A PRELIMINARY ASSESSMENT OF THE EFFECTS OF A SMALL SOUTH-AFRICAN INTER-BASIN WATER TRANSFER ON DISCHARGE AND INVERTEBRATE COMMUNITY STRUCTURE

This preliminary study (March-October, 1994) describes the effects of an inter-basin water transfer (IBT) on discharge and benthic macro-inv ertebrate community structure. The IBT from Theewaterskloof, an impoun dment on the Riviersonderend system, to the Berg River in the Western Cape, South Africa is part of the water supply system for Greater Cape Town. Transfers occur during summer for irrigation and rainbow trout (Oncorhyncus mykiss) farming in the Berg catchment and lead to greatly elevated summer discharges in the recipient river: 4500% in late summ er (March) and 830% in autumn (May). During winter and spring, when th e transfer ceases, discharges revert to normal winter volumes below th e outlet. A decrease in taxon richness of the invertebrate communities was observed below the transfer outlet, compared to the river above t he transfer. Sensitive families such as the heptageniid Ephemeroptera and leptocerid Trichoptera were not recorded below the outlet during t ransfer months (March and May). On the other hand, collector-predators such as the hydropsychid trichopterans showed large increases in numb ers during the same transfer months, when compared against above-outle t samples: 240 and 80 individuals m(-2), in March and May, from above the IBT, versus 46994 and 5600 individuals m(-2) below the IBT. This c hange was probably due to the introduction of live zooplankton to the receiving river from the source reservoir. Dendrograms and MDS-ordinat ion diagrams showed that, in summer, invertebrate communities sampled at the two sites above and below the IBT were between 52 and 56% simil ar, while in winter and spring, in the absence of water releases simil arities were higher (> 70%) in July and October. There were also seaso nal differences between invertebrate communities sampled in different months, but, during water release months, MDS-ordination diagrams show ed clear grouping of communities according to position above or below the IBT, rather than month. Results to hand indicate a 'reset' of the entire benthic invertebrate community during winter and spring when no transfers occur. Of concern is the potential for the transfer of cyan ophyte blooms (commonly of the genus Anabaena) from the source reservo ir to the Berg River: transfers of non-toxic, malodorous geosmin, a cy anophyte exudate which has affected the flesh of rainbow trout farmed in the Berg catchment, have already been reported. (C) 1998 John Wiley & Sons, Ltd.