Seasonal freeze-thaw CO2 sink 'midday rest' phenomenon in lakes: A case study of the largest freshwater lake in the Yellow River Basin

• Lake Wuliangsuhai serves as a CO 2 sink during both frozen and non-frozen periods. • Significant diurnal variations include a “midday rest in CO 2 flux”. • The CO 2 flux during the frozen period is roughly half that in the non-frozen period. • Eutrophication reduces CO 2 sink during the non-ice period, as opposed to ice period. • Lake Wuliangsuhai has a greater capacity as a CO 2 sink than other lakes. The transition between ice-covered and non-ice-covered periods in lakes is continuous. However, the scarcity of comparative CO 2 flux studies between these periods in large plateau lakes of mid-latitude regions represents a significant gap in our knowledge. This study is the first to use an eddy covariance system to directly measure year-round CO 2 flux (May 2018 to April 2019) at Lake Wuliangsuhai, the largest freshwater lake on the Inner Mongolia Plateau in the Yellow River Basin. Results indicated that Lake Wuliangsuhai was a CO 2 sink throughout the observation period. During the ice-covered period, the lake absorbed significant amounts of CO 2 (−0.82 ± 0.26gCm −2 d −1 ), about half the rate observed during the non-ice-covered period (−1.61 ± 1.23gCm −2 d −1 ). In the non-ice-covered period, the net CO 2 exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (RECO) were −382.26 gCm −2 , −519.02 gCm −2 , and 146.98 gCm −2 , respectively. Significant diurnal variations were observed between the two periods. A “midday rest” phase, attributed to stomatal closure in emergent plants, characterized the non-ice-covered period. Carbon source activity was more intense during the non-ice-covered period, showing an unimodal pattern. Monthly, NEE and GPP followed a bimodal pattern. As the lake transitioned from the non-ice-covered to the ice-covered period, daytime meteorological factors influencing CO 2 absorption decreased by nearly half, while nighttime driving forces increased substantially. Eutrophication reduced CO 2 absorption during the non-ice-covered period but enhanced it during the ice-covered period, with CO 2 flux changes showing a delayed response. Lake Wuliangsuhai demonstrated a stronger CO 2 sink capacity than other lakes during both periods.