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TECHNICAL REPORT
ATMOSPHERIC POLLUTANTS AND TRACE GASES

Greenhouse Gases in Non-Oxygenated and Artificially Oxygenated Eutrophied Lakes during Winter Stratification

^a Research and Development Unit of Environmental Health, Department of Environmental Sciences, Bioteknia 2, University of Kuopio, P.O. Box 1627, FIN-70211 Kuopio, Finland
^b North Savo Regional Environment Centre, P.O. Box 1049, FIN-70701 Kuopio, Finland
^c Department of Biology, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland

Corresponding author (jari.huttunen{at}uku.fi)

Received for publication September 14, 1999. 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 µM) than in the oxygenated ones (0.01–1.41 µM). 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 µM) than the non-oxygenated ones (0.005–0.024 µM). 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.

Abbreviations: GWP, global warming potentials • NTOT, total nitrogen • ON, organic nitrogen • PTOT, total phosphorus

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