Oxygen cycle in a hippo pool, Serengeti National Park, Tanzania

The vertical distribution of temperature and dissolved oxygen was sampled over 24 h in a pond of the Seronera River inhabited by hippos. The waters were very turbid (visibility < 2 cm) because of the high turbidity from animals trampling sediment and a permanent surface algal bloom sustained by faecal matter. Direct solar heating was restricted to the top few centimetres. This resulted in a strong thermal and density stratification inhibiting aeration of the water column. Waters at mid-depth were aerated only when hippos stirred the water. Anoxic conditions were common in bottom water, these were occasionally ventilated in daytime by mixing due to bottom heating from decaying organic matter and at night by convective cooling. Poor water quality in hippo pools may affect wildlife.