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

Abstract

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.