Applicability and consequences of the integration of alternative models for CO 2 transfer velocity into a process-based lake model

Abstract. Freshwater lakes are important in carbon cycling especially in the boreal zone, where many lakes are supersaturated with the greenhouse gas carbon dioxide (CO 2 ) and emit it to the atmosphere, thus ventilating carbon originally fixed by the terrestrial system. The exchange of CO 2 between water and the atmosphere is commonly estimated using simple wind-based parameterizations or models of gas transfer velocity (k). More complex surface renewal models, however, have been shown to yield more correct estimates of k in comparison with direct CO 2 flux measurements. We incorporated four gas exchange models with different complexity into a vertical process-based physicobiochemical lake model MyLake C and assessed the performance and applicability of the alternative lake model versions to simulate air-water CO 2 fluxes over a small boreal lake. None of the incorporated gas exchange models significantly outperformed the other models in the simulations in comparison to the measured near-surface CO 2 concentrations or respective air-water CO 2 fluxes calculated directly with the gas exchange models using measurement data as input. The use of more complex gas exchange models in the simulation, on the contrary, led to difficulties in obtaining sufficient gain of CO 2 in the water column and thus resulted in lower CO 2 fluxes and water column CO 2 concentrations compared to the respective measurement-based values. Inclusion of sophisticated and more correct models for air-water CO 2 exchange in process-based lake models is crucial in efforts to properly assess lacustrine carbon budgets through model simulations both in single lakes and on a larger scale. However, finding higher estimates for both the internal and the external sources of inorganic carbon in boreal lakes is important if the improved knowledge of the magnitude of CO 2 evasion from lakes is included in future studies on lake carbon budgets.