Inter-basin water transfer effects on the invertebrates of the Mvuzane River: A test of the SASS rapid biomonitoring protocol to assess the impact of flow modification

S.P Weerts,D.P Cyrus

doi:10.4314/wsa.v34i2.183637

Abstract

Effects of the transfer of water from the Thukela to the Mvuzane River were investigated using the SASS rapid bioassessment technique and more quantitative methods of guild composition and community analyses. Although the transfer was relatively small (< 1 m3/s) it completely dominated natural flows in the recipient system. SASS monitoring was found to be useful as a rapid, field-based assessment of water quality and can be used to act as a ‘red flag’ to problems in the future. However, it was not sensitive to changes in the community structure at different sites and was inefficient as a tool to investigate the wider ecological impacts of the transfer scheme, most notably those associated with flow modification. Changes in the invertebrate fauna were most easily explained in terms of the effect of increased flow velocities and volumes on taxa that were classified as ‘swimmers’ and ‘skaters’. Impacts via disruption to trophic pathways cannot be discounted and will most certainly occur with prolonged water transfer.

Highlights

Increasing water demands of mining, industrial, agricultural and domestic users in the greater Richards Bay – Empangeni area have largely been met by the Goedertrouw Dam, which was completed in 1980
By applying a variety of techniques of data analysis, this study provided the opportunity to comment on the impact of elevated flows on the invertebrate biota of a small subtropical river in an upper foothills reach and evaluate the efficiency of a rapid biological assessment technique (SASS) in detecting such impact
Flows differed markedly downstream of the transfer pipe outlet ranging from 0.220 to 0.710 m3/s during trial pumping in September 1999 but never exceeded 0.034 m3/s during natural flows in October 2000

Summary

Introduction

Increasing water demands of mining, industrial, agricultural and domestic users in the greater Richards Bay – Empangeni area have largely been met by the Goedertrouw Dam, which was completed in 1980. This indicated that higher SASS4 scores during 2000 were the result of higher numbers of taxa rather than more sensitive organisms, and suggested that the fauna at IBT sites did not change markedly more than that at control sites despite significant differences between flows during different years.

Results

Conclusion