Bioassessment in ephemeral rivers: constraints and challenges in applying macroinvertebrate sampling protocols

Hydrology is a major factor controlling the structure and function of river ecosystems. In contrast to perennial rivers, temporary rivers have periods where surface flow ceases resulting in a highly variable flow regime. Existing bioassessment methods in South Africa are designed for use in perennial rivers, and thus the potential application of the rapid bioassessment protocol SASS (South African Scoring System) in temporary rivers forms the basis of this paper. Temporal variation in macroinvertebrate assemblages was studied at four sites on the ephemeral Seekoei River, South Africa. The upstream Sites 1 and 2 were significantly different from the downstream Sites 3 and 4 with respect to physical and chemical properties. When data were pooled, there were no significant differences in macro invertebrate assemblages among biotopes, months, seasons or years. Assemblages grouped largely on the basis of hydrological phase. A comparison among macroinvertebrate sensitivity in perennial, ephemeral and seasonal rivers indicated that ephemeral rivers had a higher proportion of insensitive taxa. Variable and limited habitats, dominance by generalist taxa and variable flow/no-flow periods impeded the application of SASS and MIRAI (macroinvertebrate response assessment index) to temporary rivers. An alternative approach to bioassessment and interpreting data for temporary rivers is discussed.