As lakes receive and transform significant amounts of terrestrial carbon, they often act as source of atmospheric CO₂. Yet, long-term measurements of lake-atmosphere CO₂ exchange with high temporal resolution are sparse. In this study, we measured the CO₂ exchange of a small lake situated in complex mountainous topography in the Austrian Alps continuously for one year. We used the eddy covariance (EC) and the boundary layer model (BLM) approaches to estimate the lake’s CO₂ source or sink strength and to analyze differences between these methods.

Overall, CO₂ fluxes were small and EC measurements indicated influence of low-frequency contributions. 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.

During night-time, the CO₂ concentration gradient at the air-water interface decreased 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 exemplifies the relevance of the surrounding landscape for 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 systems.