Jay O'keeffe

Are Macroinvertebrate Assemblages in the Buffalo River, Southern Africa, Associated with Particular Biotopes?

Stream biotopes (habitat types) have commonly been defined using subjective recognition of hydraulic and substrate conditions. In this study the relationship between groups of macroinvertebrates and subjectively defined biotopes was examined at three sites in the upper, middle, and lower reaches of the Buffalo River. Biotopes which were recognised included riffles, leaf packs in riffles, and a waterfall (erosional biotopes); and stony backwaters, marginal vegetation and sediments (depositional biotopes). Over 100 taxa were identified from the three sites, and an hierarchical classification of 138 samples was prepared using TWINSPAN. The first division distinguished between the invertebrate assemblages of the upper reaches and those of the middle/lower reaches. In the upper reaches, the waterfall was differentiated from all the other stream biotopes. In addition, seasonal changes in assemblage composition were recognised. Divisions in the middle/lower reaches differentiated between biotopes. Of the 27 most common taxa, 13 occurred in a single biotope in more than 50% of the samples in which they had been collected. Clearest species-biotope associations were riffle-dwelling mayflies and caddisflies, and stone-dwelling planaria and ancylid gastropods. Invertebrate assemblages were not always associated with subjectively identified biotopes, particularly in the narrow headwaters, where different biotopes tended to be smaller and less distinct. However, associations of assemblages with biotopes were found in the middle/lower reaches. This study shows that stratified random sampling regimes can be useful tools in stream research if the scale at which biotopes are recognised is clearly defined.

Downstream effects of impoundments on the water chemistry of the Buffalo River (Eastern Cape), South Africa

The downstream effects of impoundments on the chemistry of pristine and polluted stretches of the Buffalo River were investigated in the context of the Serial Discontinuity Concept (Ward & Stanford, 1983). Impoundments which received water from a near-pristine upper catchment caused alterations of the water quality which were consistent with the Serial Discontinuity Concept, and recovery to riverine conditions was within 2.6 to 18.4 km of the dam, depending on flow. Impoundments which received agricultural runoff and urban effluents generally caused an improvement in the water quality of the downstream reaches (with the exception of nitrate concentrations, which were higher in the tailwaters compared to inflowing water). Impoundments with polluted inflows therefore usually ‘reset’ the river towards its natural condition, rather than acting as perturbations. This represents a reversal of the Serial Discontinuity Concept described for pristine rivers. The downstream effects of impoundments on water chemistry therefore depend on the relative impact of other catchment perturbations. These disturbances become more severe during low flow, and it is during this time that the impoundments have the greatest effect on the river.

An assessment of macroinvertebrate functional feeding groups as water quality indicators in the Buffalo River, eastern Cape Province, South Africa

The aim of this paper was to investigate the potential for using functional feeding groups (FFGs) as indicators of water quality conditions in rivers, using the Buffalo River, South Africa, as a specific example. Multivariate classification and ordination techniques were used to investigate species and FFG distributions in relation to a number of physico-chemical variables at 16 sites from the headwaters to the estuary of the Buffalo River.Two-way indicator species analysis (TWINSPAN) of species composition ranked most of the sites sequentially down the river, irrespective of water quality conditions. Ordination of FFGs from a set of riffle samples collected in mid-late summer showed only weak relationships between FFG distribution and water quality changes, except where variables changed sequentially down the river (e.g. pH and temperature). Individual species responses to water quality gradients were examined for nine riffle-dwelling species representing diverse FFGs. Following correspondence analysis of a matrix of environmental variables and species frequencies, some species showed strong associations with defined ranges of some variables. In particular, Adenophlebia auriculata (Leptophlebiidae, Ephemeroptera) from the headwater sampling site, was associated with low pH and low temperature. Simulium damnosum occurred under conditions of high turbidity, while Afronurus harrisoni was found under high concentrations of potassium, ammonium and nitrite ions.We conclude that although there was a distinct headwaters fauna in the Buffalo River, and sequential downstream changes in species composition, most FFGs (apart from shredders) were represented down the whole length of the river. FFG classifications are therefore unlikely to provide useful indications of water quality conditions in the Buffalo River.Using a categorical approach to classifying water quality variables, and by applying correspondence analysis to the resulting matrix, we recognised nine species that could be used to define water quality. These indicator species can be used to define tolerance ranges of the fauna for water quality conditions in different parts of the Buffalo river.

Feeding patterns of four macroinvertebrate taxa in the headwaters of the Buffalo River, eastern Cape

Gut content analysis, field and laboratory observations, and food choice experiments were used to assign four abundant macroinvertebrate taxa in the headwaters of the Buffalo River, eastern Cape, to functional feeding groups. The mayfly Adenophlebia auriculata (Leptophlebiidae) was classified as a collector: brusher; while the caddisflies, Dyschimus ensifer (Pisuliidae) and Goerodes caffrariae (Lepidostomatidae), and stoneflies Afronemoura spp. (Notonemouridae) were classified as shredders. The effects of organism size, season and biotope on dietary composition were tested, with size accounting for most of the dietary variability within each taxon. Larger individuals consumed more material, larger items, and, in the case of A. auriculata, a wider variety of food-types. There was little variation in the feeding of the taxa in different seasons or biotopes. Shredders ingested mainly leaf fragments, and this, rather than the size of particles in the gut, is a more useful basis for the shredder designation. A. auriculata was the most opportunistic feeder, and items in its diet additional to fine detritus varied seasonally and in the various biotopes. Of the shredders, Afronemoura spp. and D. ensifer were more varied in their diet, augmenting the staple intake of leaf material with other items. G. caffrariae was the most specialised feeder, being exclusively a shredder, regardless of biotope or season. Despite criticisms of the applicability of the FFG concept in the literature, we conclude that these taxa can reasonably be accommodated in functional feeding classes, and that the results are useful in describing the functions performed by the organisms in the river. The relationship between feeding function and river process is emphasised: we suggest that collectors contribute primarily to organic particle retention, while shredders facilitate organic particle size reduction and mobilisation, and the enhancement of substrates for microbial colonisation. An emphasis on river function is a useful context within which to view the FFG concept.