Surface CO₂ Gradients Challenge Conventional CO₂ Emission Quantification in Lentic Water Bodies under Calm Conditions

Abstract. Lakes are hotspots of inland carbon cycling and are important sources of greenhouse gases (GHGs), such as carbon dioxide (CO₂). The significant role of CO₂ in global carbon cycle makes quantifying its emission from various ecosystems, including lakes and reservoirs, important for developing strategies to mitigate climate change. The thin boundary layer method is a common approach to calculate CO₂ fluxes from CO₂ measurements in both the water and the air, and wind speed. However, one assumption for the TBL method is a homogeneous CO₂ concentration between the measurement depth and the water surface, where gas exchange takes place. This assumption might not be true under calm conditions, when microstratification below the surface slows vertical exchange of gases. We used a floating outdoor laboratory to monitor CO₂ concentrations in 5 cm and 25 cm depth, CO₂ concentration in the air, wind speed, and water temperature profiles for one week in Bautzen Reservoir, Germany. While we found homogeneous CO₂ concentrations in the two depths during wind speeds above 3 m s^-1, there was a vertical gradient observed during wind still nights. The concentrations observed temporally ranged from undersaturation to supersaturation in 25 cm and 5 cm, respectively. Fluxes calculated from the measured concentrations therefore would change from negative to positive, depending on the measurement depth. Simultaneous Eddy Covariance measurements showed that even the measurements close to the surface underestimated the actual CO₂ concentration. Oxygen measurements support our hypothesis that respirational processes at the water surface cause a temporal CO₂ concentration gradient from the surface to the underlying water. Until now, the depth of CO₂ measurements has not been questioned, as long as measurements were done in the upper mixed layer and close to the surface. Our results provide evidence that representative measurements of CO₂ in the water strongly depend on depth and time of measurements.