Eutrophication and Dissolved Organic Matter Exacerbate the Diel Discrepancy of CO Emissions in China's Largest Urban Lake.

The large variability in the emissions of carbon dioxide (CO) 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 CO, yet relationships of the CO concentration (cCO) and emission flux (FCO) with DOM in urban lakes have rarely been reported. In this study, we monitored levels of cCO, FCO, 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 FCO (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 CO emissions can increase regional estimates by 70%. This study revealed that eutrophication affected diurnal CO emissions with greater algal growth enhancing daytime CO uptake and subsequently increasing nighttime CO emissions via DOM degradation (larger protein-like DOM fraction). We anticipate that the relative magnitude of FCO 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.