A new pathway of freshwater methane emissions and the putative importance of microbubbles

Continental freshwater systems have now been shown to be globally significant sources of methane, but there are still large uncertainties associated with freshwater methane fluxes. Studies to date have mainly focused on either ebullition of bubbles originating from sediments or on diffusive fluxes of dissolved methane across the air–water interface. We examined the potential influence of a new mode of methane emission from freshwater systems by comparing the gas exchange velocities derived from carbon dioxide and methane fluxes in a set of diverse systems and environmental conditions. In more than 90% of 260 measurements, methane exhibited higher evasion rates than strictly Fickian diffusive processes would suggest. The portion of flux associated with this non-Fickian diffusion, which we attribute to the presence of semistable microbubbles, was closely related and directly proportional to the degree of methane supersaturation relative to the atmosphere. On average, microbubble-mediated flux contributed about half of the total measured diffusive efflux from the systems and could be modeled as an additive gas piston velocity of, on average, 2.1 m d−1. The microbubble-mediated flux is completely absent in calculations of diffusive methane fluxes derived from ambient pCH4, and is also not necessarily captured in current approaches used to determine ebullition rates. Our results suggest that methane evasion rates based only on measured partial pressures and exchange velocities characteristic of Fickian diffusive processes will grossly underestimate methane losses from these boreal ecosystems.