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 (FCH4d) 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 FCH4d during daytime was higher than that during nighttime. Mean daily FCH4d 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. FCH4d 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.