High Methane Emissions from a Midlatitude Reservoir Draining an Agricultural Watershed

Abstract

Reservoirs are a globally significant source of methane (CH₄), although most measurements have been made in tropical and boreal systems draining undeveloped watersheds. To assess the magnitude of CH₄ emissions from reservoirs in midlatitude agricultural regions, we measured CH₄ and carbon dioxide (CO₂) 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 CH₄ throughout the year, emitting on average 176 ± 36 mg C m^–2 d^–1, the highest reservoir CH₄ emissions profile documented in the United States to date. Contrary to our initial hypothesis, the largest CH₄ emissions were during summer stratified conditions, not during fall turnover. The river–reservoir transition zone emitted CH₄ at rates an order of magnitude higher than the rest of the reservoir, and total carbon emissions (i.e., CH₄ + CO₂) 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 CH₄ to the atmosphere than currently recognized, particularly if river deltas are consistent CH₄ hot spots. We estimate that CH₄ emissions from agricultural reservoirs could be a significant component of anthropogenic CH₄ emissions in the U.S.A