Reservoir Water-Level Drawdowns Accelerate and Amplify Methane Emission.

Water-level fluctuations due to reservoir management could substantially affect the timing and magnitude of reservoir methane (CH) fluxes to the atmosphere. However, effects of such fluctuations on CH emissions have received limited attention. Here we examine CH emission dynamics in six Pacific Northwest U.S. reservoirs of varying trophic status, morphometry, and management regimes. In these systems, we show that water-level drawdowns can, at least temporarily, greatly increase per-area reservoir CH fluxes to the atmosphere, and can account for more than 90% of annual reservoir CH flux in a period of just a few weeks. Reservoirs with higher epilimnetic [chlorophyll a] experienced larger increases in CH emission in response to drawdown (R = 0.84, p < 0.01), suggesting that eutrophication magnifies the effect of drawdown on CH emission. We show that drawdowns as small as 0.5 m can stimulate ebullition events. Given that drawdown events of this magnitude are quite common in reservoirs, our results suggest that this process must be considered in sampling strategies designed to characterize total CH fluxes from reservoirs. The extent to which (and the mechanisms by which) drawdowns short-circuit connections between methanogenesis and methanotrophy, thereby increasing net CH fluxes to the atmosphere, should be a focus of future work.