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    High Methane Emissions from a Midlatitude Reservoir Draining an Agricultural Watershed

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    Reservoirs are a globally significant source of methane (CH<sub>4</sub>), although most measurements have been made in tropical and boreal systems draining undeveloped watersheds. To assess the magnitude of CH<sub>4</sub> emissions from reservoirs in midlatitude agricultural regions, we measured CH<sub>4</sub> and carbon dioxide (CO<sub>2</sub>) emission rates from William H. Harsha Lake (Ohio, U.S.A.), an agricultural impacted reservoir, over a 13 month period. The reservoir was a strong source of CH<sub>4</sub> throughout the year, emitting on average 176 ± 36 mg C m<sup>–2</sup> d<sup>–1</sup>, the highest reservoir CH<sub>4</sub> emissions profile documented in the United States to date. Contrary to our initial hypothesis, the largest CH<sub>4</sub> emissions were during summer stratified conditions, not during fall turnover. The river–reservoir transition zone emitted CH<sub>4</sub> at rates an order of magnitude higher than the rest of the reservoir, and total carbon emissions (i.e., CH<sub>4</sub> + CO<sub>2</sub>) were also greater at the transition zone, indicating that the river delta supported greater carbon mineralization rates than elsewhere. Midlatitude agricultural impacted reservoirs may be a larger source of CH<sub>4</sub> to the atmosphere than currently recognized, particularly if river deltas are consistent CH<sub>4</sub> hot spots. We estimate that CH<sub>4</sub> emissions from agricultural reservoirs could be a significant component of anthropogenic CH<sub>4</sub> emissions in the U.S.A
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