Sub-oxycline methane oxidation can fully uptake CH produced in sediments: case study of a lake in Siberia.

It is commonly assumed that methane (CH) released by lakes into the atmosphere is mainly produced in anoxic sediment and transported by diffusion or ebullition through the water column to the surface of the lake. In contrast to that prevailing idea, it has been gradually established that the epilimnetic CH does not originate exclusively from sediments but is also locally produced or laterally transported from the littoral zone. Therefore, CH cycling in the epilimnion and the hypolimnion might not be as closely linked as previously thought. We utilized a high-resolution method used to determine dissolved CH concentration to analyze a Siberian lake in which epilimnetic and hypolimnetic CH cycles were fully segregated by a section of the water column where CH was not detected. This layer, with no detected CH, was well below the oxycline and the photic zone and thus assumed to be anaerobic. However, on the basis of a diffusion-reaction model, molecular biology, and stable isotope analyses, we determined that this layer takes up all the CH produced in the sediments and the deepest section of the hypolimnion. We concluded that there was no CH exchange between the hypolimnion (dominated by methanotrophy and methanogenesis) and the epilimnion (dominated by methane lateral transport and/or oxic production), resulting in a vertically segregated lake internal CH cycle.