ABUNDANCE, BIOMASS AND DIVERSITY OF BENTHIC MACRO-INVERTEBRATES IN A WESTERN CAPE RIVER, SOUTH AFRICA

SUMMARY

The Eerste River is situated in the south-western Cape Province, South Africa. Its upper reaches have a stony substratum, and can be divided into three physical/biotic zones: the Mountain Stream, Upper River and Lower River zones. Investigations of the abundance, biomass, species diversity and species composition of the benthic macro-invertebrates in the three zones were carried out in 1975/76. Spatial and temporal changes in these factors can be correlated with changes in the physico-chemical quality of the water. Abundance and biomass generally increased downstream, but were low below a dam construction site, because of the blanketing effects of silt on the substratum. Seasonal changes in abundance and biomass differed in the three zones, but always included a peak in spring. The highest values of both were in the Lower River in summer, reflecting the build-up of organic pollutants there during times of low flow. Diversity of the Ephemeroptera, which was felt to be representative of a change in diversity of the fauna as a whole, generally decreased downstream, with a major drop at the town of Stellenbosch, and another at a sewage-farm outfall. A misleading increase in diversity occurred, however, in the silt-laden waters below the dam site, due to the fleeting occurrence of individuals of several different species. Composition of the fauna was typical of local rivers with mountain sources.