THE BENTHOS OF SOME SOUTHERN AFRICAN LAKES PART V: THE RECOVERY OF THE BENTHIC FAUNA OF ST LUCIA LAKE FOLLOWING A PERIOD OF EXCESSIVELY HIGH SALINITY

Summary

St Lucia Lakes (Zululand, South Africa) were sampled with a Van Veen grab in January 1972, July 1972, and January 1973. The system had high salinities at the commencement of the programme (70%₀-80%₀ False Bay, 55%₀-60%₀ North Lake, 45%₀-58%₀ South Lake) which quickly dropped to less than sea-water values in the first six months. The January 1972 samples showed that only a few Polypedilum (Chironomidae) survived in False Bay, while a very reduced faunal list was shown for North Lake (9 sp.). Nevertheless South Lake showed a fairly complete faunal list (21 sp.). By July 1972 nine species were recovered in False Bay, 18 in North Lake and 23 in South Lake, indicating reinvasion consequent upon dilution of the salts in the water. No great change was shown by the result of the January 1973 samples.

Evidence suggests that most forms would tolerate salinities up to about 55%₀ salinity. Beyond this level only Chironomid larvae and Ostracods survived. Most rapid reinvasion of the denuded parts of the system was by rapidly maturing forms with planktonic larvae. There is evidence of exchanges of water between the various areas consequent upon wind induced seiches which would carry the planktonic larvae to these denuded areas.

Numerical and gravimetric analysis of standing stocks showed that sandy substrates are dominated by the small Assiminea bifasciata (Mollusca; Gastropoda) which is probably the most important food item in the benthos, apart from the larger bivalves which were not sampled. Muddy samples were similar to sandy samples when Assiminea was excluded from consideration. Estimates of standing stocks for the St Lucia system were: for sandy substrates between 1,08 and 3,26 g/m⁻² and muddy substrates 0,013 and 0,063 g/m⁻².