作者：L. Zhang, Y. J. Xu and S. Li

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 (F CO₂) 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 p CO₂ and F CO₂ 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.

（来源：Journal of Hydrology 2025 Vol. 660  DOI: 10.1016/j.jhydrol.2025.133427）