A preliminary assessment of the effects of a small south african inter-basin water transfer on discharge and invertebrate community structure

Abstract

This preliminary study (March–October, 1994) describes the effects of an inter-basin water transfer (IBT) on discharge and benthic macro-invertebrate community structure. The IBT from Theewaterskloof, an impoundment 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 summer (March) and 830% in autumn (May). During winter and spring, when the transfer ceases, discharges revert to normal winter volumes below the outlet. A decrease in taxon richness of the invertebrate communities was observed below the transfer outlet, compared to the river above the transfer. Sensitive families such as the heptageniid Ephemeroptera and leptocerid Trichoptera were not recorded below the outlet during transfer months (March and May). On the other hand, collector-predators such as the hydropsychid trichopterans showed large increases in numbers during the same transfer months, when compared against above-outlet samples: 240 and 80 individuals m−2, in March and May, from above the IBT, versus 46 994 and 5600 individuals m−2 below the IBT. This change was probably due to the introduction of live zooplankton to the receiving river from the source reservoir. Dendrograms and MDS-ordination diagrams showed that, in summer, invertebrate communities sampled at the two sites above and below the IBT were between 52 and 56% similar, while in winter and spring, in the absence of water releases similarities were higher (>70%) in July and October. There were also seasonal differences between invertebrate communities sampled in different months, but, during water release months, MDS-ordination diagrams showed 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 cyanophyte blooms (commonly of the genus Anabaena) from the source reservoir to the Berg River: transfers of non-toxic, malodorous geosmin, a cyanophyte exudate which has affected the flesh of rainbow trout farmed in the Berg catchment, have already been reported. © 1998 John Wiley & Sons, Ltd.