MACROINVERTEBRATE FUNCTIONAL FEEDING GROUPS IN THE MIDDLE AND LOWER REACHES OF THE BUFFALO RIVER, EASTERN CAPE, SOUTH-AFRICA .1. DIETARY VARIABILITY

1. The question of whether the diets of twelve abundant macroinvertebr ate taxa from the middle and lower reaches of the Buffalo River (easte rn Cape, South Africa) were too variable to allow them to be assigned to functional feeding groups (FFGs) was addressed. 2. Spatial, tempora l and developmental variations in diet were assessed. Foregut content analysis was used as an index to compare diets. 3. Foregut contents we re compared from early (small) and late (large) instar larvae collecte d from riffles at thirteen sites in summer, and from riffles, stony ba ckwaters and sediments at four of these sites in spring, summer, autum n and winter, in 1987. 4. For all individuals of all taxa, fine amorph ous detritus (0.5 - 250 mum), was the most common dietary item. Differ ences in diet followed a similar pattern for all taxa. The most freque nt differences in dietary content were between large and small larvae. The foreguts of large larvae contained more material, and a wider var iety of rare items than small larvae. Neither species nor FFGs could b e distinguished on the basis of foregut contents. 5. Two dietary types were recognized. The eight mayfly larvae were fine detritivores, havi ng ingested fine detritus almost exclusively. In contrast, the two hyd ropsychid caddisfly larvae had ingested a mixed diet; their gut conten ts included chitinous invertebrate remains and other items as well as fine detritus. 6. Intra-specific dietary variability was not so great as to prevent these taxa from being assigned to FFGs. On the contrary, inter-specific dietary content was so similar that gut content analys is could not provide a positive basis upon which to identify FFGs.