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 more 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, C₂H₆ and d¹³CH₄. During a 1-day field campaign, we performed 2D mapping of surface water of Lake Aiguebelette (France). In the littoral (pelagic) area, average dissolved CH₄ concentrations and d¹³CH₄ were 391.9 ± 156.3 (169.8 ± 26.6) nmol L^-1 and -67.3 ± 3.4 (-61.5 ± 3.6) ‰, respectively. The dissolved CH₄ concentration in the pelagic zone was fifty 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, that was further confirmed by an isotopic mass balance technique with the high-resolution transect data. This new in situ fast response sensor allows to obtain unique high-resolution and high-spatial coverage datasets within a limited amount of survey time. This tool will be useful in the future for studying processes governing CH₄ dynamics in aquatic systems.