Diurnal dynamics variations of in CO2 exchange fluxes and their influencing factors in a shallow macrophyte-dominated lake in the northeastern Qinghai-Tibetan Plateau: a case study of Hurleg Lake

Junxiang Xie^1,2Yanxiang Jin^1,2,3,4*Xin Jin^1,2,3,4Zi’ang Li^1,2Tongrui Zhang^1,2Xin Zhang^1,2Jingyun Yang^1,2

• ¹Qinghai Normal University School of Geographical Sciences, Xining, China
• ²Qinghai Normal University, Qinghai Province Key Laboratory of Physical Geography and Environmental Process, Xining, China
• ³Key Laboratory of Tibetan Plateau Land Surface Processes and Ecological Conservation (Ministry of Education), Qinghai Normal University, Xining, China
• ⁴Academy of Plateau Science and Sustainability, People's Government of Qinghai Province and Beijing Normal University, Xining, China

As an important component of inland waters, shallow macrophyte-dominated lakes significantly influence regional carbon budgets through their carbon source-sink characteristics. To investigate the diurnal dynamics of CO2 exchange fluxes (F(CO2)) at the water-air and water-sediment interfaces of shallow macrophyte-dominated lakes in Qinghai-Tibetan Plateau, this study took Hurleg Lake in the northeastern Qinghai-Tibetan Plateau, a typical lake of this type, as an example. By using the static chamber-gas chromatography method and the sediment in-situ simulation, continuous fixed-point observations of F(CO2) at both interfaces were conducted on July 19, August 10, August 21, and October 4, 2024. Combination with watershed meteorological conditions and lake water environmental parameters, their influencing factors were explored. The results revealed significant diel variations in F(CO₂) at both interfaces, characterized by peaks in the early morning and troughs in the evening or late night-a common feature of shallow macrophyte-dominated lakes. The results showed that: The F(CO2) at the water-air and water-sediment interfaces of Hurleg Lake exhibited significant diel dynamics, with peaks generally occurring in the early morning and troughs mostly in the evening and late night, which is a common feature of shallow macrophyte-dominated lakes.The composition of submerged macrophyte communities considerably affected the relative contribution of sediment-released CO₂ to the net atmospheric flux. The type of submerged macrophyte community significantly affected the relative contribution of sediment-released CO2 to the net atmospheric flux of the water body. The maximum contribution was observed in areas dominated by Potamogeton, followed by Myriophyllum zones, while the minimum occurred in Chara beds. Nocturnal F(CO₂) played a critical role in sustaining the carbon source function of the lake, accounting for 22.65%–42.90% of the total daily flux at the water-air interface and 5.57%-64.54% at the water-sediment interface across different vegetated and unvegetated zones. Neglecting nocturnal F(CO₂) would substantially increase uncertainties in estimating the lake's overall carbon budget. The F(CO₂) at the water-air interface was primarily regulated by water temperature, pH, dissolved oxygen, and atmospheric pressure, whereas F(CO₂) at the sediment-water was mainly driven by porewater CO₂ concentration, sediment porosity, and water temperature.