Is ebullition or diffusion more important as methane emission pathway in a shallow subsaline lake?

Methane (CH₄) emissions via ebullition contribute significantly to greenhouse gas emissions from freshwater bodies. According to the literature, the ebullition pathway may even be the most important pathway in some cases, particularly in shallow lakes. Ebullition rates are not often estimated because of the high uncertainty associated with episodic releases, leading to difficulties in their determination. This study provides an estimate of such emissions in a large, shallow, subsaline lake in eastern Austria, Lake Neusiedl, and compares them to the diffusion pathway. Ebullition gas sampling was conducted every 5-10 days over a period of 107 days from late March to mid-July 2021, using ebullition traps placed in three distinct locations: Reed belt, Channel and Open water/Lake. The aim was to study the temporal and spatial heterogeneity of ebullition and its contribution to total emissions. At the same time, several water quality and other environmental parameters were measured and then tested against the CH₄ ebullition rates to explore them as potential drivers for this pathway. The carbon isotope fractionation factor (α_C) of the measured CH₄ ebullition gas, ranging from 1.03 to 1.06, indicates a dominance of the acetoclastic methanogenesis in the sediments of Lake Neusiedl, regardless of the location. The Reed belt location showed the highest mean CH₄ ebullition rate (17 ± 28 mg CH₄ m^-2 d^-1), which is >340-fold higher than the mean of the other two locations, and demonstrated also a strong temperature dependency. In all locations at Lake Neusiedl, the median CH₄ fluxes via diffusion are significantly higher than via ebullition. Our analyses do not confirm the dominance of the ebullition pathway in any of the studied locations. Whereas at the Reed belt, ebullition accounts for 48 % of the CH₄ emissions, in the other two locations, is responsible only for about 1 %.