A coupled two-dimensional hydrodynamic and terrestrial input model to simulate CO₂ diffusive emissions from lake systems

Abstract. Most lakes worldwide are supersaturated with carbon dioxide (CO₂) and consequently act as atmospheric net sources. Since CO₂ is a major greenhouse gas (GHG), the accurate estimation of CO₂ exchanges at air/water interfaces of aquatic ecosystems is vital in quantifying the carbon budget of aquatic ecosystems overall. To date, lacustrine CO₂ emissions are poorly understood, and lake carbon source proportions remain controversial, largely due to a lack of integration between aquatic and terrestrial ecosystems. In this paper a new process-based model (TRIPLEX-Aquatic) is introduced incorporating both terrestrial inputs and aquatic biogeochemical processes to estimate diffusive emissions of CO₂ from lake systems. The model was built from a two-dimensional hydrological and water quality model coupled with a new lacustrine CO₂ diffusive flux model. For calibration and validation purposes, two years of data collected in the field from two small boreal oligotrophic lakes located in Québec (Canada) were used to parameterize and test the model by comparing simulations with observations for both hydrodynamic and carbon process accuracy. Model simulations were accordant with field measurements in both calibration and verification. Consequently, the TRIPLEX-Aquatic model was used to estimate the annual mean CO₂ diffusive flux and predict terrestrial dissolved organic carbon (DOC) impacts on the CO₂ budget for both lakes. Results show a significant fraction of the CO₂ diffusive flux (~30–45%) from lakes was primarily attributable to the input and mineralization of terrestrial DOC, which indicated terrestrial organic matter was the key player in the diffusive flux of CO₂ from oligotropical lake systems in Québec, Canada.