Factors driving GHG emissions in kettle ponds in NE Germany

Freshwater systems are considered important contributors to global greenhouse gas (GHG) emissions. However, most studies so far focus on relatively large systems and typically disregard small ponds (< 0.01 ha) and shallow lakes. This is surprising since some studies have shown that small aquatic systems significantly contribute to global GHG emission. A profound understanding of the factors that determine GHG emission by small ponds is therefore important, especially in the face of climate change. The present study aims to fill the knowledge gap by investigating methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) emissions in a set of 30 kettle hole ponds located in agricultural landscapes in different regions of Northeast (NE) Germany. More specifically, this study aims to assess the effect of hydroperiod on GHG emissions by ponds in relation to local environmental pond conditions, to assess seasonal and interannual variation in GHG emission, and to identify significant environmental drivers for variation in GHG emissions in the set of investigated ponds. For this purpose, we investigated a set of kettle hole ponds located in different regions in NE Germany for major local environmental pond variables and GHG emission during spring, summer and autumn during two subsequent years (2021 and 2022). Our results show that pond hydroperiod affects GHG emission and reveals considerable seasonal variation in the extent GHG emission. Fluxes of CH4 were higher in permanent ponds compared to temporal ponds, being the opposite for CO2 fluxes, and emission in summer overall higher than in spring and autumn, except N2O. We did not observe systematic differences in GHG emission within ponds between years. Overall, we observed that the investigated ponds acted primarily as CO2 and CH4 emitters, while N2O emissions were comparatively low and even negative. We also observed that the main pathway for CH4 release was through ebullition, rather than by diffusive fluxes. The emission rates of CH4 and N2O seems largely driven by sulphate concentration in the ponds. Additionally, CH4 emissions were also determined by the concentration of total phosphorus and dissolved oxygen. N2O emissions were also determined by NH4, TOC and chloride concentrations. In contrast, we could not identify a significant local pond variable explaining variation in CO2 emission between ponds.