Abstract

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]=[CO_2(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.