Oxic Methane Photoproduction in Global Great Lakes

Lingering inconsistencies in global methane (CH₄) budgets have prompted huge efforts to estimate CH₄ emissions from anthropogenic and natural sources. Herein, we report the spatiotemporal patterns of the oxic CH₄ photoproduction potential in global lakes and its contribution to diffusive CH₄ fluxes via a remote sensing based photochemical model and lab-determined photoproduction molar ratio of CH₄ to CO (ΔCH₄/ΔCO). During the years 2011-2020, the oxic CH₄ photoproduction potential in China's lakes (>100 km²) ranged from 2.21e-7 to 5.97e-6 mol (CH₄) m^-2 day^-1. Deep oligotrophic lakes in western China, particularly the Tibetan Plateau, exhibited a higher CH₄ photoproduction potential compared to shallow eutrophic lakes in the eastern regions. Globally, the oxic photoproduction potential of CH₄ from lakes (>100 km²) averaged 1.27e-6 mol (CH₄) m^-2 day^-1 during 2011-2020. Lakes in higher latitudes (>60°) exhibited generally lower CH₄ photoproduction rates, whereas those in equatorial regions (-20° ≤ latitude ≤ 20°) and high-altitude areas (>4000 m) showed overall higher rates. It was estimated that the oxic CH₄ photoproduction process of inland lakes accounted for 0.1-22% of diffusive CH₄ fluxes on a global scale, and the total oxic CH₄ photoproduction budget of global lakes is estimated to be 0.0104 Tg CH₄/year. This study highlights the non-negligible role of the oxic CH₄ photoproduction process in aquatic ecosystems, which contributes a valuable piece to the complex puzzle of global CH₄ cycles.