Pumping methane out of aquatic sediments -- forcing mechanisms that affect the temporal dynamics of ebullition.

Freshwater systems contribute significantly to the global atmospheric methane budget. A large fraction of the methane emitted from freshwaters is transported via ebullition. However, due to its strong variability in space and time, accurate measurements of ebullition rates are difficult; hence, the uncertainty of its contribution to global budgets is large. Here, we analyze measurements made by continuously recording automated bubble traps in an impounded river in central Europe and investigate the mechanisms affecting the temporal dynamics of bubble release from cohesive sediments. Our results show that the main mechanisms for bubble release were pressure changes, originating from the passage of ship-lock induced surges and ship-passages. The response to physical forcing was strongly affected by previous outgassing. Ebullition rates varied strongly over all relevant timescales from minutes to days; therefore, representative ebullition estimates could only be inferred with continuous sampling over long periods. Since ebullition was found to be episodic, short sampling intervals of a few days or weeks will likely underestimate ebullition rates, which may result in an uncertainty of over 50% in current global freshwater emission estimates. [ABSTRACT FROM AUTHOR]