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 (CH₄) fluxes to the atmosphere. However, effects of such fluctuations on CH₄ emissions have received limited attention. Here we examine CH₄ 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 CH₄ fluxes to the atmosphere, and can account for more than 90% of annual reservoir CH₄ flux in a period of just a few weeks. Reservoirs with higher epilimnetic [chlorophyll a] experienced larger increases in CH₄ emission in response to drawdown (R² = 0.84, p < 0.01), suggesting that eutrophication magnifies the effect of drawdown on CH₄ 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 CH₄ fluxes from reservoirs. The extent to which (and the mechanisms by which) drawdowns short-circuit connections between methanogenesis and methanotrophy, thereby increasing net CH₄ fluxes to the atmosphere, should be a focus of future work.