Characteristics and influence factors of carbon dioxide efflux from Lake Hongze under different hydrological scenarios

CO2 emission from inland waters is an important component in assessing the global carbon budgets, while the large uncertainty during the budget estimation not only results from incomplete spatial coverage of sampling sites, but also from the lack of studies on the linkages between hydrological heterogeneity of inland waters and CO2 emission. Here we examined the CO2 efflux from Lake Hongze under different hydrological scenarios in 2018, to investigate the characteristics and influencing factors of CO2. Our results indicated that the CO2 fluxes in the wet season ((106.9±73.4)mmol/(m2•d)) was significantly higher than in the dry season ((18.7±13.6)mmol/(m2•d)), and further higher in the wet-to-dry transition season ((5.2±15.5)mmol/(m2•d)). The alternation of CO2 efflux from the wet season to the dry, and further to the wet-to-dry transition season (310.2-32.0 mmol/(m2•d), 50.8-2.2 mmol/(m2•d), and -17.3-39.8 mmol/(m2•d), separately) suggested a shift from the net carbon source to a weak sink of CO2 emission from Lake Hongze. The CO2 efflux was generally low in the northern open water and high in the southern waterway region. The emission of CO2 from Lake Hongze was highly sensitive to the hydrological scenarios, especially the inflow runoff from the upstream Huaihe River to the lake. In the rainy wet season, the lake received large quantities of organic and inorganic carbon inputs from the Huaihe River. The labile inorganic carbon could be easily transformed into CO2, and the degradation and mineralization of terrestrial organic carbon could significantly promote the production and emission of CO2. The input of nitrogen, phosphorus and other nutrients enhanced the eutrophication and further increased the emission of CO2 from Lake Hongze, especially the upstream inflowing lake regions. Relatively low inflow runoff during the dry and the wet-to-dry transition seasons resulted in a prolonged water residence time and allochthonous input to autochthonous degradation of organic matter potentially favored the production of CO2 emission in the water. We further found the accumulation and degradation of terrestrial humic-like could promote CO2 emission, while autochthonous components of dissolved organic matter (DOM) were linked weakly to the process and emission of CO2. Our results demonstrate the importance of hydrological scenarios fueling the emission of CO2 from freshwater lakes, and it is necessary to conduct high-frequency observations to further clarify the efflux of CO2 from lakes and the corresponding influencing factors. © 2022 by Journal of Lake Sciences.