Temporary stratification promotes large greenhouse gas emissions in a shallow eutrophic lake

Davidson, Thomas A.; Søndergaard, Martin; Audet, Joachim; Levi, Eti; Esposito, Chiara; Bucak, Tuba; Nielsen, Anders

Shallow lakes and ponds undergo frequent temporary thermal stratification. How this affects greenhouse gas (GHG) emissions is moot, with both increased and reduced GHG emissions hypothesised. Here, weekly estimations of GHG emissions, over the growing season from May to September, were combined with temperature and oxygen profiles of an 11 ha temperate shallow lake to investigate how thermal stratification shapes GHG emissions. There were three main stratification periods with profound anoxia occurring in the bottom waters upon isolation from the atmosphere. Average diffusive emissions of methane (CHinline-formula4) and nitrous oxide (Ninline-formula2O) were larger and more variable in the stratified phase, whereas carbon dioxide (COinline-formula2) was on average lower, though these differences were not statistically significant. In contrast, there was a significant order of magnitude increase in CHinline-formula4 ebullition in the stratified phase. Furthermore, at the end of the period of stratification, there was a large efflux of CHinline-formula4 and COinline-formula2 as the lake mixed. Two relatively isolated turnover events were estimated to have released the majority of the CHinline-formula4 emitted between May and September. These results demonstrate how stratification patterns can shape GHG emissions and highlight the role of turnover emissions and the need for high-frequency measurements of GHG emissions, which are required to accurately characterise emissions, particularly from temporarily stratifying lakes.

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Davidson, Thomas A. / Søndergaard, Martin / Audet, Joachim / et al: Temporary stratification promotes large greenhouse gas emissions in a shallow eutrophic lake. 2024. Copernicus Publications.

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