Freshwater reservoirs are an important source of the greenhouse gas methane (CH4) to the atmosphere, but global emission estimates are poorly constrained (13.3–52.5 Tg C yr−1), partially due to extreme spatial variability in emission rates within and among reservoirs. Spatial heterogeneity in the availability of organic matter (OM) for biological CH4 production by methanogenic archaea may be an important contributor to this variation. To investigate this, we measured sediment CH4 potential production rates, OM source and quantity, and methanogen community composition at 15 sites within a eutrophic reservoir in Ohio, USA. CH4 production rates were highest in the shallow riverine inlet zone of the reservoir, even when rates were normalized to OM quantity, indicating that OM was more readily utilized by methanogens in the riverine zone than in the transitional or lacustrine zones. Sediment stable isotopes and C:N indicated a greater proportion of terrestrial OM in the particulate sediment of this zone. Methanogens were present at all sites, but the riverine zone contained a higher relative abundance of methanogens capable of acetoclastic and methylotrophic methanogenesis, likely reflecting differences in decomposition processes or OM quality. While we found that methane potential production rates were negatively correlated with autochthonous carbon in particulate sediment OM, rates were positively correlated with indicators of autochthonous carbon in the porewater dissolved OM. It is likely that both dissolved and particulate sediment OM affect CH4 production rates, and that both terrestrial and aquatic OM sources are important in the riverine methane production hot spot.
Bibliographical noteFunding Information:
This research was supported in part by a grant from the United States Geological Survey 104(b) program, sub‐award #60059469 through the Ohio Water Resources Center. Megan Berberich was supported through the Graduate Research Traineeship program sponsored by the U.S. EPA and administered through the University of Cincinnati, in addition to support from the UC Department of Biological Sciences and the Wieman Wendel Benedict Fund. We thank Joel Allen for support in sediment trap deployment and sampling, Kit Daniels for key assistance with field sample collection, Madison Duke for sediment trap work and field and lab support, Mike Elovitz for advice and insights into DOC EEMs, Jeff Havig for stable isotope and elemental analysis of sediment, Xuan Li for field and laboratory support including EEMs, and Karen White for laboratory assistance; this research was made possible with their help. We are also grateful to Aaron Diefendorf for access to and use of the Stable Isotope Geochemistry Lab in the Department of Geology at the University of Cincinnati.
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