RAPID BIOLOGICAL ASSESSMENTS - HOW MANY ANIMALS ARE ENOUGH

Rapid bioassessment data using counts of benthic macroinvertebrates fr om the littoral zones of 5 lakes in south-central Ontario were examine d to determine if subsamples of 100 animals provided sufficient statis tical power to distinguish these lakes. One-way analysis of variance a nd power analysis were completed using 17 biological indices based on counts of 100, 200, and 300 animals. Despite the common perception tha t more is better, the doubled or tripled effort required to sort, iden tify, and enumerate more animals resulted in very little improvement i n our ability to distinguish lakes. Eight of the 17 indices were of li mited value for separating the 5 lakes. High correlations, indicating redundancies among the best indices, suggested that 1 or 2 metrics wou ld be sufficient to characterize the lakes. Three indices including % amphipods, % insects, and a multivariate metric representing the Ist a xis from a correspondence analysis ordination were the best metrics fo r separating the 5 lakes. A variant of the commonly used EPT index, co mposed of the total number of individuals of Ephemeroptera, Plecoptera , and Trichoptera, was also useful for characterizing the lakes. From this comparative analysis, subsamples based on counts of 100 animals a re sufficient to distinguish the littoral benthic communities of small inland lakes in south-central Ontario.