A Novel High-Resolution In Situ Tool for Studying Carbon Biogeochemical Processes in Aquatic Systems: The Lake Aiguebelette Case Study

Abstract

Lakes and reservoirs are a significant source of atmospheric methane (CH4), with emissions comparable to the largest global CH4 emitters. Understanding the processes leading to such significant emissions from aquatic systems is therefore of primary importance for producing accurate projections of emissions in a changing climate. In this work, we present the first deployment of a novel membrane inlet laser spectrometer (MILS) for fast simultaneous detection of dissolved CH4, ethane (C2H6) and the stable carbon isotope of methane (δ13CH4). During a 1-day field campaign, we performed 2D mapping of surface water of Lake Aiguebelette (France). Average dissolved CH4 concentrations and δ13CH4 were 391.9 ± 156.3 nmol L−1 and −67.3 ± 3.4‰ in the littoral area and 169.8 ± 26.6 nmol L−1 and −61.5 ± 3.6‰ in the pelagic area. The dissolved CH4 concentration in the pelagic zone was 50 times larger than the concentration expected at equilibrium with the atmosphere, confirming an oversaturation of dissolved CH4 in surface waters over shallow and deep areas. The results suggest the presence of CH4 sources less enriched in 13C in the littoral zone (presumably the littoral sediments). The CH4 pool became more enriched in 13C with distance from shore, suggesting that oxidation prevailed over epilimnetic CH4 production and it was further confirmed by an isotopic mass balance technique with the high-resolution data. This new in situ fast response sensor allows one to obtain unique high-resolution and high-spatial coverage data sets within a limited amount of survey time. This tool will be useful in the future for studying processes governing CH4 dynamics in aquatic systems.