Atmospheric CO <sub>2</sub> Exchange of a Small Mountain Lake: Limitations of Eddy Covariance and Boundary Layer Modeling Methods in Complex Terrain
Katharina Scholz, Elisabet Ejarque, Albin Hammerle, Martin J. Kainz, Jakob Schelker, Georg Wohlfahrt
Abstract
Abstract Lakes receive and transform significant amounts of terrestrial carbon and are often considered a source of atmospheric carbon dioxide (CO 2 ). Yet, continuous direct measurements of lake‐atmosphere CO 2 exchange with high temporal resolution are sparse. In this study, we measured the CO 2 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 CO 2 with highest emissions in fall. EC flux measurements were affected by low‐frequency contributions especially during low wind conditions. The CO 2 concentration gradient at the air‐water interface decreased during night‐time due to an increase in atmospheric CO 2 above the lake, likely caused by cold and CO 2 ‐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 CO 2 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 CO 2 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.