St. George, Ontario, during thermal stratification from 1980 to 1984. NH 4 + accumulated in the hypolimnion throughout the period of stratification and provided an overall measure of organic matter mineralization. Anaerobic metabolism, particularly methanogenesis, predominated because anoxia was total within less than 2 mo of spring water column turnover and nitrogen oxides were always completely depleted before fall turnover. The time of onset and rate of NO 3 − , depletion varied considerably between years and were not always directly correlated with O 2 concentration. It was estimated that, on average, O 2 consumption, denitrification, and methanogenesïs accounted for approximately 80% of hypolimnetic organic C mineralization in the periods surveyed.