STRATIFICATION IN TROPICAL AFRICAN LAKES AT MODERATE ALTITUDES (1,500 TO 2,000 m)

Abstract

Examples of stratification from nine tropical upland lakes, two in Uganda and seven in Ethiopia, arc described. Strong thermal gradients were often developed from diurnal warming in the 0–5‐m layer. In some productive lakes, these gradients frequently trapped photosynthetic oxygen to form well‐defined maxima. They sometimes also delimit deeper, oxygen‐poor layers in some highly productive lakes. One lake showed nocturnal mixing with complete deoxygenation, followed by a superficial restratification and photosynthetic reoxygenation by day. Deeper thermal gradients often determined the course of oxygen depletion at lower levels, and in some lakes also determined other aspects of chemical stratification, including the deep accumulation of Ca ++ and HCO 3 − ions. Considerable variation of the stratification was found in the Ethiopian Lake Awassa, both in time and horizontally over the lake basin, with some evidence that the deeper layer was formed by down‐streaming from locally cooled margins. Comparison is made with other lakes at similar altitudes in equatorial Africa. Although seasonal changes arc inadequately known, the incidence of complete mixing probably varies from very frequent (polymictic lakes) to very infrequent (oligomictic or meromictic lakes), and is determined by both depth and shape of the lake basin.