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

Abstract. 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 estimation of GHG emissions were combined with high-resolution temperature and oxygen profiles of an 11 hectare shallow lake to investigate how thermal stratification shapes GHG emissions. There were three main stratification periods with profound anoxia in the bottom waters occurring quickly upon isolation from the atmosphere. Average diffusive emission of methane (CH₄) and nitrous oxide (N₂O) were larger and more variable in stratified phase, whereas carbon dioxide (CO₂) was on average lower. CH₄ ebullition was an order of magnitude greater in the stratified phase. In addition, there was a large efflux of CH₄ and CO₂ when the lake mixed after periods of extended (circa 14 days) thermal stratification. These two turnover events were estimated to have released the majority of the CH₄ emitted between May and September. These results highlight the role of turnover emissions resulting from temporary thermal stratification and also the need high frequency measurements of GHG emission in order to accurately characterise emissions from these temporarily stratifying lakes.