On the methane paradox: Transport from shallow water zones rather than in situ methanogenesis is the major source of CH 4 in the open surface water of lakes

Estimates of global methane (CH 4 ) emissions from lakes and the contributions of different pathways are currently under debate. In situ methanogenesis linked to algae growth was recently suggested to be the major source of CH 4 fluxes from aquatic systems. However, based on our very large data set on CH 4 distributions within lakes, we demonstrate here that methane‐enriched water from shallow water zones is the most likely source of the basin‐wide mean CH 4 concentrations in the surface water of lakes. Consistently, the mean surface CH 4 concentrations are significantly correlated with the ratio between the surface area of the shallow water zone and the entire lake, f A,s / t , but not with the total surface area. The categorization of CH 4 fluxes according to f A,s / t may therefore improve global estimates of CH 4 emissions from lakes. Furthermore, CH 4 concentrations increase substantially with water temperature, indicating that seasonally resolved data are required to accurately estimate annual CH 4 emissions.