Warm-wet conditions enhance nighttime methane accumulation in China's largest urban lake

Diel fluctuation of methane (CH₄) emission in urban lakes has received limited attention. In this study, we assessed diel dynamics of dissolved CH₄ concentration (dCH₄, μmol L^-1) and diffusive CH₄ emission (FCH₄, mmol m^-2 d^-1) in China's largest urban lake, Lake Tangxun in Wuhan City, Central China. Three 24-h sampling campaigns were conducted in each of a cool-dry and a warm-wet season, whereby hourly dCH₄, FCH₄, and ambient conditions were determined. In the cool-dry season, no significant day-night differences were observed in measured dCH₄ and estimated FCH₄; In contrast, during the warm-wet season, nighttime-measured dCH₄ (0.42 ± 0.23) was significantly higher than daytime values (0.31 ± 0.11) (p < 0.05; effect size |r| = 0.246; Mann Whitney U test), yet estimated FCH₄ showed no significant diel difference due to elevated daytime winds inflating gas transfer velocity. Nevertheless, the day-to-night FCH₄ ratio decreased numerically from 0.88 (cool-dry, day: 0.07; night: 0.08) to 0.74 (warm-wet, day: 0.29; night: 0.39), implying that daytime-only measurements could underestimate daily FCH₄ by up to ∼20 % if nighttime contributions are overlooked. Mean FCH₄ (0.33 ± 0.24 vs. 0.08 ± 0.07) was 4-fold higher in the warm-wet season. These results indicate that warm-wet conditions, characterized by co-occurring increases in temperature, nutrients, and trophic state, amplified nighttime dCH₄, and, after wind correction, FCH₄. Our findings emphasize the necessity of incorporating diel and seasonal variations in urban lake CH₄ emission estimates. Multi-year, multi-lake studies are essential to quantify climate change impacts on nighttime CH₄ emissions and inform urban lake management within global carbon budgets.