Daily and seasonal variations of CO2 and CH4 diffusive emissions in the largest urban lake of China

Lakes are an important source of atmospheric carbon dioxide (CO₂) and methane (CH₄), but current research is still fragmented and overlooks daily and seasonal variability of the greenhouse gas emissions in urban lakes. In this study, we conducted high-resolution field measurements on CO₂ and CH₄ diffusion during the daytime over three seasons in the largest eutrophic urban lake in China. The lake showed a clear daily sink-source shifting of CO₂ in the autumn and summer days, but acted as a source of atmospheric CO₂ in the spring days. Strong declining trends of partial pressure of CO₂ (pCO₂) and flux (FCO₂) from morning to afternoon were observed across different seasons. Specially, CO₂ measurements at 11:00–13:00 in autumn and spring and at 8:30–9:30 in summer had the least deviation to the daily means. Unlike with CO₂, the lake served as an important source of atmospheric CH₄ throughout the study period. CH₄ fluxes were higher in the morning than in the afternoon. The peak flux was typically observed in two-time intervals: 8:00–11:00 in the autumn and spring, and 15:00–19:00 in the summer. Relationships between declining rates of daily pCO₂ and FCO₂ and ambient factors revealed that water temperature, dissolved oxygen and chlorophyll a had a great potential for predicting daily variation in lake CO₂ diffusive fluxes. Ammonia-nitrogen seemed to have played a role in influencing the daily fluctuation of CH₄ fluxes. The study reveals that urbanization and eutrophication amplify the daily variability in CO₂ and CH₄ emission fluxes from urban lakes, while they also introduce additional uncertainties when single-timepoint sampling approach is employed. These findings highlight the importance of incorporating urban lakes and diel carbon dynamics for accurately estimating the carbon budget of lacustrine ecosystems.