A novel high-resolution in situ tool for studying biogeochemical processes in aquatic systems: The Lake Aiguebelette case study

Inland waters are a significant source of atmospheric methane (CH₄), a greenhouse gas (GHG) 34-85 times stronger than carbon dioxide (on 100 to 20-yr timescales) and responsible for ~23% of global radiative forcing. Of the GHGs produced by inland waters (i.e., carbon dioxide, CH₄ and nitrous oxide), CH₄ is responsible for ~75% of the climatic impact of aquatic GHG emissions with aquatic CH₄ emissions comparable to the largest global CH₄ emitters - wetlands and agriculture. Considering that aquatic systems contribute up to half of global CH₄ emissions and that CH₄ is predominantly formed in anoxic environments such as lake sediments, the source and quantification of ubiquitous surface CH₄ observed in most aquatic systems are a question of global importance.

In this work we present the first deployment of a novel membrane inlet laser spectrometer (MILS) instrument, composed of a mid-infrared spectrometer for simultaneous detection of CH₄, C₂H₆ and  d¹³CH₄ coupled with a fast response (t₉₀ < 30sec) membrane extraction system. During a 1-day field campaign, we performed a 2D mapping of dissolved CH₄, C₂H₆ and d¹³CH₄ of surface water of Lake Aiguebelette (France) highlighting the advantages of continuous high-resolution mapping of dissolved gases.

The results showed the presence of CH₄ sources less enriched in ¹³C in the littoral zone (presumably the littoral anoxic sediments). The CH₄ pool became more enriched in ¹³C with distance from shore, suggesting that oxidation processes prevailed over epilimnetic CH₄ production. The data obtained were in line with recent multi-lake studies.