Tracing the origins of metalimnetic methane in a stratified large lake: Insights from high‐resolution mapping and three‐dimensional hydrodynamic modeling

Methane (CH 4 ) accumulation in the oxic metalimnion is a common yet poorly resolved feature of deep stratified lakes. In Lake Geneva, lateral transport from the Rhône delta has been suggested but lacks spatial validation. Here, we quantify the role of lateral advection in shaping metalimnetic CH 4 using high‐resolution in situ CH 4 measurements, δ 13 C‐CH 4 analyses, and a three‐dimensional Lagrangian particle‐tracking model. Elevated metalimnetic CH 4 (up to 1143 nM) near the Rhône delta becomes isotopically enriched, likely from prolonged residence and partial oxidation during summer stratification. While additional sources may contribute, comparisons of in situ measurements with particle‐tracking simulations suggest circulation gyres may distribute deltaic CH 4 . Moreover, isotope‐ and transport‐based indicators suggest more coherent coupling in the epilimnion than in the metalimnion. These results demonstrate that horizontal advection can be an important driver of metalimnetic CH 4 and highlight the importance of circulation‐driven lateral transport for assessing CH 4 fluxes from large, stratified lakes.