Lakes receive and transform significant amounts of terrestrial carbon and are often considered a source of atmospheric carbon dioxide (CO₂). Yet, continuous direct measurements of lake-atmosphere CO₂ exchange with high temporal resolution are sparse. In this study, we measured the CO₂ exchange of a mountain lake in the eastern Austrian Alps continuously for one year using the eddy covariance (EC) and the boundary layer model (BLM) approaches. Results from both the EC and the BLM methods indicated the lake to be a small source of atmospheric CO₂ with highest emissions in fall. EC flux measurements were affected by low-frequency contributions especially during low wind conditions. The CO₂ concentration gradient at the air-water interface decreased during night-time due to an increase in atmospheric CO₂ above the lake, likely caused by cold and CO₂-rich air draining from the surrounding land. Consequently, BLM fluxes were lower during night-time than during daytime. This diel pattern was lacking in the EC flux measurements because the EC instruments deployed at the shore of the lake did not capture low nocturnal lake CO₂ fluxes due to the local wind regime. Overall, this study illustrates the effect of the surrounding landscape on lake-atmosphere flux measurements. We conclude that estimating CO₂ evasion from lakes situated in complex topography needs to explicitly account for biases in EC flux measurements caused by low-frequency contributions and local wind regimes.

中文翻译：

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一个小山地湖泊的大气 CO2 交换：复杂地形中涡度协方差和边界层建模方法的局限性

湖泊接收并转化了大量的陆地碳，通常被认为是大气二氧化碳 (CO ₂ ) 的来源。然而，对具有高时间分辨率的湖泊-大气 CO ₂交换的连续直接测量很少。在这项研究中，我们使用涡度协方差 (EC) 和边界层模型 (BLM) 方法连续测量了奥地利阿尔卑斯山脉东部山区湖泊的 CO ₂交换一年。EC 和 BLM 方法的结果表明，该湖是大气 CO ₂的小来源，秋季排放量最高。EC 通量测量受低频贡献的影响，尤其是在低风条件下。二氧化碳₂由于湖泊上方大气 CO ₂的增加，空气-水界面的浓度梯度在夜间下降，这可能是由于周围土地排出的寒冷和富含CO _{2 的}空气所致。因此，夜间 BLM 通量低于白天。EC 通量测量中缺乏这种 diel 模式，因为部署在湖岸的 EC 仪器由于当地风况而无法捕获低夜间湖泊 CO ₂通量。总的来说，这项研究说明了周围景观对湖泊-大气通量测量的影响。我们得出结论，估计 CO ₂ 避开位于复杂地形中的湖泊需要明确考虑由低频贡献和当地风况引起的 EC 通量测量偏差。