Autochthonous dissolved organic matter potentially fuels methane ebullition from experimental lakes

Shallow lakes are hotspots for carbon processing and important natural sources of methane (CH₄) emission. Ebullitive CH₄ flux may constitute the overwhelming majority of total CH₄ flux, but the episodic nature of ebullition events makes determining both quantity and the controlling factors challenging. Here we used the world's longest running shallow-lake mesocosm facility, where the experimental treatments are low and high nutrients crossed with three temperatures, to investigate the quantity and drivers of CH₄ ebullition. The mean CH₄ ebullition flux in the high nutrient treatment (41.5 ± 52.3 mg CH₄-C m^-2 d^-1) mesocosms was significantly larger than in the low nutrient treatment (3.6 ± 5.4 mg CH₄-C m^-2 d^-1) mesocosms, varying with temperature scenarios. Over eight weeks from June to August covered here warming resulted in a weak, but insignificant enhancement of CH₄ ebullition. We found significant positive relationships between ebullition and chlorophyll-a, dissolved organic carbon (DOC), biodegradable DOC, δ²H, δ¹⁸O and δ¹³C-DOC, autochthonous dissolved organic matter (DOM) fluorescent components, and a fraction of lipids, proteins, and lignins revealed using ultrahigh-resolution mass spectrometry, and a negative relationship between ebullitive CH₄ flux and the percentage volume inhabited of macrophytes. A 24 h laboratory bio-incubation experiment performed at room temperature (20 ± 2 °C) in the dark further revealed a rapid depletion of algal-DOM concurrent with a massive increased CH₄ production, whereas soil-derived DOM had a limited effect on CH₄ production. We conclude that eutrophication likely induced the loss of macrophytes and increase in algal biomass, and the resultant accumulation algal derived bio-labile DOM potentially drives enhanced outgassing of ebullitive CH₄ from the shallow-lake mesocosms.