Evaluation of the sample size used for the rapid bioassessment of rivers using macroinvertebrates

Stream biological monitoring programs using benthic invertebrates have been implemented in all States and Territories of Australia in recent years. Although some variations exist in sorting strategies, all have adopted a rapid bioassessment approach with the collection of a single large sample from specified habitats at a site. However, the adequacy of the size of the sample collected has never been assessed. In this study, we examined data collected from rivers in three different States (Victoria, Queensland and Western Australia - each from a distinct bioregion of Australia), from four habitats (riffles, edges, macrophytes and pool rocks) and collected over four sample sizes (smaller and larger than that the national standard of 10 m or 10-20 pool rocks). We also used a subset of the data to examine the interaction of taxonomic resolution (family vs species) with sample size. All samples were collected using a live sorting approach which aimed at maximizing taxa richness while collecting about 200 animals. We found that the current recommended sample size adequately described the invertebrate community at a site in comparison to samples of other sizes. There were some differences between the States and these varied with habitat. In some instances, smaller sized samples would be adequate for monitoring purposes. Taxonomic level had little effect with only the riffle species samples showing a significant difference between sizes in contrast to the family level data which showed no difference.