Methane Formation in a Remote Mountain Lake (Lake Planina, NW Slovenia)

Abstract

An incubation experiment was performed with the sediment from a eutrophic mountain Lake Planina (NW Slovenia). In the experiment chemical and stable C and H isotope analyses were used to determine the processes of CH₄ formation. The isotopic composition of dissolved inorganic carbon, δ ¹³ C_DIC, indicates three different processes occurring during incubation: (a) oxic degradation of organic matter (OM), (b) transition between oxic degradation and methanogenesis (“transition”) and (c) methanogenesis. During oxic degradation of OM δ ¹³ C_DIC values decreased from −7.9‰ to −13.5‰, and stayed relatively constant during “transition” with mean δ ¹³ C_DIC value of −12.6 ± 0.6‰. After 56 days the δ ¹³ C_DIC value reached −7.6‰ and did not change much until the end of incubation. ¹³ C enrichment coincided with the formation of CH₄, which started to increase at the same time. The methane produced had a measured average δ ¹³ C _CH4 value of −70.1 ± 1.1‰ and δD _CH4 value of −208 ± 10‰ indicating the formation of methane through bacterial CO ₂ reduction. The flux-weighted ¹³ C/ ¹² C content of DIC and CH₄ is equivalent to the isotopic composition of the deposited metabolizable organic carbon and plankton. Thus, at least at the surface of the sediment, oxidized carbon reflects the reactivity of planktonic OM, since the δ ¹³ C of remineralized carbon was estimated to be −40.1‰. A lower δ ¹³ C_rem value of −48.1‰ was calculated during methanogenesis due to selective lipid degradation of planktonic origin and/or to δ ¹³ C depleted microbial lipid degradation. According to FTIR and solid state ¹³ C NMR measurements of lake sedimentary organic matter, the most degradable components are polysaccharides and aromatic/olefinic organic compounds, while carbonyl and amide structures as well as and alkyl components seem more resistant. The contribution of the dissolution of calcite to DIC decreased during the incubation from 20% during the oxic degradation of OM to 7% during methanogenesis.

Keywords:

• extreme lacustrine system
• methane
• incubation experiment
• hydrogenotrophy

Acknowledgments

This research was conducted in the framework of programme activity P1-0143: Cycling of nutrients and contaminants in the environment, mass balances and modeling of environmental processes and risk analysis, funded by the Slovenian Research Agency (ARRS). Special thanks are given to the Slovenian NMR Centre at National Institute of Chemistry (Slovenia) for ¹³C NMR analyses and to A.R. Byrne for linguistic corrections.