Sub-oxycline methane oxidation can fully uptake CH4 produced in sediments: case study of a lake in Siberia

Abstract

it is commonly assumed that methane (cH 4) 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 4 does not originate exclusively from sediments but is also locally produced or laterally transported from the littoral zone. Therefore, CH 4 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 4 concentration to analyze a Siberian lake in which epilimnetic and hypolimnetic CH 4 cycles were fully segregated by a section of the water column where CH 4 was not detected. This layer, with no detected cH 4 , 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 4 produced in the sediments and the deepest section of the hypolimnion. We concluded that there was no CH 4 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 4 cycle.

Methanogenesis exceeds CH4 consumption in eutrophic lake sediments

“…For example, aerobic and anaerobic methanotrophic communities in the water column are dynamic and may rapidly respond to shifting CH 4 , O 2 , and other electron acceptor concentrations (Graf et al 2018;Rissanen et al 2020;Mayr et al 2020b). Stratified lakes may have sufficiently high rates of CH 4 oxidation at or near the oxycline (Morana et al 2015) or in the anaerobic water column (Thalasso et al 2020; van Grinsven et al 2020a) to compensate for potentially low oxidation efficiency in sediments. Low sediment oxidation efficiency may be a less important control on emissions during seasonal overturn of stratified lakes, which can be a hot moment for CH 4 oxidation in the water column (Kankaala et al 2007;Mayr et al 2020a).…”

Dissolved CH<sub>4</sub> coupled to photosynthetic picoeukaryotes in oxic waters and to cumulative chlorophyll <i>a</i> in anoxic waters of reservoirs

“…Traditionally, the net CH 4 production is determined by archaeal methanogenesis, which produces methane as an end product of organic matter degradation in anoxic conditions, and to methanotrophs, which consume it in oxic conditions (Schubert and Wehrli, 2018). In freshwater ecosystems, the anoxic sediments are a primary source of CH 4 (Segers, 1998), where methanogens are very sensitive to temperature and quantity and quality of the organic matter used as substrate (Marotta et al, 2014;Rasilo et al, 2015;Sepulveda-Jauregui et al, 2018;Thanh-Duc et al, 2010;West et al, 2012;Yvon-Durocher et al, 2014). They are also affected by the extent of anoxia in the sediments insomuch as they are obligate anaerobes and will not survive and produce CH 4 under aerobic conditions (Chistoserdova et al, 1998;Schubert and Wehrli, 2018).…”

A synthesis of methane dynamics in thermokarst lake environments

“…6). A recent study by Thalasso et al (2020) found that, in a stratified lake in north-central Siberia, all CH4 produced in the sediments and deep hypolimnion was removed below the oxycline, suggestive of high AOM. This case study suggests, in northern lakes that stratify, it is possible for most or all CH4 produced in sediments to be anaerobically oxidized in the hypolimnion, resulting in little to no CH4 exchange with the epilimnion and/or atmosphere.…”