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- - COinf3sup2-and Na+ were the dominant ions in all the lakes. Concentrations of K+, Cl- and SOinf4sup2-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. © 1994 Kluwer Academic Publishers.