Significant monthly and diel variations of CH4 emission from a shallow eutrophic lake with submerged aquatic vegetation

Shallow eutrophic lakes with submerged aquatic vegetation (SAV) are a large source of atmospheric methane (CH₄) emissions. However, current estimates of CH₄ emissions from lakes are uncertain owing to the lack of data on the diel and monthly variability of CH₄ emissions. In this study, we conducted monthly diel measurements in Ulansuhai Lake, China during the ice-free period (April to October, 2019) to determine the diel and monthly variability of CH₄ emissions. The diffusive CH₄ emission flux (FCH_4d) from June to September was significantly higher (∼5- to 10-fold) than that from April, May, and October, accounting for > 92% of the total emissions. Daytime measurements would overestimate emissions if extrapolated to the whole day because FCH_4d during daytime was higher than that during nighttime. Mean daily FCH_4d emission estimates are overestimated by 25%, ranging from 11% in summer to 46% in spring based solely on daytime measurement. The sampling sites were net sources of CH₄ emissions, with total FCH₄ of 9.74 ± 9.06 mmol m⁻² d⁻¹. FCH_4d increased sharply when the water temperature was above 20 °C. FCH₄ (CO₂ eq) accounted for over 90% of the total greenhouse gas emissions (CH₄ plus CO₂), with the majority occurring from June to September. Our findings indicate that diel and monthly variations should be considered for more accurate estimation of CH₄ emissions from the lakes with SAV.