The Magnitude and Drivers of Methane Ebullition and Diffusion Vary on a Longitudinal Gradient in a Small Freshwater Reservoir

Reservoirs emit large amounts of methane (CH4) to the atmosphere relative to their small surface area globally. Among the different pathways of reservoir CH4 emissions, bubbling from the sediments (ebullition) and diffusion from the water surface are major contributors of CH4 efflux. The magnitude of ebullition and diffusion can vary substantially over space and time in large reservoirs. However, it is unclear how the drivers of ebullition and diffusion vary along a reservoir's longitudinal gradient, particularly in small reservoirs. We measured ebullition, diffusion, and eight environmental driver variables at four transects along a longitudinal gradient within a small, eutrophic reservoir. We used time series modeling to examine how the drivers of ebullition and diffusion varied among transects. Sediment-water interface temperature, inflow discharge, and wind speed were the most important drivers of CH4 ebullition in upstream transects of the reservoir, while phytoplankton biomass was the most important driver of ebullition in the downstream transect closest to the dam. Strikingly, CH4 ebullition dynamics were extremely well captured by the time series models, as the modeled rates for the furthest upstream transect closely matched the observed rates throughout the monitoring period. In contrast, CH4 diffusion dynamics were harder to model, with phytoplankton biomass as the primary driver of diffusion across all transects. Our results indicate that multiple drivers affect CH4 emissions along a small reservoir's longitudinal gradient and should be considered when upscaling site measurements to reservoir-wide CH4 emissions and ultimately regional or global estimates.