THE BENTHOS OF SOME SOUTHERN AFRICAN LAKES PART III: THE BENTHIC FAUNA OF LAKE NHLANGE, KWAZULU, SOUTH AFRICA

Summary

Lake Nhlange (KwaZulu, South Africa) is connected to the sea through the Kosi estuary and has a salinity between 36%₀ and 5%₀. The benthos of the lake was sampled at approximately six-monthly intervals from July 1965 to January 1969. The substrates are mainly sandy in shallow water and also on steeply sloping profiles even in the deepest water of 30 m. Flat areas are mainly silty. In December 1965, following the closure of the estuary mouth and severe rains, the lake level rose to about 2 m above its normal level. Although the salinity changed little (5%₀, to 3%₀), quantities of peat-stained water and detritus were brought into the lake. A waning pattern of low oxygen conditions below about 12 m in the hot season, followed by reasonably well-oxygenated conditions in the cold season, was found. Particularly severe oxygen depletions were discovered at the mud water interface, even where well-oxygenated conditions pertained in the open water column at equivalent depths.

Evidence that the fauna was driven into shallower waters by the influx of detritus, with its high oxygen demand, is given and a slow recolonization of the deeper water is shown. Large fluctuations in standing stocks were noted, together with changes in dominant forms. The question as to whether this was entirely due to the effects of the flood or whether fluctuations in fairly stable conditions are to be expected is examined with reference to some Northern hemisphere lakes. It is concluded that fluctuations are to be expected under fairly stable conditions, but that those shown in the present study were in the main due to the effects of the flood.