Primary productivity of five Ethiopian Rift Valley lakes

Summary

From a very limited set of data, L. Zwei, L. Abiata and L. Awassa show moderate to high primary productivity and crops of phytoplankton, while L. Shala and L. Langano are of very low productivity.

The productive lakes include those with the highest and the lowest content of dissolved solids, while the least productive, L. Shala, has a gross chemistry remarkably similar to one of the most productive. An important influence on primary production is probably the depth of lake (the two least productive are the deepest) taken in conjunction with the generally poor penetration of light. In the absence of detailed studies of water circulation, L. Shala and L. Langano must be presumed to be optically deep (Talling 1971).

Principal interest in the sustainable production of Ethiopia's inland waters must surely rest with fish rather than algae but it would be wrong to leap directly to the conclusion that L. Awassa, L. Zwei and L. Abiata should be vigorously exploited without detailed studies of the fish and of the principal paths and process rates linking sunlight to fish. However, there is reason to anticipate that the 3 productive lakes would be the most obvious starting point for the research resources available. The present, albeit limited, scientific findings are consistent with the existence, over the past 20 years at least, of small scale fishing on the three productive lakes and the known populations of large colonies of fish-eating birds especially on L. Abiata (BROWN & URBAN 1969;

Urban 1970; Urban & Jefford 1977).