Spatial variability in macroinvertebrate assemblages: comparing regional and multivariate approaches for classifying reference sites in South Africa

Abstract

Spatial variability in macroinvertebrate assemblages was examined with the aim of evaluating the utility of regional classification systems in aquatic bioassessment. Sampling was undertaken at reference sites in the Western Cape and Mpumalanga, South Africa, using the rapid bioassessment method SASS4 (South African Scoring System Version 4). Multivariate analysis of macroinvertebrate assemblages showed that assemblages varied regionally with differences most apparent in upland areas, i.e. mountain streams and foothill-cobble beds, with lowland areas less regionally distinct. Within regions, longitudinal zonation into upland and lowland areas was important, with sites grouping on the basis of broad geomorphological zones or subregions. Of the upland sites, differentiation into mountain streams and foothill-cobble beds was not evident, although overall variability of assemblages within upland areas, in particular the Western Cape, was very high. In general, a priori regional classification of sites using the hierarchical spatial framework developed in South Africa provided a useful framework for preliminary classification of reference sites. Groups of sites based on a posteriori analysis of macroinvertebrate data, however, provided a more robust classification than any regional classification. Spatial classifications therefore offer geographic partitions within which to expect somewhat similar conditions, and regional reference sites selected within the context of the spatial framework are likely to be more representative of specific river types than those selected without using the framework. Classification of sites needs to be an iterative process that allows for subjective a priori regional classifications to be modified on the basis of independent, objective a posteriori classification of biological assemblages.