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

Concentrations of dissolved methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) were measured in the water columns of non-oxygenated and artificially oxygenated, ice-covered eutrophied lakes in the mid-boreal zone in Finland during late winter 1997 and 1999. Sampling was conducted during winter stratification, the critical period for oxygen (O2) deficiency in seasonally ice-covered, thermally stratified lakes. Oxygen concentrations were maintained at least at a moderate level throughout the oxygenated water columns, whereas the non-oxygenated columns suffered anoxic hypolimnia. The mean concentrations of dissolved CH4 exceeding the atmospheric equilibrium were greater in the non-oxygenated water columns (20.6-154 microM) than in the oxygenated ones (0.01-1.41 microM). In contrast, the mean excess CO2 concentrations varied less between the non-oxygenated and oxygenated sites (0.28-0.47 and 0.25-0.31 mM, respectively). Oxygenated water columns had greater mean excess concentrations of N2O (0.018-0.032 microM) than the non-oxygenated ones (0.005-0.024 microM). If the accumulated greenhouse gas stores in the water columns 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 ranged from 177 to 654 g CO2 equivalent (CO2-e) m-2 compared with 144 to 173 g CO2-e m-2 at the oxygenated sites. The increase in the accumulation of CH4 was the main reason for the higher GWP of the non-oxygenated sites. Anthropogenic eutrophication of lake ecosystems can generate increased CH4 emissions due to associated O2 depletion of their sediment and water column.