Significant methane ebullition from large shallow eutrophic lakes of the semi-arid region of northern China.

Eutrophic lakes are a major source of the atmospheric greenhouse gas methane (CH), and CH ebullition emissions from inland lakes have important implications for the carbon cycle. However, the spatio-temporal heterogeneity of CH ebullition emission and its influencing factors in shallow eutrophic lakes of arid and semi-arid regions remain unclear. This study aimed to determine the mechanism of CH emission via eutrophication in Lake Ulansuhai, a large shallow eutrophic lake in a semi-arid region of China.To this end, monthly field surveys were conducted from May to October 2021, and gas chromatography was applied using the headspace equilibrium technique with an inverted funnel arrangement. The total CH fluxes ranged from 0.102 mmol m d to 59.296 mmol m d with an average value of 4.984 ± 1.82 mmol m d. CH ebullition emissions showed significant temporal and spatial variations. The highest CH ebullition emission was observed in July with a grand mean of 9.299 mmol m d, and the lowest CH ebullition emissions occurred in October with an average of 0.235 mmol m d. Among seven sites (S1-S7), the maximum (3.657 mmol m d) and minimum (1.297 mmol m d). CH ebullition emissions were observed at S2 and S7, respectively. As the main route of CH emission to the atmosphere in Lake Ulansuhai, the CH ebullition flux during May to October accounted for 69% of the total CH flux. Statistical analysis showed that CH ebullition was positively correlated with temperature (R = 0.391, P < 0.01) and negatively correlated with air pressure (R = 0.286, P < 0.00). Temperature and air pressure were found to strongly regulate the production and oxidation of CH. Moreover, nutritional status indicators such as TP and NH-N significantly affect CH ebullition emissions (R = 0.232, P < 0.01; R = -0.241, P < 0.01). This study reveals the influencing factors of CH ebullition emission in Lake Ulansuhai, and provides theoretical reference and data support for carbon emission from eutrophic lakes. Nevertheless, research on eutrophic shallow lakes needs to be further strengthened. Future research should incorporate improved flux measurement techniques with process-based models to improve the accuracy from regional to large-scale estimation of CH emissions and clarify the carbon budget of aquatic ecosystems. In this manner, the understanding and predictability of CH ebullition emission from shallow lakes can be improved.