Temperature and electrical conductivity of water in Lake Nyos transmitted by an automatic observation buoy

An automatic observation buoy (AOB) was installed at Lake Nyos in March 2014. The temperature and electrical conductivity (EC) of lake water were continuously observed from March 10, 2014 to April 15, 2016. The thermocline and chemocline located deeper than −90 m subsided 15 m in the observation period due to the bottom water removal by the CO2 degassing pipes. The velocity of subsidence was estimated quantitatively by correlating the time variation of EC obtained by AOB and the depth profile of EC. The velocity of subsidence was not constant, namely, it was fast in dry season and slow in rainy season. The time variation in the velocity of subsidence can be attributed to the seasonal variation of water flux in the degassing pipes. Considering the water balance in the deep part of lake, the flux of ground water supplied to lake was estimated to be 6.5*106 (m3/year), which is comparable to the recharge rate of ground water within the catchment area surrounding Lake Nyos. Abrupt drops in temperature and conductivity were observed at −120 m in January 2016, when the temperature at -4 m reached 21.8 °C as the lowest value. The abrupt drops are due to the sinking of surface cold water.