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Lakes and reservoirs are a significant source of atmospheric methane (CH4), with emissions comparable to the largest global CH4emitters. 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 CH4concentrations and δ13CH4were 391.9 ± 156.3 nmol L−1and −67.3 ± 3.4‰ in the littoral area and 169.8 ± 26.6 nmol L−1and −61.5 ± 3.6‰ in the pelagic area. The dissolved CH4concentration in the pelagic zone was 50 times larger than the concentration expected at equilibrium with the atmosphere, confirming an oversaturation of dissolved CH4in surface waters over shallow and deep areas. The results suggest the presence of CH4sources less enriched in 13C in the littoral zone (presumably the littoral sediments). The CH4pool became more enriched in 13C with distance from shore, suggesting that oxidation prevailed over epilimnetic CH4production 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 CH4dynamics in aquatic systems. High‐resolution mapping of surface methane and its isotopic signature enables accurate characterization of aquatic systems and discrimination of biochemical processes at work. At Lake Aiguebelette, this new in situ tool allowed us to conclude that methane present at the surface comes mainly from shallow littoral areas, where sediments, which are a source of methane, are closer to the surface. During lateral transport of water masses from the littoral zone, the change in isotopic signature reveals that methane oxidation prevails over local in situ production. Comparison with previous studies validates the importance of high‐resolution measurements (particularly to capture the high variability in the littoral zone) and showed that smaller lakes experience stronger methane isotopic signature changes for a given methane concentration variation. This can be explained by the fact that the smaller lake has a larger littoral‐to‐total surface area. This new tool will be useful in the nearby future to study the processes governing CH4dynamics in aquatic systems. Fast in situ measurements of dissolved methane and its stable carbon isotopeHigh‐spatial resolution mapping of dissolved methane and its stable carbon isotopeImproved production/oxidation process identification over discrete sampling Fast in situ measurements of dissolved methane and its stable carbon isotope High‐spatial resolution mapping of dissolved methane and its stable carbon isotope Improved production/oxidation process identification over discrete sampling