![]() Reservoir sediments are often enriched with organic matter derived from river inflows and internal algal production, among other sources. Methane is created in reservoirs through methanogenesis, a type of microbial metabolism that yields energy from the degradation of organic matter in low oxygen environments. Reservoirs are extremely numerous, covering more than 30 million ha of the earth’s land surface (Downing and others 2006), and the global surface area of reservoirs is expected to increase substantially over the coming decades as developing countries turn to hydropower to meet growing energy demands (Zarfl and others 2015). Reservoirs, which are created when landscapes are flooded behind dams, are a globally significant source of CH 4 to the atmosphere (Barros and others 2011 Bastviken and others 2011 Deemer and others 2016). Methane (CH 4) is a potent greenhouse gas and the second largest contributor to climate change (IPCC 2013). Although drawdown emissions were found to be a minor component of annual CH 4 emissions in this reservoir, our findings demonstrate a link between water-level change and CH 4 ebullition, suggesting that CH 4 emissions may be mitigated through water-level management in some reservoirs. 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 4 flux during the 7-month monitoring period due to the naturally high ebullitive CH 4 flux rates that persist throughout the warm weather season. ![]() We found that the ebullitive CH 4 flux rate was among the highest ever reported for a reservoir (mean = 32.3 mg CH 4 m −2 h −1). We explored this hypothesis by monitoring CH 4 ebullition in a eutrophic reservoir over a 7-month period, which included an experimental water-level drawdown. We hypothesized that periods of water-level decline trigger the release of CH 4-rich bubbles from the sediments and that these emissions constitute a substantial fraction of the annual CH 4 flux. ![]() Here we investigate CH 4 bubbling (that is, ebullition) during periods of falling water levels in a eutrophic reservoir in the Midwestern USA. However, emission rate estimates may be biased low due to inadequate monitoring during brief periods of elevated emission rates (that is, hot moments). Reservoirs are a globally significant source of methane (CH 4) to the atmosphere.
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