Characteristics of carbon sink and the influence factors in Ngoring Lake, Qinghai-Tibet Plateau

Lakes, as a fundamental component of the Earth's surface system, play a crucial role in the carbon cycle, closely linked to climate change. However, understanding carbon flux in Qinghai-Tibet Plateau (QTP) lakes is restricted by environmental factors and limited observations, hindering insights into regional and global climate change. Continuous annual carbon dioxide (CO₂) flux, encompassing ice-covered periods, has been monitored in the largest freshwater lake on the QTP. Utilizing continuous eddy system data, the characteristics and mechanisms influencing carbon flux at various temporal scales in this lake were investigated. Findings revealed Ngoring Lake as predominantly a carbon sink year-round, with two CO₂ absorption peaks in spring and autumn, respectively. These peaks were associated with mixing state triggered by cooling processes. In spring, as temperatures rose above the lake water's maximum density temperature (3.98 ℃ for freshwater lake), subsequent rapid cooling and mixing occurred upon ice melt. In autumn, cooling and mixing were induced by decreasing air and water temperatures alongside strong winds. These cooling processes facilitated significant CO₂ absorption. As the lake transitioned from stratification to mixing, lake mixing played a dominant role. Biochemical reactions driven by water temperature play a dominant role during stable stratification and complete mixing phases.