A new biogeochemical modelling framework (FLaMe v1.0) for lake methane emissions on the regional scale: Development and application to the European domain

Abstract. This study presents a new physical-biogeochemical modelling framework for simulating lake methane (CH₄) emissions at regional scales. The new model, FLaMe v1.0 (Fluxes of Lake Methane), rests on an innovative, computationally efficient lake clustering approach that enables the simulation of CH₄ emissions across a large number of lakes. Building on the Canadian Small Lake Model (CSLM) that simulates the lake physics, we develop a suite of biogeochemical modules to simulate transient dynamics of organic Carbon (C), Oxygen (O₂), and CH₄ cycling. We first test the performance of FLaMe by analyzing physical and biogeochemical processes in two representative lakes (an oligotrophic, deep lake driven by cold climate versus a trophic, shallow lake driven by warm climate). Next, we evaluate the model by comparing simulated and observed timeseries of CH₄ emissions in four well-surveyed lakes. We then apply FLaMe at the European scale to evaluate simulated diffusive and ebullitive lake CH₄ fluxes against in-situ measurements in both boreal and central European regions. Finally, we provide a first assessment of the spatio-temporal variability in CH₄ emissions from European lakes smaller than 1000 km² (n=108407, total area = 1.33x10⁵ km²), indicating a total emission of 0.97±0.23 Tg CH₄ yr^-1, with the uncertainty constrained by combining FLaMe and machine learning techniques. Moreover, 30 % and 70 % of these CH₄ emissions are through diffusive and ebullitive pathways, respectively. Annually averaged CH₄ emission rates per unit lake area during 2010–2016 have a South-to-North decreasing gradient, resulting in a mean over the European domain as 7.39 g CH₄ m^-2 yr^-1. Our simulations reveal a strong seasonality in European lake CH₄ emissions, with late summer emissions nearly ten times higher than winter values. This pronounced seasonal variation highlights the importance of accounting for the sub-annual variability in CH₄ emissions to accurately constrain regional CH₄ budgets. In the future, FLaMe could be embedded into Earth System Models to investigate the feedback between climate warming and global lake CH₄ emissions.