Large-scale assessment of oxic methane production in lakes using an isotopic mass balance approach

Widespread methane (CH ₄ ) supersaturation in lake surface water is a key contributor to natural greenhouse gas emissions. Oxic CH ₄ production (OMP) has recently been recognized as a potential CH ₄ source, challenging the long-held view that surface CH ₄ originates solely from anoxic sediments. Yet most studies are limited to individual systems, leaving broader-scale dynamics uncertain. Here, we apply surface-CH ₄ mass balance approaches incorporating CH ₄ isotopes across 260 lakes spanning diverse landscapes. OMP accounts for 32% of total surface CH ₄ sources on average, at a rate of 1.2 mmol m ^-2 d ^-1 , comparable to air-water diffusive CH ₄ emissions. Our results provide insight into cyanobacterial photosynthesis as an important contributor to OMP in nutrient-rich lakes at the continental scale. Moreover, OMP exhibits a higher temperature sensitivity than any other surface CH ₄ processes, overall indicating its importance for future CH ₄ emissions under eutrophication and climate change.