Photosynthesis‐driven methane production in oxic lake water as an important contributor to methane emission

Recent discovery of methane (CH 4 ) production in oxic waters challenges the conventional understanding of strict anoxic requirement for biological CH 4 production. High‐resolution field measurements in Lake Stechlin, as well as incubation experiments, suggested that oxic‐water CH 4 production occurred throughout much of the water column and was associated with phytoplankton especially diatoms, cyanobacteria, green algae, and cryptophytes. In situ concentrations and δ 13 C values of CH 4 in oxic water were negatively correlated with soluble reactive phosphorus concentrations. Using 13 C‐labeling techniques, we showed that bicarbonate was converted to CH 4 , and the production exceeded oxidation at day, but was comparable at night. These experimental data, along with complementary field observations, indicate a clear link between photosynthesis and the CH 4 production‐consumption balance in phosphorus‐limited epilimnic waters. Comparison between surface CH 4 emission data and experimental CH 4 production rates suggested that the oxic CH 4 source significantly contributed to surface emission in Lake Stechlin. These findings call for re‐examination of the aquatic CH 4 cycle and climate predictions.