Limnological studies on the Pretoria Salt Pan,a hypersaline maar lake

The Pretoria Salt Pan is shallow (maximum depth 2.85 m) and alkaline (surface water pH varied from 9.6 to 10.9) with pronounced mesothermy (38.2 °C in spring) at a depth of between 0.55 and 0.7 metres. Secchi disc transparencies ranged from 7 to 19 cm. A total ionic concentration gradient increasing from 59 500 mg · ?^?1 near the surface to 298 000 mg · ?^?1 at 2.75 metres stabilized the thermally inverted water column. Sodium was the major cation and nearly equal proportions of chloride plus carbonate and bicarbonate accounted for over 98% of the anions. The lake was meromictic with a steep chemocline, persistent thermally inverted temperature profile and complete anoxia at depths greater than 50 cm. The diel pattern of dissolved oxygen distribution involving a nocturnal deoxygenation and diurnal reoxygenation was unusual.