Autochthonous dissolved organic matter potentially fuels methane ebullition from experimental lakes

Abstract

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 theepisodic 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 experimentaltreatments are low and high nutrients crossed with three temperatures, to investigate thequantity and drivers of CH4 ebullition. The mean CH4 ebullition flux in the high nutrient treatment(41.5 ± 52.3 mg CH4eC m2 d1) mesocosms was significantly larger than in the low nutrient treatment(3.6 ± 5.4 mg CH4eC m2 d1) mesocosms, varying with temperature scenarios. Over eight weeks fromJune 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, dissolvedorganic carbon (DOC), biodegradable DOC, d2H, d18O and d13C-DOC, autochthonous dissolved organicmatter (DOM) fluorescent components, and a fraction of lipids, proteins, and lignins revealed usingultrahigh-resolution mass spectrometry, and a negative relationship between ebullitive CH4 flux and thepercentage volume inhabited of macrophytes. A 24 h laboratory bio-incubation experiment performed atroom temperature (20 ± 2 C) in the dark further revealed a rapid depletion of algal-DOM concurrentwith 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 algalbiomass, and the resultant accumulation algal derived bio-labile DOM potentially drives enhancedoutgassing of ebullitive CH4 from the shallow-lake mesocosms.