1.The 2018 intercalibration campaign aimed at quantifying the methane (CH₄) and carbon dioxide (CO₂) gas content and the recharge rate of CH₄ in Lake Kivu using a range of different measurement methods. Measurements were performed by research teams from the Helmholtz Centre for Environmental Research (UFZ) in Magdeburg (Germany), the Swiss Federal Institute for Aquatic Science and Technology (Eawag, Switzerland), the French National Centre for Scientific Research (CNRS) in Grenoble (France), and from KivuWatt Ltd (Kigali, Rwanda).
2. The following measurement methods were applied: two sensors for the in-situ observation of total dissolved gas pressure; two sensors for the in-situ observation of the partial pressure of dissolved CH₄; and two methods for quantifying the concentrations of dissolved CH₄ and CO₂ in samples retrieved from the lake either using sampling bags or a tubing system. These methods were specifically customized for the application under the special conditions in Lake Kivu. Since some of the methods quantify partial pressures of CH₄ and/or CO₂, and other methods quantify their concentrations, a procedure for converting between partial pressures and concentrations was developed and implemented.
3. The observations yielded a consistent picture of the vertical profiles of dissolved concentrations of CH₄ and CO₂ as well as the total gas pressures in Lake Kivu. The observed variability between the datasets is related to the limited accuracy of the different measurement methods.
4. The observed CH₄ concentrations were within the range of previous observations. However, in the resource zone (below 260 m depth), they were approximately 5-20 % below the concentrations measured by M. Halbwachs and J.-C. Tochon in 2003, which had previously been used as the standard for estimating the CH₄ content in the lake.
5. The CH₄ content in the resource zone (between 260 and 480 m depth) was estimated to ~40 km³ STP (volume of gas at a temperature of 0°C and a pressure of 1 atm) (range, 36.4 - 42.2 km³). These numbers are somewhat lower than the previous estimate of 44.7 km³ by Wüest and Schmid (2012) based on the measurements of M. Halbwachs and J.-C. Tochon in 2003. The CH₄ content in the potential resource zone (200-260 m) was estimated to range between 8.2 and 8.6 km³ for all methods, which agrees with the previous estimate of 8.5 km³. The whole-lake CO₂ content was estimated to 285 km³ STP.
6. The differences in CH₄ concentrations and content compared to previous estimates are due to the limited accuracy of the different measurement methods and were not caused by the comparably small amount of CH₄ removed by the past and ongoing gas extraction operations.
7. The 2018 measurements do not confirm the previous hypothesis that the CH₄ concentrations were increasing during the last decades in Lake Kivu. They rather indicate approximately constant concentrations since the first observations in the 1950's within the uncertainty range of the present and previous measurements.
8. Recommendations for monitoring the evolution of gas concentrations in the lake cover two important aspects. Required monitoring equipment and frequency are defined based on different monitoring purposes. In addition, a need for building up sampling routines and skills is identified.