Temporary thermal stratification and mixing drive variation in CO2 and CH4 dynamics in a shallow lake

Fresh waters are known to be significant, but highly uncertain, sources of greenhouse gas (GHG) emissions to the atmosphere. In particular, lakes and ponds have been identified as hotspots of methane (CH₄) emissions, with large variation across systems.  One reason for this variability in estimates of emissions may be a mismatch between the scale of observation (often monthly) and how variable emissions are over relatively short timescales. Whilst studies have highlighted the importance of nutrient concentration, primary production and temperature in shaping fluxes of GHG, it is increasingly clear that physical limnology in small and shallow lakes may play a significant role.  Recent research has highlighted the prevalence of temporary thermal stratification in lakes previously classified as non-stratifying lakes, i.e. smaller shallow lakes and ponds. Here we present high frequency measurements from automatic flushing chambers and a novel CH₄ sensor measuring dissolved concentrations to quantify diffusive and ebullitive emissions of CH₄ along with diffusive fluxes of carbon dioxide (CO₂) from an 11 hectare lake, with a maximum depth of 5 m. We compare these emissions with patterns of thermal stratification, that featured partial and full mixing events, over a three-month period from mid-June to late September. This shows that diffusive emissions of both CH₄ and CO₂ are shaped primarily by partial and full mixing events between the GHG-rich hypolimnetic waters and the lower concentrations in the surface waters and by full mixing events. Whereas ebullition is largely a function of the extent of the duration of anoxia in the bottom waters, with large releases triggered by atmospheric pressure changes.  The results show that the variable emissions found in syntheses of spatial data from multiple lakes can be found within this single lake over time. Thus, the high variability in emissions reported across lakes may be the result of infrequent sampling of temporally highly dynamic systems, rather than the systems having such variable emissions.