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

Abstract

The transition between ice-covered and non-ice-covered periods in lakes is continuous. However, the scarcity of comparative CO₂ 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 CO₂ 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 CO₂ sink throughout the observation period. During the ice-covered period, the lake absorbed significant amounts of CO₂ (−0.82 ± 0.26gCm⁻²d⁻¹), about half the rate observed during the non-ice-covered period (−1.61 ± 1.23gCm⁻²d⁻¹). In the non-ice-covered period, the net CO₂ 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 CO₂ absorption decreased by nearly half, while nighttime driving forces increased substantially. Eutrophication reduced CO₂ absorption during the non-ice-covered period but enhanced it during the ice-covered period, with CO₂ flux changes showing a delayed response. Lake Wuliangsuhai demonstrated a stronger CO₂ sink capacity than other lakes during both periods.