Lakes of south‐western Uganda

Summary Lake Bunyonyi was formed about 18,000 years B.P. by a volcanic eruption which blocked a steep‐sided valley. Earlier this century fish were introduced into the lake but recently there have been mass fish deaths. Although the thermocline was inconspicuous, density profiles showed the lake to the stratified and chemical evidence suggested stratification was for many years. The water was clear with little plankton and the epilimnioncontained low concentrations of dissolved mineral ions. The hypolimnion was anoxic and had a relatively high mineral ion concentration. The mass fish deaths were attributed to a violent shallow mixing, probably caused by wind, but mixing was insufficient to remineralize the epilimnion.