An integrative analysis of global patterns of CH4 ebullition in inland waters

CH4 ebullition is one of the least well-constrained components of ecosystem CH4 budgets in inland waters, despite its clear dominance in many habitats. Studies have shown that CH4 ebullition varies greatly in space, both within and among ecosystems, and in time, being extremely episodic, and this variability poses significant challenges for quantifying and modelling the process. Global syntheses of CH4 or greenhouse gases have not explicitly focused on CH4 ebullition, and the literature on the topic is still highly fragmented. Although there is evidence of links between ebullition and environmental factors, particularly at local scales, there is still considerable uncertainty on the magnitude and regulation of ebullition at regional and global scales. Here, we present an integrative analysis of CH4 ebullition in inland waters based on a systematic literature search, where we have combined a qualitative review of CH4 ebullition studies, a meta-analysis of effect sizes between environmental factors and ebullition in freshwater ecosystems, and a quantitative data analysis of global ebullition fluxes, bubble related variables and associated environmental factors, to gain a broader understanding of CH4 ebullition and its global regulation. We show that despite the intrinsic variability in CH4 ebullition, this emission pathway has clear global patterns across climatic and environmental gradients. Our meta-analysis reveals that waterbody morphometry, carbon source availability, and climate-driven variables are key drivers of CH4 ebullition, but their effect varies. For example, the positive effect of temperature on CH4 ebullition varied across climatic zones and ecosystem types. The quantitative analysis of the database we assembled complemented these results by showing that global CH4 ebullition fluxes are higher in shallow ponds, eutrophic systems, and habitats in tropical and subtropical regions. This database also yielded global-scale empirical models, demonstrating that, in addition to climate and carbon sources, system productivity and trophic status also play key roles in the global regulation of CH4 ebullition in inland waters. Our results further revealed that bubble CH4 concentrations and isotopic signatures associated with CH4 ebullition fluxes provide essential insights into sediment CH4 dynamics.