Vertical profiles of sound speed (SS) in lake water were measured at lakes Nyos and Monoun (Cameroon) in 2012 and 2014. A significant linear correlation with Pearson's r of 0.987 to 0.995 between total CO2 concentration ([CO2]=[CO2(aq)]+[HCO3−]) and SS excess (Δv) in water was found. Based on this correlation, we propose the SS method as a new simple tool to monitor CO2 concentration in lakes Nyos and Monoun. We applied this method to multipoint measurements of CO2 in lakes Nyos and Monoun, and found horizontal differences between CO2 distributions in the two lakes. The results indicated that Lake Nyos water was stably stratified, and the total CO2 decreased from 2012 to 2014. At Lake Monoun, which consists of three basins (the western basin, the central basin, and the main basin), waters in the western and central basins exhibited a higher dissolved CO2 content than those in the main basin at the same depth. An increase in CO2 was also detected in the main basin from 2012 to 2014.
A new method to measure the CO2 concentration in lakes Nyos and Monoun using sound speed (SS) and electrical conductivity (EC) of the lake water was developed. Limnic eruptions at lakes Monoun and Nyos in Cameroon, characterized by a risk related to possible sudden release of magmatic CO2 gas dissolved in the lake water, occurred in 1984 and 1986 respectively, killing about 1800 residents. CO2 monitoring of the lake waters was initiated to prevent further disasters. Classical methods of CO2 measurement, which require chemical analysis of water samples, are not suitable for frequent and multi-point monitoring. Inspired by the method of measuring salt concentration using SS, we obtained data for dissolved CO2 (CO2(aq)) assuming the following formula , where Δv is additional SS due to dissolved ions and k1 and k2 are coefficients determined through experiments. The coefficient k1 was determined with temperature T (°C) dependence as 0.0323–0.000523T [l m s−1 mmol−1]. k2 was estimated to be 0.132 at Lake Nyos and 0.0930 at Lake Monoun. Once k1 and k2 values are obtained, [CO2(aq)] and are calculated using the SS, T and EC.
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