Characteristics of Carbon Sink and the Influencing Factors in Ngoring Lake, Qinghai-Tibet Plateau

Continuous annual carbon dioxide (CO₂) flux, encompassing ice-covered periods, has been monitored in Ngoring Lake, the largest freshwater lake on the Qinghai-Tibet Plateau (QTP). By utilizing continuous eddy system data, the characteristics and mechanisms influencing CO₂ flux at various temporal scales in the lake were investigated. Findings revealed that Ngoring Lake was predominantly acting as a carbon sink year-round. The average annual CO₂ sink value was maximum in 2016, about −1.46 g C m⁻² d⁻¹. There were two CO₂ absorption peaks in spring and autumn, respectively. The multi-year average monthly mean CO₂ absorption peaks occurred in April (−1.70 g C m⁻² d⁻¹) and October (−1.75 g C m⁻² d⁻¹), respectively. These peaks were associated with the freeze-thaw process and were caused by the mixing process due to water cooling. The continuous warming during the ice-covered period led to a high-water temperature, and the maximum value reached 6°C. In spring, mixing occurred upon ice melt, and the water temperature at 2 m depth decreased rapidly to 4°C because it was about 5°C higher than the air temperature. In autumn, cooling and mixing were induced by decreasing air and water temperatures alongside strong wind. These cooling processes facilitated significant CO₂ absorption. The CO₂ absorption process was controlled by wind speed, lake ice, lake mixing and stratification.