Importance of the Autumn Overturn and Anoxic Conditions in the Hypolimnion for the Annual Methane Emissions from a Temperate Lake

Abstract

Changes in the budget of dissolved methane measured in a small temperate lake over 1 year indicate that anoxic conditions in the hypolimnion and the autumn overturn period represent key factors for the overall annual methane emissions from lakes. During periods of stable stratification, large amounts of methane accumulate in anoxic deep waters. Approximately 46% of the stored methane was emitted during the autumn overturn, contributing ∼80% of the annual diffusive methane emissions to the atmosphere. After the overturn period, the entire water column was oxic, and only 1% of the original quantity of methane remained in the water column. Current estimates of global methane emissions assume that all of the stored methane is released, whereas several studies of individual lakes have suggested that a major fraction of the stored methane is oxidized during overturns. Our results provide evidence that not all of the stored methane is released to the atmosphere during the overturn period. However, the fraction of stored methane emitted to the atmosphere during overturn may be substantially larger and the fraction of stored methane oxidized may be smaller than in the previous studies suggesting high oxidation losses of methane. The development or change in the vertical extent and duration of the anoxic hypolimnion, which can represent the main source of annual methane emissions from small lakes, may be an important aspect to consider for impact assessments of climate warming on the methane emissions from lakes.

Assessment of methane and carbon dioxide emissions in two sub‐basins of a small acidic bog lake artificially divided 30 years ago

“…Encinas Fernández et al. () attributed these low CH 4 emissions during the spring turnover to continuous oxic conditions in the hypolimnia following the fall turnover, similar to the results of this study. Additionally, the low spring emissions could be the result of lower temperatures and reduced supply of bio‐labile DOC mitigating the methanogenic activity during the short winter stratification.…”

“…Fall turnover is the major source of CH 4 and CO 2 emissions to the atmosphere in both basins. Similar tendencies were described in small, sheltered boreal humic lakes in southern Finland and in a small mesotrophic dimictic lake in southern Germany; in both lakes, major CH 4 emissions were observed during the fall turnover, and no significant amounts of CH 4 were emitted during the spring turnover (Encinas Fernández, Peeters, & Hofmann, 2014;Kankaala, Huotari, Peltomaa, Saloranta, & Ojala, 2006;Kankaala et al, 2007;López Bellido et al, 2013). Studies in Finnish lakes attributed these low emissions during the spring turnover to short or incomplete mixing of water masses, which in turn is a result of the high absorption of solar radiation by the coloured fraction of DOC in the first few metres of water (Kankaala et al, 2007).…”

“…High temperatures amplified CH 4 emissions (Sepulveda‐Jauregui et al., ), which led to markedly higher amounts of CH 4 stored and emitted from the basins in the hotter summer of 2014 with a clear correlation between CH 4 emissions and temperature. Similar responses to regional temperature have been shown in ponds, in small shallow, and medium‐sized lakes (Bartosiewicz, Laurion, & Macintyre, ; Bartosiewicz et al., ; Encinas Fernández et al., ; Liikanen, Huttunen, Valli, & Martikainen, ; Miettinen et al., ; Natchimuthu et al., ; Room et al., ) and in enclosure experiments (Audet et al., ; Yvon‐Durocher et al., ), suggesting that in predicted warming scenarios, an increase in carbon greenhouse gas emissions can be expected in lake ecosystems regardless of DOC inputs and their biogeochemical effects in the water column (e.g., in stratification, temperature regimes, pH, microbial metabolism and CO 2 dynamics).…”

“…() and Encinas Fernández et al. () also found dissolved O 2 in the water column to be an indicator of CH 4 and CO 2 dynamics in summer. Moreover, Sabrekov et al.…”

An inter-regional assessment of concentrations and δ13C values of methane and dissolved inorganic carbon in small European lakes

“…Furthermore, late summer CH 4 concentrations in stratified lakes are important for assessing the amount of CH 4 released from dimictic and monomictic lakes during the autumn overturn, when CH 4 accumulated in the deepwater layers under anoxic conditions is released to the atmosphere (e.g. Schubert et al 2012;Encinas Fernández et al 2014).…”