A comparison of several methods of assessing river condition using benthic macroinvertebrate assemblages

This paper attempts to address some of the problems that are faced in the endeavour to minimise the time and financial costs of environmental monitoring. At the same time, it aims to assess the usefulness to freshwater studies of some of the advances being made in the marine sciences. The paper examines the results obtained and the conclusions drawn using several recognised techniques for measuring aquatic macroinvertebrate communities. The physical, chemical and macroinvertebrate data used were collected upstream and downstream of a land-based trout farm situated on a tributary of the Elands River in the Molenaars River catchment, south-western Cape, South Africa. Five measures of community stress were examined to determine the extent to which they summarised community changes in response to the trout farm effluent. They ranged from those requiring quantitative sampling and species-level identification to those requiring only qualitative sampling and an ability to distinguish between taxa. The results obtained suggest that: the incorporation of an abundance measurement can greatly enhance the sensitivity of an index; combining the results from different microhabitats can artificially inflate the scores obtained at a site; change in community structure following a perturbation may be more marked at higher taxonomic levels, because the community response is more evident above the natural environmental noise; identification to resolution lower than family (e.g. order) may result in important information being missed, and increase the chances of the effects of a perturbation going unnoticed. It is concluded that quantitative or semi-quantitative family-level data probably represent the best compromise between the costs of collecting and processing the samples, and the information content of the resultant data, but that the use of several different techniques would enhance the reliability of the conclusions drawn. In addition, it is recommended that some of the advances being made in the marine sciences, such as the 'marine' ranked Abundance Biomass Curves (ABCs) should be developed and implemented, since they can provide a much needed functional underpinning for results obtained. Keywords: biomonitoring; water quality; mountain streams; macroinvertebrates; community data; trout farms (Afr J Aqua Sci: 2001 26(2): 135- 147