The Ethiopian Rift Valley lakes: chemical characteristics of a salinity-alkalinity series

The study on 10 lakes within the Ethiopian Rift Valley during March–May 1991 covered a range of conductivity (K₂₅) between 286 and 49100 µS cm^–1. HCO₃^– — CO_inf3^sup2–and Na⁺ were the dominant ions in all the lakes. Concentrations of K⁺, Cl^– and SO_inf4^sup2–increased with increasing salinity and alkalinity, whereas Ca²⁺ and Mg²⁺ decreased. Comparison of these data with previous records showed that a ten-fold dilution of total ionic concentration occurred over 30 years in Lake Metahara and about three-fold increase occurred over 65 years in Lake Abijata. Concentrations of soluble silica were generally high (12–222 mg SiO₂ 1^–1) and increased with increasing salinity, except for Lake Chamo which showed SiO₂ depletion (to < 1 mg SiO₂ 1 ^–1) over the past three decades.The relationship between ionic concentration and phosphorus was irregular although high phosphorus concentrations generally corresponded with increasing salinity. Fitting data to the Dillon & Rigler (1974) chlorophyll a — total phosphorus relationship suggested that lakes Zwai, Awassa and Chamo are phosphorus-limited, whereas others have surplus phosphorus.