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

Estimates of global methane (CH sub(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 sub(4) fluxes from aquatic systems. However, based on our very large data set on CH sub(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 sub(4) concentrations in the surface water of lakes. Consistently, the mean surface CH sub(4) concentrations are significantly correlated with the ratio between the surface area of the shallow water zone and the entire lake, fA,s/t, but not with the total surface area. The categorization of CH sub(4) fluxes according to fA,s/t may therefore improve global estimates of CH sub(4) emissions from lakes. Furthermore, CH sub(4) concentrations increase substantially with water temperature, indicating that seasonally resolved data are required to accurately estimate annual CH sub(4) emissions. Key Points * Shallow water zones are the most likely source of the basin-wide mean CH sub(4) concentrations in the surface water of lakes * The categorization of CH sub(4) fluxes according to the fraction of shallow water zones may improve global estimates of CH sub(4) emissions from lakes