Applicability and consequences of the integration of alternative models for CO₂ transfer velocity into a process-based lake model

• Main Authors: P. Kiuru, A. Ojala, I. Mammarella, J. Heiskanen, K.-M. Erkkilä, H. Miettinen, T. Vesala, T. Huttula
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-09-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/3297/2019/bg-16-3297-2019.pdf
• ISSN: 1726-4170; 1726-4189

<p>Freshwater lakes are important in carbon cycling, especially in the boreal zone where many lakes are supersaturated with the greenhouse gas carbon dioxide (<span class="inline-formula">CO₂</span>) and emit it to the atmosphere, thus ventilating carbon originally fixed by the terrestrial system. The exchange of <span class="inline-formula">CO₂</span> between water and the atmosphere is commonly estimated using simple wind-based parameterizations or models of gas transfer velocity (<span class="inline-formula"><i>k</i></span>). More complex surface renewal models, however, have been shown to yield more correct estimates of <span class="inline-formula"><i>k</i></span> in comparison with direct <span class="inline-formula">CO₂</span> flux measurements. We incorporated four gas exchange models with different complexity into a vertical process-based physico-biochemical lake model, MyLake C, and assessed the performance and applicability of the alternative lake model versions to simulate air–water <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span> concentrations or respective air–water <span class="inline-formula">CO₂</span> 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 a sufficient gain of <span class="inline-formula">CO₂</span> in the water column and thus resulted in lower <span class="inline-formula">CO₂</span> fluxes and water column <span class="inline-formula">CO₂</span> concentrations compared to the respective measurement-based values. The inclusion of sophisticated and more correct models for air–water <span class="inline-formula">CO₂</span> exchange in process-based lake models is crucial in efforts to properly assess lacustrine carbon budgets through model simulations in both single lakes and on a larger scale. However, finding higher estimates for both the internal and external sources of inorganic carbon in boreal lakes is important if improved knowledge of the magnitude of <span class="inline-formula">CO₂</span> evasion from lakes is included in future studies on lake carbon budgets.</p>