Atmospheric CO2 Exchange of a Small Mountain Lake

Lakes and rivers are considered to be substantial sources of carbon dioxide (CO2)
Image: Lunzer See (Credit: K. Scholz)

Lakes and rivers are considered to be substantial sources of carbon dioxide (CO2). Yet, continuous measurements of lake-atmosphere CO2 exchange are sparse. To better understand the magnitude, temporal variability, and driving factors of CO2 fluxes of a small mountain lake, a team of ecologists of the University of Innsbruck and WasserCluster Lunz monitored the CO2 exchange at the air-water interface of Lake Lunz for an entire year. The results were recently published in JGR Biogeosciences.

To capture CO2 fluxes at the air-water interface, several methods with differing temporal and spatial resolution exist. In this study, the scientists compared the results of CO2 fluxes measured with two different methods – the Eddy Covariance (EC) and the Boundary Layer Model (BLM) approach. In addition, the local wind regime and its influence on the CO2 exchange and on the respective measurement method was analyzed. Overall, the measured CO2 fluxes were low with highest emissions in fall during lake turnover. Furthermore, the results demonstrated that the existence of a local land-lake wind regime directly influenced the CO2 exchange at the air-water interface by determining the atmospheric CO2 concentration. During night, the air was typically draining down the slopes of the surrounding land towards the lake which was accompanied by a substantial increase in atmospheric CO2. During the day, strong westerly winds persisted above the lake and atmospheric CO2 was typically lower.  Because the gas transfer at the air-water interface is largely determined by the concentration difference at the interface, this resulted in a diel pattern in CO2 fluxes with higher CO2 emissions throughout the day and lower emissions or CO2 uptake during the night.

EC data were biased towards higher fluxes, because with the EC approach an upwind signal is captured and the instruments were set up on the east shore of the lake, therefore measuring lake fluxes only during westerly winds.

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