A plateau freshwater shallow lake as a significant CO₂ sink during the ice-covered period: A case study of Lake Wuliangsuhai, China

Lakes are essential for estimating the global CO₂ budget. However, approximately 50 % of lakes undergo periodic freezing, and there is limited research on the factors influencing the CO₂ cycle and ice formation in freshwater lakes located in middle- and high-latitude plateaus during ice-covered periods. Using high-frequency meteorological-flux data collected over six consecutive months during the 2018-2019 freezing period of Lake Wuliangsuhai, this study explored the diurnal variation, daily accumulation, and monthly accumulation of the CO₂ cycle and its influencing mechanisms at a half-hour scale. The key findings are as follows. Lakes are CO₂ sinks during the ice-covered period, with the fluxes being -1.28 ± 4.79 gCm^-2d^-1. Net CO₂ exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (RECO) during the monitoring period were -116.93 gCm^-2,190.36 gCm^-2, and 86.09 gCm^-2, respectively. The lake ice-air CO₂ cycle exhibited significant (p < 0.05) diurnal variation, with daytime contributing 92.89 %, 78.31 %, and 56.91 % to NEE, GPP, and RECO, respectively. From December 2018 to March 2019, the monthly total GPP in the whole lake exceeded 10,000 tons, and high autotrophy was observed in February 2019. During the freezing period, 45.22 % of plant assimilation was consumed by autotrophic and heterotrophic respiration. The capacity of the lake CO₂ sink was primarily driven by evaporation, latent heat, radiative sensible heat, and air pressure inhibition, whereas snow cover reduced the CO₂ sink capacity by 73.01 %. CO₂ capture by surface ice was mainly affected by external factors such as snow cover and solar radiation, whereas bottom ice capture was influenced by internal factors such as acid-base balance, the carbonate pump, and biological primary production. Bubble storage and ice crevice transport significantly affected CO₂ migration. In summary, further research should focus on elucidating the CO₂ capture mechanisms in seasonally frozen lakes located at middle and high latitudes, within subsequent lake‑carbon sink studies.