Kai Jinlei, Wang Junbo, Ju Jianting, Huang Lei, Daut Gerhard, Zhu Liping
State Key Laboratory of Tibetan Plateau Earth System, Environment and Resources (TPESER), Nam Co Observation and Research Station (NAMORS), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China.
State Key Laboratory of Tibetan Plateau Earth System, Environment and Resources (TPESER), Nam Co Observation and Research Station (NAMORS), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China.
Sci Total Environ. 2024 Oct 15;947:174682. doi: 10.1016/j.scitotenv.2024.174682. Epub 2024 Jul 11.
The Tibetan Plateau (TP) has the world's largest distribution of high-alpine and saline (generally hardwater) lakes, which are expected to affect regional carbon cycling profoundly. However, the variability, and especially underlying factors controlling CO dynamics, across widespread hardwater lakes is poorly understood on the TP. Here, we present year-round records of surface water pCO from a representative hardwater lake (Nam Co) on the TP, and analyze relationships between ambient variables and pCO during open water (i.e., ice-free) and ice-covered months. Surface pCO (233.3 μatm on average) was a little oversaturated to atmosphere (219 μatm on average) during the open water season. As a CO source, Nam Co emitted 8.73 ± 1.06 Gg C annually, but this flux only accounted for 0.53 ± 0.06 ‰ of its total dissolved inorganic carbon pool (1.64 × 10 g C). Regression results indicate that, during open water months, both seasonal and diurnal varying patterns of surface pCO were influenced predominantly by water temperature, in a quasi-marine mode, by controlling gas solubility and dissolved carbonate equilibria. Therefore, CO evasion was elevated during summer months, despite the lake being autotrophic (i.e., CO consumption via photosynthesis). By contrast, during ice-covered months the surface pCO was strongly related to under-ice thermodynamics, and declined nonlinear with increased inversed stratification. In the hypolimnion, as a result of extremely weak metabolism (as indicated by low dissolved oxygen depletion rates) and a combined high carbonate buffering effect, accumulation of CO was negligible, leading to an absence of peak effluxes of CO during turnover periods, compared to eutrophic freshwater lakes. We argue that, under future global warming scenarios, consideration of the impact of gradually warming lake water on thermodynamics and dissolved carbonate equilibria are vital in order to understand the future CO dynamics of these widespread high-altitude oligotrophic-hardwater lakes situated across the TP.
青藏高原拥有世界上分布最广的高海拔和咸水（通常为硬水）湖泊，预计这些湖泊会对区域碳循环产生深远影响。然而，青藏高原上广泛分布的硬水湖泊的变异性，尤其是控制二氧化碳动态的潜在因素，目前了解甚少。在此，我们展示了青藏高原上一个具有代表性的硬水湖泊（纳木错）地表水pCO₂的全年记录，并分析了在无冰期和冰封期，环境变量与pCO₂之间的关系。在无冰期，地表水pCO₂（平均为233.3微大气压）略高于大气（平均为219微大气压），呈过饱和状态。作为一个二氧化碳源，纳木错每年排放8.73±1.06 Gg碳，但该通量仅占其总溶解无机碳库（1.64×10⁶ g碳）的0.53±0.06‰。回归结果表明，在无冰期，地表水pCO₂的季节性和昼夜变化模式主要受水温影响，通过控制气体溶解度和溶解碳酸盐平衡，呈现出类似海洋的模式。因此，尽管该湖泊是自养型（即通过光合作用消耗二氧化碳），但夏季的二氧化碳逸出量仍有所增加。相比之下，在冰封期，地表水pCO₂与冰下热力学密切相关，并随着逆温层的增加而非线性下降。在湖下层，由于代谢极其微弱（低溶解氧消耗率表明）以及高碳酸盐缓冲作用的综合影响，二氧化碳的积累可以忽略不计，与富营养化淡水湖泊相比，在水体翻转期没有出现二氧化碳排放峰值。我们认为，在未来全球变暖的情况下，考虑湖水逐渐变暖对热力学和溶解碳酸盐平衡的影响，对于理解青藏高原上这些广泛分布的高海拔贫营养硬水湖泊未来的二氧化碳动态至关重要。
