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. We report the first high-resolution continuous profile ofdissolved methane in the shallow water of Lake Kivu, Rwanda. Themeasurements were performed using an in situ dissolved gas sensor, calledSub-Ocean, based on a patented membrane-based extraction technique coupledwith a highly sensitive optical spectrometer. The sensor was originallydesigned for ocean settings, but both the spectrometer and the extractionsystem were modified to extend the dynamical range up to 6 orders ofmagnitude with respect to the original prototype (from nmol L −1 to mmol L −1 detection) to fit the range of concentrations at Lake Kivu. Theaccuracy of the instrument was estimated to ±22  % ( 2σ ) fromthe standard deviation of eight profiles at 80 m depth, corresponding to ±0.112  mbar of CH4 in water or ±160  nmol L −1 at25  ∘ C and 1 atm. The instrument was able to continuously profilethe top 150 m of the water column within only 25 min. The maximum observedmixing ratio of CH4 in the gas phase concentration was 77 %, which at150 m depth and under thermal conditions of the lake corresponds to 3.5 mmol L −1 . Deeper down, dissolved CH4 concentrations were too large forthe methane absorption spectrum to be correctly retrieved. Results are ingood agreement with discrete in situ measurements conducted with thecommercial HydroC® sensor. This fast-profiling feature ishighly useful for studying the transport, production and consumption of CH4 and other dissolved gases in aquatic systems. While the sensor iswell adapted for investigating most environments with a concentration of CH4 up to a few millimoles per liter, in the future the spectrometer could bereplaced with a less sensitive analytical technique possibly includingsimultaneous detection of dissolved CO2 and total dissolved gaspressure, for exploring settings with very high concentrations of CH4 such as the bottom waters of Lake Kivu.

Continuous in situ measurement of dissolved methane in Lake Kivu using a membrane inlet laser spectrometer