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

中文翻译：

---

使用膜入口激光光谱仪连续原位测量基伍湖中的溶解甲烷

我们报告了卢旺达基伍湖浅水区中溶解甲烷的第一个高分辨率连续剖面。测量是使用称为Sub-Ocean的原位溶解气体传感器进行的，该传感器基于已获专利的基于膜的提取技术以及高灵敏度的光谱仪。该传感器最初是为海洋环境设计的，但是光谱仪和提取系统都经过了修改，以相对于原始原型（从nmol L ^-1到mmol L ^-1检测）将动态范围扩展到6个数量级。适合基伍湖的浓度范围。仪器的精确度估计为±22  ％（2 σ）从八个型材在80米的深度的标准偏差，对应于 ±0.112 毫巴的CH ₄在水或±160 纳摩尔大号^-1在25  ^∘ ℃和1个大气压。该仪器仅在25分钟内即可连续描绘出水柱顶部150 m。在气相浓度中CH ₄的最大观察混合比为77％，在湖深150 m处和热条件下，该浓度为3.5 mmol L ^-1。深入溶解CH ₄浓度太大，无法正确检索甲烷吸收光谱。结果与与商业HydroC进行现场测量离散吻合^®传感器。这种快速分析功能对于研究水生系统中CH ₄和其他溶解气体的运输，生产和消耗非常有用 。尽管该传感器非常适合用于调查CH ₄浓度高达每升几毫摩尔的大多数环境 ，但将来该光谱仪可能会被灵敏度较低的分析技术所取代，该技术可能包括同时检测溶解的CO ₂以及总溶解气体压力，用于探索CH ₄浓度很高的环境， 例如基伍湖的底水。