MACROINVERTEBRATE COMMUNITY STRUCTURE AND ALTITUDINAL CHANGES IN THE UPPER REACHES OF A WARM TEMPERATE SOUTHERN AFRICAN RIVER

1. The Buffalo River rises 1200 m above sea level, drops 600 m in the first 7 km, and a further 100 m in the next 30 river kilometres. Macro invertebrates were sampled, and environmental variables measured month ly in 1987, at four sites along this part of the river. 2. Flow at the headwater site (1120 m a.s.l., 1 km from the source) was seasonal, th ough pools remained and subterranean flow was continuous. Twelve macro invertebrate taxa were found exclusively at this site, where conductiv ity, pH and nutrient concentrations were low. 3. Flow at the foothill site (530 m a.s.l., 7 km from the source) was perennial. The invertebr ate community, although distinct from that at downstream sites, lacked the unique taxa of the headwater site. Conductivity, pH and nutrient concentrations were higher. 4. The two sites downstream of the foothil ls (450 m a.s.l., 18 km from the source, and 410 m a.s.l., 31 km from the source), had similar invertebrate communities. Conductivity, pH an d nutrient concentrations were higher than at the upper sites. 5. Comm unity structure changed most between the headwater and foothill sites. This paralleled changes in river steepness rather than changes in mea sured physicochemical variables.