Eutrophication and\nDissolved Organic Matter Exacerbate\nthe Diel Discrepancy of CO<sub>2</sub> Emissions in China’s\nLargest Urban Lake

The large variability in the emissions of carbon dioxide\n(CO<sub>2</sub>) from urban lakes remains a challenge for partitioning\nthese\nsources at meaningful spatial and temporal scales. Dissolved organic\nmatter (DOM) governs the spatial and temporal variations in CO<sub>2</sub>, yet relationships of the CO<sub>2</sub> concentration (cCO<sub>2</sub>) and emission flux (FCO<sub>2</sub>) with DOM in urban lakes\nhave rarely been reported. In this study, we monitored levels of \ncCO<sub>2</sub>, FCO<sub>2</sub>, and the composition of DOM over\na 24 h period at three sites during the dry and wet seasons in China’s\nlargest urban lake, Tangxun Lake. Our study found the ratio of day/night\nFCO<sub>2</sub> (millimoles per square meter per day) decreased from\nthe dry season (0.79; 7.68/9.68) to the wet season (0.25; 6.05/24.16),\naveraging 0.42 (6.77/15.97), implying that accounting for nighttime\nCO<sub>2</sub> emissions can increase regional estimates by 70%. This\nstudy revealed that eutrophication affected diurnal CO<sub>2</sub> emissions with greater algal growth enhancing daytime CO<sub>2</sub> uptake and subsequently increasing nighttime CO<sub>2</sub> emissions\nvia DOM degradation (larger protein-like DOM fraction). We anticipate\nthat the relative magnitude of FCO<sub>2</sub> between day and night\nfrom lakes is likely to increase due to urbanization and climate change,\nunderscoring the importance of treating urban lakes as a distinct\ngroup and integrating DOM dynamics into carbon cycling in future research.