Effects of an Experimental Water-level Drawdown on Methane Emissions from a Eutrophic Reservoir

Reservoirs are a globally significant source of methane (CH₄) to the atmosphere. However, emission rate estimates may be biased low due to inadequate monitoring during brief periods of elevated emission rates (that is, hot moments). Here we investigate CH₄ bubbling (that is, ebullition) during periods of falling water levels in a eutrophic reservoir in the Midwestern USA. We hypothesized that periods of water-level decline trigger the release of CH₄-rich bubbles from the sediments and that these emissions constitute a substantial fraction of the annual CH₄ flux. We explored this hypothesis by monitoring CH₄ ebullition in a eutrophic reservoir over a 7-month period, which included an experimental water-level drawdown. We found that the ebullitive CH₄ flux rate was among the highest ever reported for a reservoir (mean = 32.3 mg CH₄ m^-2 h^-1). The already high ebullitive flux rates increased by factors of 1.4-77 across the nine monitoring sites during the 24-h experimental water-level drawdown, but these emissions constituted only 3% of the CH₄ flux during the 7-month monitoring period due to the naturally high ebullitive CH₄ flux rates that persist throughout the warm weather season. Although drawdown emissions were found to be a minor component of annual CH₄ emissions in this reservoir, our findings demonstrate a link between water-level change and CH₄ ebullition, suggesting that CH₄ emissions may be mitigated through water-level management in some reservoirs.