Greenhouse gases in non-oxygenated and artificially oxygenated eutrophied lakes during winter stratification

Concentrations of dissolved methane (CH4), carbon dioxide (CO2), and nitrou s oxide (N2O) were measured in the water columns of nonoxygenated and artif icially oxygenated, ice-covered eutrophied lakes in the mid-boreal zone in Finland during late winter 1997 and 1999. Sampling was conducted during win ter stratification, the critical period for oxygen (O-2) deficiency in seas onally ice-covered, thermally stratified lakes. Oxygen concentrations were maintained at least at a moderate level throughout the oxygenated water col umns, whereas the nonoxygenated columns suffered anoxic hypolimnia. The mea n concentrations of dissolved CH4 exceeding the atmospheric equilibrium wer e greater in the non-oxygenated water columns (20.6-154 muM) than in the ox ygenated ones (0.01-1.41 muM) In contrast, the mean excess CO2 concentratio ns varied less between the non-oxygenated and oxygenated sites (0.28-0.47 a nd 0.25-0.31 mM, respectively). Oxygenated water columns had greater mean e xcess concentrations of N2O (0.018-0.032 muM) than the non-oxygenated ones (0.005-0.024 muM). If the accumulated greenhouse gas stores in the water co lumns during winter are assumed to be released to the atmosphere during the spring overturn, the global warming potentials (GWP, time horizon 100 yr) of these potential emissions at the non-oxygenated, eutrophic study sites r anged from 177 to 654 g CO2 equivalent (CO2-e) m(-2) compared with 144 to 1 73 g CO2-e m(-2) at the oxygenated sites. The increase in the accumulation of CHI was the main reason for the higher GWP of the non-oxygenated sites. Anthropogenic eutrophication of lake ecosystems can generate increased CHI emissions due to associated O-2 depletion of their sediment and water colum n.