The annual variability of CO₂ exchange in most ecosystems is primarily driven by the activities of plants and soil microorganisms. However, little is known about the carbon balance and its controlling factors outside the growing season in Arctic regions dominated by soil freeze/thaw processes, long-lasting snow cover, and several months of darkness. This study presents a complete annual cycle of the CO₂ net ecosystem exchange (NEE) dynamics for a high Arctic tundra area at the west coast of Svalbard based on eddy covariance flux measurements. The annual cumulative CO₂ budget is close to 0 g C m⁻² y^r−1, but displays a strong seasonal variability. Four major CO₂ exchange seasons have been identified. (1) During summer (snow-free ground), the CO₂ exchange occurs mainly as a result of biological activity, with a dominance of strong CO₂ assimilation by the ecosystem. (2) The autumn (snow-free ground or partly snow-covered) is dominated by CO₂ respiration as a result of biological activity. (3) In winter and spring (snow-covered ground), low but persistent CO₂ release occurs, overlayed by considerable CO₂ exchange events in both directions associated with high wind speed and changes of air masses and atmospheric air pressure. (4) The snow melt season (pattern of snow-free and snow-covered areas) is associated with both meteorological and biological forcing, resulting in a carbon uptake by the high Arctic ecosystem.

Data related to this article are archived at http://doi.pangaea.de/10.1594/PANGAEA.809507.