Phytoplankton species composition and biomass of 10 lakes were studied in relation to environmental variables, especially with respect to salinity and alkalinity. In addition, laboratory experiments were done on SPIRULINA PLATENSIS, the dominant phytoplankter in the saline, alkaline lakes. Growth response to variations in light, ionic composition, and salinity level were studied in continuous culture.

A total of 206 phytoplankton species/taxa were recorded from all lakes, in samples taken once during the short rains, in March-May 1991. The number of species declined as salinity and alkalinity increased in the series, reaching an almost unialgal population in Lake Chitu, the most saline (45 grams per liter salinity), alkaline lake in the series. Cyanophytes, chlorophytes, and diatoms were the most important groups in the lakes, and flagellated groups were not common. The most dilute Lake Koka (0.2 g/l salinity) had the highest species richness. There was no apparent relationship between phytoplankton biomass and salinity. In a seasonal study on Lake Awassa, three phases of thermal stratification and mixing were recognised. Increase in phytoplankton biomass was associated with inflow of nutrients and mixing during the rainy season. Seasonal variation in total biomass was relatively low (coefficient of variation < 20%). Phytoplankton production could be limited by nitrogen at the end of the dry season, during the stratification period, and by light, at least during the mixing periods.

SPIRULINA PLATENSIS isolated from Lake Chitu, showed a maximum specific growth rate of 1.78 per day at steady state, at about 330 micromol photons per square meter per second. The optimum nitrogen:phosphorus (N:P) ratio in cells was also reached at this irradiance level. Salinity stress with sodium salts showed that response in terms of growth rate and quantum yield was best in NaHCO3, less in NaCl, and least in Na2SO4.

Results from this study suggest that salinity-alkalinity, as well as inorganic turbidity and mixing depth affecting the light climate, play an especially important role in species composition and phytoplankton biomass of the Ethiopian lakes.