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

Reservoirs are a globally significant source of methane (CH) 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 CH 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 CH-rich bubbles from the sediments and that these emissions constitute a substantial fraction of the annual CH flux. We explored this hypothesis by monitoring CH ebullition in a eutrophic reservoir over a 7-month period, which included an experimental water-level drawdown. We found that the ebullitive CH flux rate was among the highest ever reported for a reservoir (mean = 32.3 mg CH m h). 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 CH flux during the 7-month monitoring period due to the naturally high ebullitive CH flux rates that persist throughout the warm weather season. Although drawdown emissions were found to be a minor component of annual CH emissions in this reservoir, our findings demonstrate a link between water-level change and CH ebullition, suggesting that CH emissions may be mitigated through water-level management in some reservoirs.