Eutrophication and Dissolved Organic Matter Exacerbate the Diel Discrepancy of CO2 Emissions in China’s Largest Urban Lake

The large variability in the emissions of carbon dioxide (CO₂) from urban lakes remains a challenge for partitioning these sources at meaningful spatial and temporal scales. Dissolved organic matter (DOM) governs the spatial and temporal variations in CO₂, yet relationships of the CO₂ concentration (cCO₂) and emission flux (FCO₂) with DOM in urban lakes have rarely been reported. In this study, we monitored levels of cCO₂, FCO₂, and the composition of DOM over a 24 h period at three sites during the dry and wet seasons in China’s largest urban lake, Tangxun Lake. Our study found the ratio of day/night FCO₂ (millimoles per square meter per day) decreased from the dry season (0.79; 7.68/9.68) to the wet season (0.25; 6.05/24.16), averaging 0.42 (6.77/15.97), implying that accounting for nighttime CO₂ emissions can increase regional estimates by 70%. This study revealed that eutrophication affected diurnal CO₂ emissions with greater algal growth enhancing daytime CO₂ uptake and subsequently increasing nighttime CO₂ emissions via DOM degradation (larger protein-like DOM fraction). We anticipate that the relative magnitude of FCO₂ between day and night from lakes is likely to increase due to urbanization and climate change, underscoring the importance of treating urban lakes as a distinct group and integrating DOM dynamics into carbon cycling in future research.