Abstract
Abstract. Methane (CH4) emissions from reservoirs are responsible for most of the atmospheric climatic forcing of these aquatic ecosystems, comparable to emissions from paddies or biomass burning. Primarily, CH4 is produced during the anaerobic mineralization of organic carbon in anoxic sediments by methanogenic archaea. However, the origin of the recurrent and ubiquitous CH4 supersaturation in oxic waters (i.e., the methane paradox) is still controversial. Here, we determined the dissolved CH4 concentration in the water column of 12 reservoirs during summer stratification and winter mixing to explore CH4 sources in oxic waters. Reservoir sizes ranged from 1.18 to 26.13 km2. We found that dissolved CH4 in the water column varied by up to 4 orders of magnitude (0.02–213.64 µmol L−1), and all oxic depths were consistently supersaturated in both periods. Phytoplanktonic sources appear to determine the concentration of CH4 in these reservoirs primarily. In anoxic waters, the depth-cumulative chlorophyll a concentration, a proxy for the phytoplanktonic biomass exported to sediments, was correlated to CH4 concentration. In oxic waters, the photosynthetic picoeukaryotes' abundance was significantly correlated to the dissolved CH4 concentration during both the stratification and the mixing. The mean depth of the reservoirs, as a surrogate of the vertical CH4 transport from sediment to the oxic waters, also contributed notably to the CH4 concentration in oxic waters. Our findings suggest that photosynthetic picoeukaryotes can play a significant role in determining CH4 concentration in oxic waters, although their role as CH4 sources to explain the methane paradox has been poorly explored.
Highlights
Lakes and reservoirs are significant sources of methane (CH4), affecting the atmospheric climatic forcing (Deemer et al, 2016)
The wide range in CH4 concentrations found in this study covers values reported in temperate lakes (Donis et al, 2017; Grossart et al, 2011; Tang et al, 2014; West et al, 2016), to those found in tropical lakes and reservoirs (Murase et al, 2003; Naqvi et al, 2018; Okuku et al, 2019; Roland et al, 2017)
Since we did not detect the existence of the methyl-coenzyme reductase (mcrA) gene in the water column, we considered that the production of methane by methanogenic Archaea occurred primarily in the sediments and was affected by the sedimentation of organic matter derived from phytoplankton
Summary
Lakes and reservoirs are significant sources of methane (CH4), affecting the atmospheric climatic forcing (Deemer et al, 2016). The anoxic sediments are a primary source of CH4 (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 affected by the extent of anoxia in the sediments insomuch as they are obligate anaerobes and will not survive
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