Spatial and temporal heterogeneity of lotic systems was examined by assessing variability in macroinvertebrate assemblages in two geographic regions, the Western Cape and Mpumalanga, in South Africa. Assemblages were assessed using the South African Scoring System (SASS), a rapid bioassessment method. The implication of spatial variability at regional, subregional and habitat levels, and temporal variability for defining reference conditions for macroinvertebrates was evaluated.

Macroinvertebrate assemblages varied both regionally and subregionally, and assemblages showed distinct geographic separation into Western Cape and Mpumalanga regions. These differences were most distinct in upland areas, i.e. mountain streams and foothill-cobble beds, with lowland areas less regionally distinct. 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 of the regional classifications. Spatial variability at the level of habitat, specifically SASS-biotopes, revealed that several taxa exhibited a degree of biotope specificity. Several families in both regions showed a preference for stones-in-current and stones-out-of-current (SIC/SOOC) although the availability and relative importance of SASS-biotopes to aquatic organisms varied regionally and seasonally. All three SASS metrics, i.e. SASS4 Score, Number of Taxa and average score per taxon (ASPT), differed significantly between biotope-groups, with highest scores consistently recorded in SIC/SOOC. SIC/SOOC was also the most consistent in terms of its associated macroinvertebrate assemblage. There was a significant positive relationship between SASS4 Score and number of taxa with number of biotopes sampled and a negative correlation between ASPT and number of biotopes sampled. Temporal variability, specifically seasonal differences, of macroinvertebrate assemblages was less pronounced than biotope-related differences and varied regionally. Temporal variability did not curtail the detection of disturbance at monitoring sites. Environmental variables at all spatial scales (i.e. catchment, site and habitat) were identified as potential predictor variables of macroinvertebrate assemblages.

Notwithstanding the spatial and temporal variability of lotic systems, it is possible to define a reference condition for macroinvertebrates. Adopting a regional framework, within which reference sites are selected and reference conditions defined, facilitates initial partitioning of variability resulting from differences at the regional and subregional levels. Further spatial partitioning is necessary at the habitat level, specifically separation of SASS-biotopes during the bioassessment and analysis phase. This study has shown that a reference condition comprised of biocriteria in the form of SASS scores, presented as biological bands, and expected SASS-taxa allows the identification of disturbed sites. The use of biological bands allows intrinsic variability in macroinvertebrate assemblages to be incorporated in data interpretation. Validation of these biological bands with data from monitoring sites is, however, an important step prior to finalisation of the biological bands.